I could never get my car to idle at an RPM low enough to use the idle-mixture screws and still "run" from a cold start. But adding vaccum advance rreeked havoc on my detonation woes. So, I met things halfway. Result was a much better idle, around 800RPM with ~14"HG manifold vacuum and 12mpg on the interstate turning 3500 RPM for 2 hours. Up from 9mpg of before this modification...

Here is the summary from my other post:

Alright here's the skinny:

HEI distributor with MSD6AL box and blaster coil.
My mechanical advance is 20 degrees.
This, with a base timing of 16 degrees made for a total of 36 degrees. No vacuum involved. My full throttle sprints had some detonation in 3rd gear. My idle quality was crap and my idle-screws did nothing. so a change was in order...

My vacuum advance was about 17 degrees.
I had a hand actuated gauge thingy that allowed me to islote the vaccum and how much the change in "Hg netted me. So, at only 5"Hg, the module would advance timing 5 degrees. At 10"Hg was 10 degrees, and by 15"Hg it was pulled fully to 17 degrees of advance.

With the stock vacuum advance installed the timing was too high for part throttle (read greater than 5"Hg) acceleration. Detonation city! So I modified the module to limit how much advance it pulled in. Now, it pulls in 10 degrees.

I also decided to drop down my overall mechanical advance (and base) to make for a total FULL THROTTLE timing of 34 degrees and cure high speed sprinting detonation

So:
Base timing: 14
Vacuum advance: 8
mechanical advance: 20

total timing at full throttle and 0"Hg is 34 degrees BTDC.
Cruise timing at between half and 3/4 throttle (20"Hg to 5"Hg) is 42 degrees. This quicklyu drops to 34 because the vacumm modual has less travel to rebound.

Thanks guys for the help and motivation to finally give my "race motor" some better street manners! Now the fans come on and the idle doesn't drop 400 RPM! Just holds strong at 800RPM loben away with no LOAD UP!



http://www.chevelles.com/forums/showthread.php?t=132750

Great that you finally joined the civilized side of the street beasts. I pretty much did the same things when I first built the engine and put it in the beast. I do have a bit more total at 38 degrees but if I go racing I do need to add a bit of race fuel to ensure no detonation. With only 9.6 compression I have no issues on the street. Mileage and manners are much better when you iron out the details!