Big Block open vs. closed chamber optimum spark advance

[Duke W.]

My oldest Chevrolet Power Manual (2nd. Ed. 1976) recommends maximum WOT spark advance in the range of 36-40 degrees for the big block engine.

It's not clear if they are talking about the closed or open chamber heads, but the head prep section talks about the open chamber heads, which were the latest and greatest when the Power Manual first came out.

For you guys who have experience tuning both closed and open chamber head big blocks, do you have any data on what you found to be the optimum WOT total spark advance for best torque and power?

Duke

[Ken A.]

Re: Big Block open vs. closed chamber optimum spark advance

38 degrees for gasoline, more (40+) for methanol

[Clem Z.]

Re: Big Block open vs. closed chamber optimum spark advance

depends on the RPMs turned because we found the fastest time at the strip was with 42/44 degrees when turning 7500+ rpm thru the lights with OC heads with 14:1 CR. now days with different plug location,smaller chamber and smaller dome pistons they need less total timing.

[Duke W.]

Re: Big Block open vs. closed chamber optimum spark advance

38 degrees for gasoline, more (40+) for methanol

...closed or open chamber heads? And can you give me an idea where your number comes from.

Duke

[Duke W.]

Re: Big Block open vs. closed chamber optimum spark advance

depends on the RPMs turned because we found the fastest time at the strip was with 42/44 degrees when turning 7500+ rpm thru the lights with OC heads with 14:1 CR. now days with different plug location,smaller chamber and smaller dome pistons they need less total timing.

How about on an OE or mildly modified engine - flat or slight piston dome that turns 5000-6500.

Is there a difference in the optimum spark advance between the two designs?

Theoretically, because of the larger chamber and single quench area, open chamber heads should need more advance than the closed chamber double quench area early heads.

I'm curious if this boils down to a measurable difference based on experiment and experience.

Duke

[Clem Z.]

Re: Big Block open vs. closed chamber optimum spark advance

How about on an OE or mildly modified engine - flat or slight piston dome that turns 5000-6500.

Is there a difference in the optimum spark advance between the two designs?

Theoretically, because of the larger chamber and single quench area, open chamber heads should need more advance than the closed chamber double quench area early heads.

I'm curious if this boils down to a measurable difference based on experiment and experience.

Duke

on street driven cars i would guess the grade of gasoline would enter into the picture. i remember back when the TI ignition came out it retarded the total ignition timing 2 degrees at max RPM. was it the electronics or was it a designed feature in the dist. i could never find out

[Ken A.]

Re: Big Block open vs. closed chamber optimum spark advance

Either head, stock to L-88 specs, 93-100 octane gas. Source any Chevy Power book up to 1978+many years of trial & error.