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AMC 360 Timing Setups

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360CJ7 View Drop Down
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    Posted: Jan/09/2011 at 8:37pm
Hello All,

I just completed the install of a HEI distributor purchased from Bulltear.com. Everything went smooth and now I am trying to dial in my timing. Right now I am running at 14* BTDC without the vacuum advance hooked up. I run my vacuum advance off of manifold vacuum, not ported vacuum. Once I hook the vacuum advance back up I am not sure where my timing is because it advances off the scale on my particular engine. With the vacuum advanced hooked up I am at about 750 rpm and 19-20 Hg manifold vacuum. The engine seems to be quite peppy and I don't hear any pinging under WOT. In order to get maximum performance, should I continue to advance the timing bit by bit until I experience pinging during WOT? Once I dial in the timing I plan on taking it to a local dyno to see what type of numbers it puts up. I appreciate any input on this matter.
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lead69 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lead69 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/09/2011 at 8:45pm
You really need to know how much total advance you have and where it comes in at,if you can buy or borrow an adjustable timing light then you will know for sure.As far as advancing the timing until detonation I personally would not do that,it is not always audible and I have seen engines with as much as 40+ degrees timing and the owner did not know it by using this method.The only way to be sure is the adjustable light or timing tape on the balancer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rixcj Quote  Post ReplyReply Direct Link To This Post Posted: Jan/09/2011 at 8:52pm
Those numbers are nearly identical to mine, also running off of manifold vacuum. Personally, I'm thrilled with the way mine runs, as a daily driver.
 
Total timing is  36, all in at 2200 rpms.
 
 
Rich


Edited by rixcj - Jan/09/2011 at 8:57pm
'79 cj5/401/ t-18/dana20/Holley TA 670/HEI/PS/PB/glass body/lots more to come...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 360CJ7 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/10/2011 at 8:11am
Originally posted by rixcj rixcj wrote:

Those numbers are nearly identical to mine, also running off of manifold vacuum. Personally, I'm thrilled with the way mine runs, as a daily driver.
 
Total timing is  36, all in at 2200 rpms.
 
 
Rich
Rich,

I am pleased with how it runs as well, but want to make sure this is the best setup. I did some research last night on hooking the vacuum advance to mnifold or ported vacuum. I have to say that both sides have very good argumants. I am going to try setting initial timing to 10* and then run off of ported vc advance to see how it feels. I like that ported vacuum increases as you roll into the throttle. I will post my results after trying this.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jan/10/2011 at 11:25am
Understanding is ya need to determine how much Mechanical Advance is available in the distributor.
If the dizzy has say 24 deg max mech adv, then 10 to 16 Initial Advance should be ok.
Goal is 34 to 40 degrees total Initial and Mechanical Advance combined,
No Vacuum Advance included.
Need to make sure no Mech Adv is starting to come in at idle
when sitting the Initial timing.
Have Mech Adv Springs in the dizzy that gives full mech adv by 3k engine rpm or less.
As mentioned, "best" way is a dial-back timing light, and set total advance
at 34 to 40 with the engine at an rpm level where mech advance is fully advanced
and the vac adv is disconnected.
That way the initial is whatever the initial is and it doesn't matter if a little mech adv
is coming in at idle, the total initial and mech adv is set to a known 34-40 deg.
The vac adv adds on top of the 34-40 intial and mech adv.
It only comes in when there's decent vacuum levels, which means off the throttle
or while maintaining a constant rate of speed.
There's umpteen different vac adv cans with different vacuum start levels, full adv vac levels,
and total amount of vacuum advance.
Challange is getting one that doesn't cause ping when cruising a steedy speed
after getting the total mech and initial adv set up to the 34-40 total coming it a 3k or less.
imho
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 360CJ7 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/10/2011 at 11:36am
Originally posted by PHAT69AMX PHAT69AMX wrote:

Understanding is ya need to determine how much Mechanical Advance is available in the distributor.
If the dizzy has say 24 deg max mech adv, then 10 to 16 Initial Advance should be ok.
Goal is 34 to 40 degrees total Initial and Mechanical Advance combined,
No Vacuum Advance included.
Need to make sure no Mech Adv is starting to come in at idle
when sitting the Initial timing.
Have Mech Adv Springs in the dizzy that gives full mech adv by 3k engine rpm or less.
As mentioned, "best" way is a dial-back timing light, and set total advance
at 34 to 40 with the engine at an rpm level where mech advance is fully advanced
and the vac adv is disconnected.
That way the initial is whatever the initial is and it doesn't matter if a little mech adv
is coming in at idle, the total initial and mech adv is set to a known 34-40 deg.
The vac adv adds on top of the 34-40 intial and mech adv.
It only comes in when there's decent vacuum levels, which means off the throttle
or while maintaining a constant rate of speed.
There's umpteen different vac adv cans with different vacuum start levels, full adv vac levels,
and total amount of vacuum advance.
Challange is getting one that doesn't cause ping when cruising a steedy speed
after getting the total mech and initial adv set up to the 34-40 total coming it a 3k or less.
imho

So from what it sounds like, you advise to run the vacuum advance off of manifold vacuum, not ported vacuum. The way I understand it, manifold vacuum is high at idle and steady speeds, but drops considerably on acceleration and WOT. Ported vacuum is very low at idle and increases as you accelerate and then once again drops to nearly zero at WOT. It seems to me that you would want the vacuum advance to come in as you accelerate, so this would have the dizzy's vacuum advance running on ported, not manifold vacuum. Am I confused on this?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jan/10/2011 at 1:00pm
Not 100% sure, thought only difference between Manifold and Ported
is with Ported there is no vacuum signal at idle.
Otherwise Ported vacuum is pretty much about the same as Manifold Vacuum.
Ported vacuum signal, since it's picked up in the sidewall of the primary carb venturi,
may rise at a little bit quicker rate than Manifold vacuum, but eventually they'll equalize.
 
If there is any vacuum signal at all on a Ported Vacuum port at idle,
then the primary butterflies are too far open at idle
and the secondary butterflies should be cracked open a little bit more
so the primaries can be closed down a little bit at idle.
This prevents any vacuum signal at all on the Ported Vacuum port, while at idle.
 
If at idle the primary 'flies are opened enough to get a signal on the Ported Vac port,
then most likely the primary Transfer Slots are also over exsposed at idle
causing idle mixture, main circuit activation, venturi fuel dripping, and off idle stumble problems.
 
Understanding is Vac Adv on Manifold vacuum is a way to get an engine with
a large cam with a LOT of overlap to idle down well and give "useable" idle vacuum levels.
 
A lean mixture burns slower than a ricj mixture.
This is what Vac Adv is "meant" to do, advance the timing during "lean" conditins,
like when cruising at steady state speeds when the 'flies are closed down,
engine "choked off", the power valve closed, and running on a lean mixture.
As soon as the 'flies are cracked open, vacuum drops, power valve opens,
main boosters come online, mixture goes "rich", ~12:1 for power,
and less advance is needed to light the fire at the proper point
to maximize cylinder pressure at the optimum piston in bore rotational location. 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jan/10/2011 at 1:42pm
There are 3 vacuum advance possibilities.......
Manifold (obvious, drops at WOT, etc.
Ported - where the port is just above the throttle plates, meaning no vacuum at idle, it comes on as you give throttle.
And venturi vacuum (*A lot of folks believe it's ported, but it's not the same)
Venturi vacuum applied to the vacuum can INCREASES with load and speed and throttle, not decreases like ported and manifold.
Other than idle and off-idle conditions, ported and manifold are the same.
Venturi, OTOH, is quite different. There are some manufacturers that used venturi vacuum for various reasons.

"Ported" and venturi are used for street - drivability, overcome some stumble due to timing not keeping up with needs when you "give it gas". Not so much for lean, but to offset the lag in mechanical advance.
Ford and Chrysler were first to use it 70 years ago - I have their reasoning in a textbook at home somewhere...........

Manifold vacuum to the advance is better when using a "large cam" than something like I've got - mild to stock.

Dunno if you can still get them, but Crane had a nice adjustable vacuum advance unit for GM distributors a few years back. You could set the amount of vacuum that it took to trigger it, and set a high-limit on how far it would go once it was triggered. So with little hassle, you could set the maximum advance, and tell it - don't advance until you get xx" vacuum.
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