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Re: HEMI Ignition Timing Question
[Re: az426john]
#713683
06/01/10 10:07 PM
06/01/10 10:07 PM
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Joined: Jan 2003
Posts: 1,379
Houston,Tx.
Lee446
pro stock
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pro stock
Joined: Jan 2003
Posts: 1,379
Houston,Tx.
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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.
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Re: HEMI Ignition Timing Question
[Re: az426john]
#713684
06/01/10 11:22 PM
06/01/10 11:22 PM
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Joined: Oct 2003
Posts: 7,506
Az
Crizila
master
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master
Joined: Oct 2003
Posts: 7,506
Az
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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
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Re: HEMI Ignition Timing Question
[Re: Crizila]
#713685
06/02/10 01:24 AM
06/02/10 01:24 AM
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Joined: Jun 2004
Posts: 3,319
Chicago Burbs
sthemi
master
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master
Joined: Jun 2004
Posts: 3,319
Chicago Burbs
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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... 
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Re: HEMI Ignition Timing Question
[Re: az426john]
#713686
06/02/10 07:18 AM
06/02/10 07:18 AM
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Joined: Nov 2003
Posts: 2,715
closer to Canadian beer!
torkrules
I'm neurotic
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I'm neurotic
Joined: Nov 2003
Posts: 2,715
closer to Canadian beer!
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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.
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Re: HEMI Ignition Timing Question
[Re: 52savoy]
#713691
06/02/10 11:44 AM
06/02/10 11:44 AM
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Joined: Dec 2005
Posts: 1,495
Shelby mi.
JAKE68
pro stock
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pro stock
Joined: Dec 2005
Posts: 1,495
Shelby mi.
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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
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Re: HEMI Ignition Timing Question
[Re: JAKE68]
#713696
06/02/10 04:51 PM
06/02/10 04:51 PM
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Joined: Nov 2003
Posts: 2,715
closer to Canadian beer!
torkrules
I'm neurotic
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I'm neurotic
Joined: Nov 2003
Posts: 2,715
closer to Canadian beer!
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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.
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with jake ..i also did the BB thing on all my cars.. dan
hm,what is the bb thing(beebee)??please educate me 
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