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Vacuum advance benefits?

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ccowx View Drop Down
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    Posted: Nov/23/2011 at 8:20pm
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!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote turbo Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 8:27pm
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 Fun.......!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:24pm
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!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote poormansMACHINE Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:26pm
Vacuum advance at idle shouldn't make anything run cooler at idle since there isn't any vacuum available.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:41pm
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 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote poormansMACHINE Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:46pm
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73hornut Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:53pm
Most of my amc's experienced more ping and run-on with vacuum advance connected, disconnected, plugged port, ran fine.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 9:54pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote 69 ambassador 390 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 10:04pm
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

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69 Cougar XR7

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 69 ambassador 390 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/23/2011 at 10:41pm
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



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