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Vacuum advance benefits?

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Topic: Vacuum advance benefits?
Posted By: ccowx
Subject: Vacuum advance benefits?
Date Posted: Nov/23/2011 at 7:20pm
Gentlemen,

I am debating about whether to use a full mechanical advance distributor or to stick with the stock Delco Remy one that has both mechanical and vacuum advance. I am well aware of the advantages of having the additional advance at idle and steady cruise situations, ie better economy and running cooler at idle. My reason for doubting that it will help much in my case is that my car is a 4 speed and has 4.10 gears. On the highway(where I rarely am anyway) it is doing about 3800 rpm's or so and I don't sit and idle in traffic for long periods of time. Between the stick and the gears it is on and off the throttle and since I use it as an around town fun car, most of it's time is spent on city streets at non-rush hour times. 

I figure that given the type of driving I do, the vacuum advance will have little to no effect. Any thoughts?

Thanks,

Chris 

PS: Happy Thanksgiving from a Canadian and his American wife!



Replies:
Posted By: turbo
Date Posted: Nov/23/2011 at 7:27pm
Kudos on the american wife (I think). The question you pose is an interesting one.....Rather than pretend I know the exact answer if it were mine I would just drive it for a while in each configuration and see how it goes.  Sometimes the answer becomes clearer with just a bit of time.  That's how I roll.....

-------------
they call me Capt RETIRED!


Posted By: ccowx
Date Posted: Nov/23/2011 at 8:24pm
Makes sense. Time will tell! Given the rpm's involved and the already poor mileage, I am not expecting miracles either way. My main wonder was whether the high rpm's on the freeway will basically eliminate any real benefits. Kind of hard to imagine an "economical cruise" at nearly 4000 rpms!

Thanks,

Chris 

PS: I am happy with her. Means two turkeys in the house, mind you!


Posted By: poormansMACHINE
Date Posted: Nov/23/2011 at 8:26pm
Vacuum advance at idle shouldn't make anything run cooler at idle since there isn't any vacuum available.


Posted By: ccowx
Date Posted: Nov/23/2011 at 8:41pm
Vacuum is high at idle, is it not? As I understand it, the more efficient burn from the additional advance makes for a slightly cooler running engine at idle. Probably a minor benefit at best, but in some cases perhaps valuable. I am really more concerned about any significant mileage benefits while driving. 

Chris 


Posted By: poormansMACHINE
Date Posted: Nov/23/2011 at 8:46pm
correctly installed, the vacuum advance isn't connect to the intake manifold. It's connected to a ported vacuum source that only gets vacuum at part throttle.


Posted By: 73hornut
Date Posted: Nov/23/2011 at 8:53pm
Most of my amc's experienced more ping and run-on with vacuum advance connected, disconnected, plugged port, ran fine.

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71 Javelin
74 Gremlin
79 Spirit AMX
Rogue Valley Rumblers
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Posted By: PHAT69AMX
Date Posted: Nov/23/2011 at 8:54pm
I lay no claim to being an expert, but this is what has been read.
Understanding is Vacuum Advance keeps the spark plugs clean.
A lean mixture burns slower so needs ignition fired sooner.
While at any given steady state cruise speed, the carb is choked down,
barely opened only enough to maintain the given steady state speed.
In this situation two things are taking place, high manifold vacuum
and a lean cruise condition, possibly running only on the idle circuit and transfer slots.
This is when Vacuum Advance is actuated, firing the plugs sooner, keeping them clean.
 
Ignition advance for "performance" is usually thinking for at WOT.
At WOT there is more or less no manifold vacuum and the Vac Adv is doing nothing.
Total Advance usually is discussing Intial and Mechanical Advance only, Vac Adv not included.
This is the 34 to 40 Total Adance value usually brought up, Initial and Mechanical only.
 
Vacuum Advance is applied in addition to the 34 to 40 total Initial plus Mechanical.
Vacuum Advance Canisters are available in a range of added advance amounts,
generaly in the range of 16 to 24 crank degrees.
They also vary in the amount of vacuum needed to Start Adv and give Full Advance.
There is, or may have been , a Limiter Sleeve on the Vac Adv Can actuator pin
to reduce the amount of advance the vac can can provide.
The Delco points style distributor Mechanical Advance Limiter Pin
can/did/does/may also have a Limiter Sleeve on it to reduce the amount of total mech advance.
Mr. Gasket Re-Curve Kits for Delco Points distributors include a brass Mech Adv Limiter Sleeve.  
 
For this example we'll use a Vac Adv Can with 20 crank degrees of advance.
Looking at stock distributor specs listed in the TSM's will show similar combined totals, but
they are usually achieved with less Intial and more Mech Adv coming in later, at higher rpms. 
+13 deg Initial Advance
+24 deg Maximum Mechanical Advance
=37 deg Total Initial + Mechanical Advance target, midrange of 34 to 40 range.
+20 deg Vacuum Advance
=57 deg Actual Ppotential Total Adv, Initial + Mechanical + Vacuum
 
Anyways, sorry to go on and on about it.
Understanding is the main thing the Vacuum Advance does is keep spark plugs clean.
 


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Posted By: 69 ambassador 390
Date Posted: Nov/23/2011 at 9:04pm
A properly set up vacume advance will be more responsive on the street and reacts to load as well as rpms.  A mechanical only sytem is totaly rpm dependant and engine load has no effect on it.  A properly set up mechanical system that give a full 38 or so degrees advance will likely advance too fast for a street car and cause detonation issues.  If you set it up with stiffer springs so it advances later you end up under advanced at lower rpms of street driving.  You will also end up running more initial and this usually leads to cranking issues and short starter life.  The vacume/mechanical set up gives a good compromise for a street car.  If however you are road racing and never getting below 3500 rpms a mechanical works very well.  I know This post will generate a lot of veiws to the contrary so let me also state that I have run full mechanical on the street in the past and had it run o.k., just always better with the vacume.  As far as mileage, you will do better with vacume also.  Vacume should only be run on ported and anyone who says otherwise is not in possesion of the facts.  The O.E. systems used a ported vacume switch that was temperature sensative to feed manifold vac. to the advance when overheated to raise the engine speed and help cool the engine.  When operating normally they still all run ported.

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: 69 ambassador 390
Date Posted: Nov/23/2011 at 9:41pm
Way off base PHAT, Sorry.  Spark plugs have nithing to do with it and the math is all wrong.  Total includes all advance and initial is just the starting point.  Ported is zero at idle and max at higher rpms.  The signal is generated above the throttle plates and has nothing to do with manifold vac.  The vac advance uses ported and is airflow generated and not generated by the pumping action of the engine directly.  If you hook to manifold vacume then you get detonation at light throttle becaus of over advancing.  Under acceleration you lose the advance when you need it most because vacume drops.  A vehicle that detonates unless you plug the vac. advance is probably running lean or has other issues.  If you have to plug the advance off than you have tunning issues.  Thats a bandaid because you don't get correct advance but it stops the symptons.  This is costing power and mileage and possibly engine damage.  Late timing causes elevated exhaust valve and port temps and leads to recession and port/manifold cracking.  It also heats the heads and is one of the main causes of damaged seats.  There is little coolant in these areas compared to the block so you may not even notice the elevated coolant temps you might expect.  The localised hot spot from late timming can even cause deiseling.  This is why the OEMs used anti deisel solenoids when they retarded the factory specs. to help lower NOX formations.

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: ccowx
Date Posted: Nov/23/2011 at 10:22pm
Thanks guys, much useful information! I have a clearer understanding of the factors that I need to take into account. I think at this point I just need to try it and see. I have a stock DC type in place now and running well, with vacuum advance. The distributor I would like to try is the old Mallory DP Group 19 job. There are several peculiarities that come into play, such as the DP does not use a straight advance curve but rather a sudden amount of timing initially and then the rate of advance slows down to keep things safe, detonation wise.

You have all given me useful info and now I think I just need to try it. Every combo is different! 

Thanks!

Chris 


Posted By: PHAT69AMX
Date Posted: Nov/23/2011 at 10:51pm
69 Ambassador 390, and all, thank you, I try to be open minded about it, always more to learn.
In all the Holley carbs I've re-built numbering well over 100, the Ported Vacuum pick-up point
is located in the Base Plate just barely above the top of one primary butterfly blade at idle position.
I lack the specific knowledge to know if this location would be considered "venturi vacuum" but
logically see it as close to being manifold vacuum since it lacks being up in the main body venturi.
Very often I find a vacuum signal, albeit low, at the Ported Vacuum port because the
Primary Idle Speed Screw is opened too far, prematurely exposing the base plate pick-up port.
Experience is that anything more than 3/4 of a turn opened from fully closed on the
Primary Idle Speed Screw will prematurely expose the Ported Vacuum base plate pick-up port.
By first setting the Primary Idle Speed Screw at 1/2 to 3/4 turn opened from fully closed,
and then setting the final desired Idle RPM by opening the Secondary Idle Speed Screw,
elimination of the premature vacuum signal at the Ported Vacuum port at idle can be eliminated.
 
Occasionally I will set one up with Vacuum Advance on Manifold Vacuum,
but only if the cam duration and overlap are considerable, causing reversion,
poor idle vacuum, and the need to open both Primary & Secondary Idle Speed Screws too far
and over exposing the Transfer Slots at idle to achieve the desired Idle RPM.
To do this a B28 Vacuum Advance Can or something with very similar specs
must be used since the B28 Vacuum Advance Canister starts at only 4" of vacuum,
gives full Advance at only 8" of vacuum, and provides only 16 deg crank of total vacuum advance.
The 16 degrees crank can be limited further by use of a Limiter Sleeve on the Vac Adv Can Pin.
A Limiter Sleeve can also be utilized on any other Vac Adv Can, but it will only limit
the total amount of advance that Vacuum Advance Canister can provide, but it will not change
the required vacuum level to start vac adv or the vacuum level required for full vacuum advance.
If setting one up to run vacuum advance on manifold vacuum, the Vacuum Advance Canister used
should / must achieve full advance at a vacuum level lower than the manifold vacuum at idle
in order to prevent hunting of the vacuum advance.  With the additional advance provided at idle
by the Vacuum Advance Canister attached to manifold vacuum it allows closing down of
both Primary and Secondary Idle Speeds Screws and reducing Transfer Slot exposure
while maintaining the desired Idle RPM without the negative affects on hot starting
that can be caused by dialing in too much intial advance.
 
I need to learn to understand how vacuum advance on ported vacuum can improve
throttle response since as soon as the throttle is moved very far both the manifold vacuum
and ported vacuum, at least picked-up where it is on a Holley, drop to zero
and the vacuum advance is all lost or goes away when connected to either source.
Now for light to part throttle acceleration I can see how vacuum advance on ported vacuum
will maintain a vacuum signal and some amount of vacuum advance will remain applied
and take the place of some amount of mechanical advance not yet engaged due to engine rpm.
 
I always find this type of tuning information and education very interesting.
Long long ago when I first made changes to the stock advance curve and parts
in my '69 343 AMX the difference it made was significantly noticeable.
Was even more intrigued once a understanding of checking and setting
the total initial and mechanical advance at high rpm with a dial-back timing light
was practiced rather than only looking at the timing advance setting only at idle.
 
Changes to these parts and settings are pretty easy, so a person can easily try
different parts and set-ups while carefuly keeping an eye on things imho.


-------------


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Posted By: purple72Gremlin
Date Posted: Nov/23/2011 at 10:59pm
If the engine is stock.....leave it on......but now, if its been hopped up, and you are running a 4 speed....and 4.10 gears.....turning 3800RPM's and highway speeds......I dont see how you are going to get any kind of mileage.......I would tune it to run as best as the combo will permit and not worry about fuel economy......Just my opinion....


Posted By: nda racer
Date Posted: Nov/23/2011 at 11:57pm

What size mill and what chassis is it in? With a 4spd and 4.10s, unless your car is 3800pds or so, I don't see any benefit. But hey, you can always try.



Posted By: ccowx
Date Posted: Nov/24/2011 at 12:27am
1970 Javelin 390. The engine is running the whole group 19 batch of stuff such as cam, R4B, Thorley headers, etc. Not radical but definitely an increase over factory stock. My guess is that I am unlikely to get a massive increase in mileage, given that the rpm's are so high. Economy kind of went out the window when the 4.10's went in! I will try it out and see if I have any massive sacrifice in either mileage or drivability.

Thanks,

Chris  


Posted By: 6PakBee
Date Posted: Nov/24/2011 at 7:15am
69 Ambassador 390 is on the mark.  The only reason for vacuum advance is part throttle fuel economy.  To completely burn the air/fuel mix you have to have a lot of ignition advance at cruising speeds.I looked at PHAT's numbers and reviewed what a typical HP Mopar distributor would have (all are crank degrees):

Initial Advance:  8 BTDC
Centrifugal: 21 @ 4600 rpm
Vacuum:  24 @ 16 InHg

Total: 53 degrees

This is the way I see it based on my limited Mopar experience with both full mechanical and combination vacuum/mechanical distributors.  The total advance necessary for peak horsepower at full throttle is not enough for peak fuel economy at reduced throttle.  You have to decide which route you are going to take.  If you run a full mechanical advance so you are at 34 degrees total at 3200+ rpm, this will be a good performance combination but when you are cruising at 3000 rpm at 30 degrees and you could stand 50 degrees, you are going to pay for it through reduced fuel consumption.

As to ported versus manifold, I have never done it myself but I have read a Mopar tech bulletin that said the only difference between ported and manifold vacuum sources was that the ported source is closed at idle.  Other than that they are the same value.  The reason this was a concern for the Mopars was that with a more radical cam that required a higher throttle opening for curb idle, the advance port was uncovered and brought the vacuum advance into play.  This caused an erratic unstable idle.  The solution was the common drilling of the primary throttle butterflies to allow them to be closed further and close the advance port.

Just my TwoCents


-------------
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

Forum Cockroach


Posted By: 70amcpwr
Date Posted: Nov/24/2011 at 8:38am
X2 w/Phat. I always unhook the vacuum advance when setting full advance timing, some here say it doesn't matter but timing lights don't lie (usually). As for ported advance, you let off the throttle and vacuum is gone, right now, manifold vacuum will run on due to reserve vacuum needing to be depleted, then its the same. I believe ported will react quicker on throttle openning but I don't know if that would be noticable.

-------------
70amcpwr You just can't fix stupid.
SOLD 1970 BBO Javelin, wifes 73 AMX 360 4spd. Next project 1969 AMX


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 8:48am
The problem with cracking the secondaries a bit to raise the idle without bringing the vac. advance into play is that the secondaries have no idle passages.  You end up lean in the rear part of the engine on a single plane and way screwy on the mixture on a dual plane.  the solution to this on non-stock large cam engines is a four corner idle system.  Some hi-po Holley carbs have this, most do not.  A properly set up vac advance distributor will eliminate the need for this.  You cannot directly add both the total mechanical and the vacume advances together.  They work in parallel to each other and never actually acheive that over 50 number unless set up incorrectly and without limiters.  Unfortunately most store bought rebuilds are way wrong.  Advance at idle or just above it from the cannister allows a more retarded initial setting to aid in easier starting and reduction of cold related lean backfires.  This set up is not really suited to H.P. use.  An adjustable cannister can be used to set Vac. advance so that it starts to come in above idle by a few hundred rpm.  You would when run more initial when setting the base timing.  I also add weight to the flyweights and try to bring in the mechanical sooner.  All by about 3200-3500.  This is just above where I cruise on the hwy.    

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: DesertAMX
Date Posted: Nov/24/2011 at 9:14am
I agree with 69 ambassador 390,  a properly setup distributor with vacuum advance is best for street.  Best for drivability and throttle response.   As you are looking at distributor choices, consider the MSD Ready to Run.  It is easy to setup and taylor your total mecanical advance and rate and has vacuum advance.  It comes with a vacuum advance lockout if you choose later to disable. It does not need the MSD box.  I replaced my stock distributor with the MSD on my 69 AMX 390 several years ago and after a little tuning, very happy.   Rather than cracking the secondaries, one trick to getting a little more idle air is to drill a 3/32 hole in each primary plate. This will allow a little more air, but keep the plates closer to the idle ports.  Once done you can't easily go back.


Posted By: Bowtie70SS
Date Posted: Nov/24/2011 at 10:14am
Here is an article about the vacuum advance written by a GM engineer. I found it helpful.


Part 1 - TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

Part 2 - Distributor Vacuum Advance Control Units

I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for driveability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

Part 3 - Vacuum Canister Part Number & Specs

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

--------------------------------------------------------------------------------------------------------------------------------------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8

--------------------------------------------------------------------------------------------------------------------------------------------

VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
1979 305 El Camino Calif.
1978-79 350 Blazer & Suburban
1979 Buick 305/350

VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
1975 454 Caprice, Chevelle, Monte, Suburban

VC1826 AR2 5-7 12 @ 10-13

VC1827 AR3 5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
1976 350 Pontiac

VC1831 AR7 6-8 12 @ 14-16

VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14

VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16

VC1834 AR9 4-6 13 @ 13-16

VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
1977 350 All exc. Calif.
1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305 All Exc. Calif
1976 350 All Exc. Vette, Exc. Calif
1976 350 Vette Exc. Calif., Exc. Hi Perf
1975 262, 350 All w/2-bbl carb
1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
1977 305 Chev Truck Light Duty
1975-76 350 El Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl

VC1839 AR13 4-6 12 @ 11-13

VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14

VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15

VC1844 AR16 3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
1977 301 Pontiac

VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
1977-79 350/403 Pontiac Hi Alt

VC1848 AR18 4-6 12 @ 9-12

VC1849 AR19 4-6 12 @ 7-10

VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11

VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
1978-79 350 Pontiac

VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty

VC1854 AR24 3-5 13 @ 10-13

VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12

VC1856 AR25 3-6 15 @ 10-14

VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
1978 350 Camaro
1978 305 Chev Truck, M/T, Light Duty
1978 350 Chev Truck Hi Alt
1978 305/350 Buick & Olds
1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
1978-79 305 w/1103282 Distr., Incl. El Camino A/T
1979 350 Camaro, Impala, Nova, Malibu, Monte
1979 350 Suburban
1979 350 Buick Century
1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi Alt.

VC1860 AR29 3-6 12 @ 10-13

VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
1979 301 Olds
1978-79 301 Pontiac

VC1863 AR32 2-4 10 @ 11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13

VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15

VC1867 AS9 2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
1979 305 Buick & Olds
1979 305 Pontiac A/T

VC1869 AS10 2-4 12 @ 8-11 __________________


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 11:48am
Pretty good article but off in some areas.  Hc emissions are actually reduced with the advanced idle but NOX emissions are increased.  The ported signal is not created in the venturi but in the throat of the carb just above the plates.  He is correct on Cannister set up.  The reason to run ported is exectly that it has no signal at idle.  This allows more initial.  The oems used various pvs and check valves to allow more vac advance from manifold during both warm up and then when overheated also.  This was the reason for the air inlet temp valves and the two ported swithces on our AMCs.  I'm not sure a G.M. engineer is the right guy to listen to.  Have you ever driven the late seventies or early eighties crap they produced?  Find an AMC engineer to listen to.  They had the most advanced yet uncomplicated systems on carbs right up until the last one in 1991.  The 91 wagoneer had the highest mileage of any full size vehicle and they were the only one running a carb.  If you are running a street car and don't run all the valves that control the mix of ported to manifold signal than you should run only ported on the street.  But in the end, run what you want.  More people run ported than manifold no matter what make you have.  FACT.  The reason is it works better.  Remember when people thought the world was flat or the center of the universe.  Experts argued that it was true even in the face of overwhelming evidence to the contrary.  And they weren't stupid people.  They were smart people who were wrong. 

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: Red Devil
Date Posted: Nov/24/2011 at 1:08pm
Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

Remember when people thought the world was flat or the center of the universe.  Experts argued that it was true even in the face of overwhelming evidence to the contrary.  And they weren't stupid people.  They were smart people who were wrong. 
 
.... you mean the world isn't flat and we're not the centre of the universe Shocked ... say it ain't so Cry
 
Understanding is manifold and ported vacuum are the same above idle, once the the throttle blades are open enough to expose the ported vacuum port.   So the debate about manifold vs. ported vacuum is mostly about idle / off idle.
 
If you've got a lumpy cam or very little initial timing, manifold vacuum may help idle and transition to part throttle ... or you could just run more initial timing.   Problem with using manifold vacuum is idle vacuum may not be too stable, so timing may jump around and become erratic.  This is where using a vacuum advance unit that's fully deployed at less than idle vacuum will help ... but it will have limited ability to fine tune advance at varying loads and throttle openings at higher rpms like ported, if using a vacuum advance unit with higher vaccum setting @ full advance. 


Posted By: Bowtie70SS
Date Posted: Nov/24/2011 at 1:20pm
www.fueleconomy.gov   In 1991 the Grand Wagoneer got 11mpg city and 12mpg hwy Dodge Ramcharger 11city 12hwy while the Blazer got 13 city 15hwy, the Ford Bronco got 12mpg city and 15hwy. The Suburban got 12city 15hwy. Not hatin on  Jeeps but they weren't the most fuel efficient according to the EPA.


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 1:26pm
You are correct sir.  That is why there is no reason to run manifold vac at all.  It's during the transitions that ported comes into play.  And what has not been said so far is that the speed of the burn remains virtually constant no matter what rpm we are at.  What does change is the time for it has to burn.  At higher rpms there is simply less time to complete the burn and thus we need more advance.  This condition is usually encountered at or near WOT and the vac. signal is low.  This is where the centrifigal comes into play.  Most conditions in normal driving the ported vac. runs the show.  Also, the vacume advance can't pull more than the breaker plate allows, so again a properlt designed set up works best.  Put the limiters were they belong.

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 1:28pm
Originally posted by Bowtie70SS Bowtie70SS wrote:

www.fueleconomy.gov   In 1991 the Grand Wagoneer got 11mpg city and 12mpg hwy Dodge Ramcharger 11city 12hwy while the Blazer got 13 city 15hwy, the Ford Bronco got 12mpg city and 15hwy. The Suburban got 12city 15hwy. Not hatin on  Jeeps but they weren't the most fuel efficient according to the EPA.
Check the late 6 Cyl.

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: Bowtie70SS
Date Posted: Nov/24/2011 at 1:36pm
 A 6cyl is available in only the Cherokee and it was rated at 16 and 18 while a full size 4.3 Chevy truck was 16 and 18. Commendable but it doesn't exactly leave the others in the dust.


Posted By: turbo
Date Posted: Nov/24/2011 at 1:55pm
man, that article above NEEDS to be made a sticky, BIG TIME.    Great overview that needs to be revisited from time to time to stay current unless you're doing this EVERY DAY and even then I think the builders need to sometimes be reminded.  trust me I have had some great builders and some pogues that I would not let work on my lawnmower.  so how bout that sticky?

-------------
they call me Capt RETIRED!


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 2:19pm
The Grand Wagoneer was offered in Base or Limited trim levels in two and four door models and was built with a 6 Cylinder, 4.2 Liter,102 Horsepower engine. There was also a V8 engine available as an option for the single trim Grand Wagoneer. The Grand Wagoneer was offered in ten colors. The 1991 model was available with a "Final Edition" badge on the dashboard.

Despite its advancing age the Grand Wagoneer remained popular. Instrument panel, grille, and tail lamps were redesigned for 1986, followed by minor revision to the wood grained sides in 1987, the year that ownership of the company passed to Chrysler Corporation. Chrysler largely left the Grand Wagoneer alone, and even continued to build the Grand Wagoneer with the carbureted AMC V8 instead of its own, and arguably, more modern fuel-injected V8s. Year-to-year changes were minimal; Chrysler added new features such as an overhead console taken from Chrysler's popular minivans and a rear-window wiper/washer system for 1989, but otherwise new model years through 1991 were marked only by new paint colors.



-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: turbo
Date Posted: Nov/24/2011 at 2:35pm
Originally posted by Bowtie70SS Bowtie70SS wrote:

www.fueleconomy.gov   In 1991 the Grand Wagoneer got 11mpg city and 12mpg hwy Dodge Ramcharger 11city 12hwy while the Blazer got 13 city 15hwy, the Ford Bronco got 12mpg city and 15hwy. The Suburban got 12city 15hwy. Not hatin on  Jeeps but they weren't the most fuel efficient according to the EPA.
heck no, anyone who was awake for a portion of their lives knows jeeps have always been thirsty.  so were the dodges until the new hemi.  just the way it was.....

-------------
they call me Capt RETIRED!


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 2:52pm
I actually had the dates wrong.  The sixs got huge mileage for a full size vehicle.  I found this from a little research.  Note,  this is a carbed enging with vac. advance.  From Alpars.com, history of Jeep Wagoneer.  My own experience with a six wagoneer supports this.  They actually had more bottom end grunt than the V8s.
 
" 1983 continued with few changes; the Wagoneer was available only in the Brougham and Limited models. A 4.2 liter six or 5.9 liter V8 were available, both with two-barrel carbs; the V8 was not available in California; and automatic transmissions were standard. The Wagoneer Six was rated at 18/23 mpg, the V8 at 13/18 mpg.

The Cherokee was sold in two-door base, Pioneer, Chief, and Laredo models, with the standard 258 cid (4.2 liter) six or 360 V8, both carrying a two-barrel carb; gas mileage was 18/24 for the six, 14/20 for the V8 (presumably tested with the four-speed manual). The V8 was no longer available in California.

Based on Cherokee gas mileage, the "automatic transmission penalty" was roughly 1 mpg for the six and 2 mpg for the V8."

 


-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: 70amxvegas
Date Posted: Nov/24/2011 at 3:34pm
The ? he had was not about MPG is just a simple ? as  does a high performance engine need the vac. at all,  since he drives it hard. This got side tracked to 6 cly MPG, I still have mine hooked up but was told i do not need it since MPG means nothing to me but i still have it hooked up since the dist, was curved with it and it runs good as is.for now anyway


Posted By: billd
Date Posted: Nov/24/2011 at 4:00pm

IS this debate back up alive again?

We've gone through this many times, I bet there are at least a dozen threads here on the topic, and most of it mis-information.
There are 3 vacuum types, manifold, ported and venturi.
Before the emissions controls and complex solenoids, stats and all, it was ported and venturi.
 They went to manifold vacuum controlled via other means on many engines (not all) for emissions reasons. Otherwise, the ORIGINAL vacuum advance as installed and designed in the 1930s was ported from the carb, there was no manifold vacuum used until much much later.
Venturi vacuum has also bee used as well as ported, and it's the opposite of the others at some stages - full throttle high speed is HIGH vacuum, which matches the time you want more advance.
Originally, it was designed to help in-town, street driving, take care of stumbles and issues that accelerator pumps did not or could not cover. In short, the design specs and reasons at the time were "improve drivability, performance".
I drive my car hard, too - no, I don't race it, but it's not babied. I see a difference on my car in town.
 
PS - that article will not be made a sticky here as most of it is pure bunk and has been debunked at no small number of other sites and forums. It's one of those urban legand things where some s-called engineer wrote and it became law. Much is wrong.  Sorry, the bad information will not be perpetuated here.
It's been around a lot and folks see it and see the word "engineer" attached....... I myself in this very forum have found information to directly contradict much of it - from solid sources, and have info myself in books backed by GM, Ford and Chrysler that contradict too much of it to be able to trust much of it at all.
 
PPS - do a search in the forum - you'll see this hashed so many times it's right up there with oiling mods - maybe even more.........  Sleepy
 


-------------


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Posted By: 17tamx
Date Posted: Nov/24/2011 at 4:09pm

The tech article from the GM Engineer has been hashed out here so many times it is boring.  So much of it has been proven wrong over the years and some of the theories are based on wrong assumptions.

The argument about ported vacuum and manifold vacuum is decades old and most OEM’s went with ported and Chevy went with manifold. The antique cars that used vacuum advance first went with ported vacuum.

I for one will stay with ported vacuum on all of my old cars for both street and strip. Why, because it works. It doesn’t bother me if someone has their car set up with manifold vacuum.  I like the fact that my car runs the way it does when I get on it and how good the throttle response is when I get on it hard. Not everybody has the talent to set-up their ignition and fuel system correctly to get the peck performance that their engines are capable of.

I would say try it both ways and see which works best for you. I am tired of the arguments.

Happy Thanksgiving everyone, it’s time to go eat some Turkey and drink some spirits.



-------------
Kirk P. Fletcher
70 AMX BBG w/Shadow 390 4sp
71 SC360 Wild Plum Ram Air 4sp
67 Rogue Convert 343 4sp
66 Rogue Hardtop 290 Auto
66 440 Convert 232 Auto


Posted By: carnuck
Date Posted: Nov/24/2011 at 4:13pm
The late '70s Prestolite distributors ran manifold vacuum because they operated backwards (advanced with spring power when the vacuum dropped off)

-------------
Got an Eagle?
http://forums.amceaglesden.com" rel="nofollow - http://forums.amceaglesden.com


Posted By: 69 ambassador 390
Date Posted: Nov/24/2011 at 4:39pm
It just goes to prove that wherever you go, There you are.  This turkey is basted.  Deads birds tell no tails.  Happy turkey day.  I'm eatin!!!!!Cheers!

-------------
Steve Brown

Algonac, Mi.

69 Ambassador sst 390

84 Grand Wagoneer

69 Cougar XR7

65 Fairlaine 500XL

79 F-350 Super Camper Special





Posted By: billd
Date Posted: Nov/24/2011 at 5:29pm

Already ate - had the most people ever in the house - Barbara's parents for the first time, and for the first time her parents met my parents and 2 fridges are still FULL after sending left-overs home with everyone. (and the cats stayed clear for the most part, except Yawnie who soaked up all the attention he could get by pretending to be cute and cuddly.)



-------------


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Posted By: PHAT69AMX
Date Posted: Nov/24/2011 at 6:54pm

Beg to differ, but respect the opinions of others.

Holley carbs, both 2-Corner and 4-Corner, have Secondary Idle Circuits and delivery.
2-Corner carbs have Secondary Idle Air Bleeds, Idle Feed Restrictions, and Discharge ports.
2-Corners have small Constant Idle Feed ports located just below the Transfer Slots.
Transfer Slot fuel for both Primay and Secondary is delivered from the Idle Circuit.
Drilling holes in butterflies is less than a good idea imho and should be avoided.

It is possible to achieve full combined Intial + Mechanical + Vacuum Advance.
if Vacuum is high enough, RPM's high enough, and the Initial is locked in.
Dialing in more Initial without also limiting Mechanical can result in too much.

Vac Adv changes Distibutor Phasing when actuated, Initial and Mechanical do not.
Initial Timing moves the housing and Point Plate "Switch" relative to the crank.  
Mechanical Advance moves the Rotor Button "Trigger" relative to the crank.
Vac Adv moves the Point Plate "Switch" relative to the Housing and changes Phasing,
that is it changes Rotor Button to Distributor Cap Post alignment at the time of firing.
Mechanical and Initial Advance do not affect or change Distributor Phasing.

Adjustable Vac Adv Cans fail to allow adjustment of the Full Advance vacuum level
except indirectly by mechanicaly limiting the amount of total Vacuum Advance
by means of a Limiter Sleeve or an Adjustable Travel Limiter Stop.
A Point Plate alone is unable to limit advance other than the wire lengths
from the pick-up and ground, and the Vacuum Advance Can rod.



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Posted By: DocCreer
Date Posted: Nov/24/2011 at 7:24pm
Well in my experience with vacc advance is that lighter springs arent always better..when i upgrded my 232 to HEi i got a mr.gasket recurve kit and installed the new weights and lightest springs...it pulled away from lights very fast and smooth then....PING PING PING....so i took it apart and put the stiffest in the pack...still pinging...so finally i went to the junkyard and got fresh looking weights and springs from a wrecked gmc van...installed the GM weights and one gm stiff spring and one medium spring from the mr.gasket pack.So far Mileage has been creeping up..no more pinging,and more cruising power.Just dont use the Mr.Gasket weights.

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61 American
82 eagle limited


Posted By: firefly
Date Posted: Nov/25/2011 at 4:12am

I don't run ANY vacuum advance on any of my AMC's and they run great.



Posted By: carnuck
Date Posted: Nov/25/2011 at 9:23am
then you probably have crappy gas in your area.

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http://forums.amceaglesden.com" rel="nofollow - http://forums.amceaglesden.com


Posted By: firefly
Date Posted: Nov/25/2011 at 10:24am
I use 91 octane,no ethanol. The guy that dyno's my engines said that I didn't need it and he's been right. On the dyno he's run it on 100 octane too. I only have manual trans. if that would make any difference.


Posted By: PHAT69AMX
Date Posted: Nov/25/2011 at 12:38pm

Not that it's gospel, but here's one literary reference that states
in the last sentence that Vacuum Advance helps keep the spark plugs clean.
Feb 2009 HotRod Magazine article "Setting Timing Curves".

http://www.hotrod.com/techarticles/setting_timing_curves_tech/ - http://www.hotrod.com/techarticles/setting_timing_curves_tech/



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