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Old 04-23-2010, 07:06 AM
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Rate of Advance of dist

I was wondering if someone can explain the difference in the rate at which the advance comes in on a mechanical distributor. I noticed on my msd distributor that you can change the springs to make it come in quick or slow.

How does this affect engine performance etc. Is slow or quick good?

I have 25* of mechanical advance and 12* of initial.

Also I have heard have all advance in by 3000. what determines this? torque curve? Stall on converter? Thanks in advance. sorry for all the questions just a very curious individual
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Old 04-23-2010, 08:23 AM
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advance

A lot of things affect advance that works best, Cam, compression, Fuel, combustion chamber design, your foot on the pedal, rear end ratio, moisture content in the air, ambient air temperature and pressure. It can be run on a dyno, matching best advance thru the Rpm range, a steady run on the dyno doesn't always give the best advance for what you need for a drag race.
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Old 04-23-2010, 12:51 PM
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Usually, you`ll want full mechanical all in by 3000 RPM. The usual is around 36 to 38 degrees. I don`t think your using a vac advance, and if your not, advance the base timing to 18 degrees. If you are using a vac advance, leave the base set at 12, then connect the vac advance to a full time manifold vacuum source, one that pulls vacuum full time, even at idle. Recheck the timing, depending on which vac advance you have, it can land from 18 to 24 degrees. Since your running a higher compression ratio, you would likely need a adjustable vac advance so you can taylor in the correct amount to keep it out of detonation.
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Old 04-23-2010, 01:00 PM
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Quote:
Originally Posted by zildjian4life218
I noticed on my msd distributor that you can change the springs to make it come in quick or slow.

How does this affect engine performance etc. Is slow or quick good?
You want the ignition advance to come in as early and as quickly as the engine will allow, w/o detonation.

Quote:
Also I have heard have all advance in by 3000. what determines this? torque curve? Stall on converter?
The RPM where the advance is all in should be around 2500 RPM in a "normal" application. There is a leeway w/this, and it will depend on the fuel quality, CR, gear ratio, vehicle use, etc.

The best way to do the ignition curve is to make a change then test it- by driving, running the quarter mile (if that's your 'thing') to see what or how each change affects ET/speed or how the vehicle drives.

The main thing to avoid is an over advanced condition- anywhere. It will cause detonation as you know, and can damage parts plus kill power when it happens.

Not knowing the specs of your engine, it's hard to say whether the initial advance of 12 is enough- this is the amount of advance that I would expect to see in a mildly cammed SBC. If this is being used w/a vacuum advance connected to manifold vacuum, it could be enough, or even too much- depending on how much the vac. adv. adds.
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Old 04-24-2010, 10:14 PM
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Quote:
Originally Posted by cobalt327

Not knowing the specs of your engine, it's hard to say whether the initial advance of 12 is enough- this is the amount of advance that I would expect to see in a mildly cammed SBC. If this is being used w/a vacuum advance connected to manifold vacuum, it could be enough, or even too much- depending on how much the vac. adv. adds.
I have a 350 .030 over with 9:1cr
performer rpm cam 234/244@050 .488/.510
441 heads
thunder series avs 650cfm carb
The msd instruction said the vacuum advance canister should pull 10* at 15" of vacuum but its for ported or timed vacuum. I am running it to a full vacuum source and i have nearly 36* at idle! I think i might need to swap for an adjustable vacuum canister.
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Old 04-25-2010, 10:52 AM
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Quote:
Originally Posted by zildjian4life218
I have a 350 .030 over with 9:1cr
performer rpm cam 234/244@050 .488/.510
441 heads
thunder series avs 650cfm carb
The msd instruction said the vacuum advance canister should pull 10* at 15" of vacuum but its for ported or timed vacuum. I am running it to a full vacuum source and i have nearly 36* at idle! I think i might need to swap for an adjustable vacuum canister.
The ACCEL 31035- adjustable vacuum advance can for GM HEI allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

To limit the amount of advance w/an OEM or some adjustable (for tip-in only- NOT total advance applied) cans like the Crane can, you will need to physically limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1: 99619-1 INSTRUCTIONS.

For that matter, there's no law that says you cannot use ported vacuum to supply the can. If you idle good w/the initial you have, w/o the added advance from the can hooked to manifold vacuum, try running it ported.

Last edited by cobalt327; 04-26-2010 at 03:54 PM.
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