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HEMI Ignition Timing Question #713682
06/01/10 09:48 PM
06/01/10 09:48 PM
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az426john Offline OP
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I have a 67 GTX 426 HEMI automatic car that weighs about 4000 lbs. and has a 3:23 gear.

The motor is freshly rebuilt with 9.6:1 (theoretical) compression ratio.

The cam is a Comp cam Street HEMI grind 278 with 227 Degrees duration @0.50

Stock Prestolite dual distributor.

All numbers are on the engine running checked with a back dial timing light.

I set the centrifugal/mechanical timing to 34 degrees total without the vacuum advance hooked up @ 3000 engine rpm. The timing is all in at about 2800 RPM.

This setting gives me a initial advance of 19 degrees.

When I hook up the vacuum advance the timing goes to 58 degrees at 3000 rpm.

The car starts fine and I can not hear any pinging.

Does this sound right to the experts or have I possibly gone deaf?

Re: HEMI Ignition Timing Question [Re: az426john] #713683
06/01/10 10:07 PM
06/01/10 10:07 PM
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Lee446 Offline
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I am not an expert, but, if it is not pinging you are fine. Your total timing sounds right, I am surprised the vac advance has that much in it, I would think that is a lot, you might be able to cut it back some by adjusting the allen screw in the vac pod to limit it, I would think 50 ought to be enough to maximize mileage. When you hammer the gas, the vac advance ceases working and then you have your total of 34*. Just my .02.

Re: HEMI Ignition Timing Question [Re: az426john] #713684
06/01/10 11:22 PM
06/01/10 11:22 PM
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Crizila Offline
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Quote:

I have a 67 GTX 426 HEMI automatic car that weighs about 4000 lbs. and has a 3:23 gear.

The motor is freshly rebuilt with 9.6:1 (theoretical) compression ratio.

The cam is a Comp cam Street HEMI grind 278 with 227 Degrees duration @0.50

Stock Prestolite dual distributor.

All numbers are on the engine running checked with a back dial timing light.

I set the centrifugal/mechanical timing to 34 degrees total without the vacuum advance hooked up @ 3000 engine rpm. The timing is all in at about 2800 RPM.

This setting gives me a initial advance of 19 degrees.

When I hook up the vacuum advance the timing goes to 58 degrees at 3000 rpm.

The car starts fine and I can not hear any pinging.

Does this sound right to the experts or have I possibly gone deaf?


Something isn't right there. On the mechanical side, the total setting is ok, but if it is a stock prestolite dist, there should be more than 7.5 degrees of advance in the distributor. Should be more like 12 degrees in the distributor. That would give you an initial of around 10 degrees at idle instead of 19. Ether someone modified the distributor, or you were already in to the mechanical advance at idle - due to one light spring or a high idle speed. Get it to idle as slow as possible and check the timing again. You will probably find it is closer to 10 degrees than 19 degrees advance. On the vacuum side, If you are running ported vacuum, 58 degrees minus your 34 degrees of total mechanical = 24 degrees in the vacuum can. I highly doubt that - and adjusting the rate won't change the total vacuum. It will only change the curve (vacuum comes in sooner or later). Most vacuum advance cans have around 16 degrees in them. 16 + 34 = 50 total, which is about ideal for cruising around town. Run the car @ idle with the vacuum line plugged. Take a timing reading. Then plug the vacuum line in to a manifold vacuum source and take another reading. Subtracting the second reading from the first reading will give you the actual vacuum advance amount. Again, should be around 16 degrees. IMO, I would not run a total of 58 degrees. You might not hear it pinging, but it's boarder-line. Slight change in altitude, temperature, lean condition, and you will be there.


Fastest 300
Re: HEMI Ignition Timing Question [Re: Crizila] #713685
06/02/10 01:24 AM
06/02/10 01:24 AM
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sthemi Offline
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at 3000 and no load, you are probably seeing both vacuum and centrifugal advance.
If you were actually driving and hammered it, there would be no vacuum advance and you total timing would be right on at 34.
Vacuum advance is to help with low throttle fuel burn and mileage.
A dyno is really the only way to properly tune it., everything else is seat of the pants...

Re: HEMI Ignition Timing Question [Re: az426john] #713686
06/02/10 07:18 AM
06/02/10 07:18 AM
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torkrules Offline
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Quote:

I have a 67 GTX 426 HEMI automatic car that weighs about 4000 lbs. and has a 3:23 gear.

The motor is freshly rebuilt with 9.6:1 (theoretical) compression ratio.

The cam is a Comp cam Street HEMI grind 278 with 227 Degrees duration @0.50

Stock Prestolite dual distributor.

All numbers are on the engine running checked with a back dial timing light.

I set the centrifugal/mechanical timing to 34 degrees total without the vacuum advance hooked up @ 3000 engine rpm. The timing is all in at about 2800 RPM.

This setting gives me a initial advance of 19 degrees.

When I hook up the vacuum advance the timing goes to 58 degrees at 3000 rpm.

The car starts fine and I can not hear any pinging.

Does this sound right to the experts or have I possibly gone deaf?




58 is a bit much. Should be more like 50.

Re: HEMI Ignition Timing Question [Re: az426john] #713687
06/02/10 10:16 AM
06/02/10 10:16 AM
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az426john Offline OP
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Thank you all for your guidance.

I took my wife with me today as we all know their hearing is much better than ours especially if you are discussing a good looking girl, and mine also knows what pinging or spark knock sounds like. It is a convertible so there are a lot of rattles and wind noise.

When I put the car under load in second or high gear and stand on it there is no ping or knock, possibly because it is pulling in the secondary’s on the rear carb and the front carb. However, I can make it ping at about 40 MPH cruising with a light throttle. If I stab the throttle really fast and stay out of the secondary’s and the front carb.

When I do this you get exactly what you would expect a couple of really light clicks and then it goes away.

I tried this several times with the same results. Additionally, if I do not stab it really quick it will not ping at all.

The car was fully warmed up and it was 71 Degrees with 35% humidity. The gas is a good local gas (QT) 91 octane that the dyno folks says is one of the brands they get the least amount of detonation from.

The car starts fine, no kick back.

I did a crude check of TDC with a screwdriver when I was priming the motor it was close additionally, the engine builder did not indicate that there was a problem with the timing marks on the vibration dampener.

At this moment I am thinking that this is where the car wants to run regardless of what the numbers show so I am going to carefully try it for awhile and see how she reacts. Additionally, I am going to take it back to the dyno as soon as I can get an appointment.

Thanks again for your help.

Re: HEMI Ignition Timing Question [Re: az426john] #713688
06/02/10 10:32 AM
06/02/10 10:32 AM
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renton, Washington
ph23vo Offline
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spark knock in a hemi sounds different than others.. have a friend drive next to you in different speed and load sitautions and listen... a friend of mine thought he had his ok too...but when i was next to him on the freeway i could hear it pinging!!! be careful dan

Re: HEMI Ignition Timing Question [Re: ph23vo] #713689
06/02/10 11:32 AM
06/02/10 11:32 AM
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az426john Offline OP
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Thanks Dan,

Good point. I will give that a try also.

Many thanks,

John

Re: HEMI Ignition Timing Question [Re: az426john] #713690
06/02/10 11:33 AM
06/02/10 11:33 AM
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52savoy Offline
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I had the very same thing with my Hemi roadrunner when I restored it years ago. I couldn't find the problem so I decided closing up the advance weight slots to take some timing out. I soldered them and filed to get the total timing down to where I wanted it. It still wasn't running right so I took it back apart and removed the solder. When I started it back up and checked the timing, it was where it's supposed to be..
never did figure it out. Maybe the advance pot stuck??

good luck..

Re: HEMI Ignition Timing Question [Re: 52savoy] #713691
06/02/10 11:44 AM
06/02/10 11:44 AM
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JAKE68 Offline
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I have a dist. machine and with todays fuels for the street I set up most hemi dists. with 31-34 total mechanical. With about 16-18 at idle. Hemis do not like much more than that for peek power. With todays pump fuel I do not use the vacumm advance. I usually put a beebee in the line. With a hemis loud valve train,piston slop ect it would be real hard ot hear it detinate.

Last edited by JAKE68; 06/02/10 01:06 PM.

JAKES AUTOMOTIVE
Re: HEMI Ignition Timing Question [Re: JAKE68] #713692
06/02/10 12:44 PM
06/02/10 12:44 PM
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renton, Washington
ph23vo Offline
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with jake ..i also did the BB thing on all my cars.. dan

Re: HEMI Ignition Timing Question [Re: ph23vo] #713693
06/02/10 03:31 PM
06/02/10 03:31 PM
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hm,what is the bb thing(beebee)??please educate me .

Re: HEMI Ignition Timing Question [Re: cuda-sweden] #713694
06/02/10 03:34 PM
06/02/10 03:34 PM
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renton, Washington
ph23vo Offline
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just blocks off the vacuum so you can still have the hose hooked up for the show pooches.. dan

Re: HEMI Ignition Timing Question [Re: ph23vo] #713695
06/02/10 03:53 PM
06/02/10 03:53 PM
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az426john Offline OP
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Thanks Guys.

I just took it out for a run with the vacuum line plugged and I am pretty sure that is what I will do. For the way I drive the car, 3 miles to the freeway, 70 MPH on the freeway and 3 miles back home on surface streets there is not much need for the vacuum advance.

The mechanical specs are: (in distributor degrees)

0 degrees @ 425 - 575 RPM
0 - 3.6 degrees @ 575 RPM
3.75 - 5.75 @ 740 RPM
7.5 - 9.5 @ 1400 RPM

I am pretty sure the mechanical advance is set correctly.

In a test I ran today it was as follows:

(In Engine RPM)

Initial - 15 degrees @750 RPM
1100 - 25 degrees
1500 - 25 degrees
1900 - 31 degrees
3000 - 32 degrees
3500 - 32 degrees

I have a dyno appointment set up for tomorrow and I will try it with and without the vacuum advance hooked up and a couple degrees either way on the timing and see what the resulas are.

More to follow after the dyno run.

Re: HEMI Ignition Timing Question [Re: JAKE68] #713696
06/02/10 04:51 PM
06/02/10 04:51 PM
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torkrules Offline
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Quote:

I have a dist. machine and with todays fuels for the street I set up most hemi dists. with 31-34 total mechanical. With about 16-18 at idle. Hemis do not like much more than that for peek power. With todays pump fuel I do not use the vacumm advance. I usually put a beebee in the line. With a hemis loud valve train,piston slop ect it would be real hard ot hear it detinate.




I tried to use the vacuum advance on mine. I have it set up 16 initial, 16 mechanical. I kept adjusting the vacuum can over and over again because I was getting a surge during acceleration. Removed practically all of the vacuum advance before it stopped, so I just unplugged it. I know all the gurus say to us it on the street, but there are so many mitigating factors, like cam timing, aftermarket carbs, intakes that it is very difficult if not impossible to get it to work on anything but a bone stock engine.

Re: HEMI Ignition Timing Question [Re: torkrules] #713697
06/02/10 05:17 PM
06/02/10 05:17 PM
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az426john Offline OP
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I am off to Harbor Freight to buy a vacuum pump and see exactly what I have, but based on how I drive the car (almost all freeway) I will probably use the BB fix.

I am not sure with the Prestolite vaccum advance module how much can really be changed by adding or subtracting washers & spacers. Does anyone know if it affects rate of vacuum advance or total?

Re: HEMI Ignition Timing Question [Re: az426john] #713698
06/02/10 07:02 PM
06/02/10 07:02 PM
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az426john Offline OP
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Here are the results of the vacuum advance module test

Factory Specification (FS) (in distributor degrees):

FS - 0 degrees between 6 - 9" of vacuum
Actual - 0 degrees @7" of vacuum - Result OK

FS - 4.5 - 7.5 Degrees @ 12" of vacuum
Actual -10 engine degrees @ 12" of vacuum = 5 distributor degrees - Result OK

FS - 8.25 - 11 degrees @ 15" of vacuum
Actual - 21 engine degrees @ 15" of vacuum = 10.5 distributor degrees - Result OK

No Factory Spec.
Max actual attainable, 42 engine degrees @20" of vacuum = 21 distributor degrees.

Re: HEMI Ignition Timing Question [Re: az426john] #713699
06/02/10 07:08 PM
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Mine is set up at 12 initial, 34 total mechanical, and a further 6 from the vacuum pot. Any more from the vacuum advance and the car surges. Gerry

Re: HEMI Ignition Timing Question [Re: az426john] #713700
06/02/10 07:31 PM
06/02/10 07:31 PM
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Crizila Offline
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Quote:

Here are the results of the vacuum advance module test

Factory Specification (FS) (in distributor degrees):

FS - 0 degrees between 6 - 9" of vacuum
Actual - 0 degrees @7" of vacuum - Result OK

FS - 4.5 - 7.5 Degrees @ 12" of vacuum
Actual -10 engine degrees @ 12" of vacuum = 5 distributor degrees - Result OK

FS - 8.25 - 11 degrees @ 15" of vacuum
Actual - 21 engine degrees @ 15" of vacuum = 10.5 distributor degrees - Result OK

No Factory Spec.
Max actual attainable, 42 engine degrees @20" of vacuum = 21 distributor degrees.


The vacuum can is just pulling the advance plate too far and the curve is a little too fast. You need to limit the total amount of vacuum advance to about 16 crankshaft degrees ( ether in the can if you can open it, or close up the slot that the rod rides in )and you will be good to go. The BB thing in the vacuum line is not the way to go for a streeter - unless the engine is running a real big cam ( reads vacuum under 10"Hg @ idle ). Your motor will like you much better if it can cruise down the highway with 50 degrees of advance rather than 32 dgerees - and so will your wallet


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Re: HEMI Ignition Timing Question [Re: Crizila] #713701
06/02/10 08:29 PM
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az426john Offline OP
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Hi Crizila,

I am kind of curious why you say the curve is a little too fast as it meets the factory specs. You must be speaking from experience which is far more valuable than the generalized factory specs.

The Vacuum Advance unit on a Prestolite dual point is a chamber with a long spring inside it and some washers behind the vacuum hose nipple nut.

I assume the adjustments are made to that spring length with the amount of washers. Does that change the advance rate and limit the total amount of advance or does it just change the rate? I would assume if I add more washers it would effectively strengthen the spring and slow the rate of advance, but not necessarially the total.

Any guideance you can provide will be much appreciated.

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