I've verified (timing light while commanding a fixed timing at the ECM) that my ECM and distributor are in sync, such that 30 degrees commanded == 30 degress actual.

I checked this because my motor seems to want a LOT of initial timing at idle to run well... on the order of about 25 degrees. It'll get by with less, but that's where the smoothest idle and least amount of throttle/IAC are required.

My question is should I be aiming for the best quality idle even if it means I'm running what, to me, is an absurd amount of timing?

The spark map is completely under my control (BS3) so it's not like an old mechanical distributor where a lot of initial timing means I have to change the curve, of course. The total timing at peak torque is still set around 32-34.

I guess my question is: what would make a motor want so much initial timing? I have very large intake runners (340cc) and a very short runner (Hogan sheet metal intake). So I imagine the motor's no especially efficient down low, but this still seems like a lot more timing than you'd normally see (stock being in the 6-12 range).

Thanks for any ideas!

- Dave

ZZ502, ZZ502 cam, AFR340 heads, short runner intake, 90mm TB, BigStuff3, 85# injectors

 

Maybe you have to advance it that much to get a decent burn because compression is low as a function of the valve timing and no load on the engine. I run about 26 degrees of advance at idle. In my engine (OBD1 LT1), at least, the computer will still be able to retard it for cranking, and you couldn't do that with a mechanical distributor unless you moved it by hand or used some other trickery.

 

I originally built this motor with the intent of adding an F2, but never have (it's been 'coming soon' in my sig for about 6 years). So it's running only about 8:1 compression. So if poor compression will demand extra timing, that may well be it. Combined with a head-intake combo that's really optimized for top end, I can imagine it's not very efficient down low.

 

My app is... 97ss 383 w/ 1 7.6/3.4 pd pulley with a MM6 trans. OBD2 PCM.
Cam is similar 231/240 duration @ 0.050". AFR 190 heads and 9.5:1 static compession. I see about 14 psi on the boost gage @ 6k+rpm.
It too likes alot of advance timing (34* btdc).

I have tried reducing the timing (20 to 24*), and, it's no fun. I also changed to a crankcase vac pump an poly-glycol cooling media to prevent detonation. Looks like it works.
I did change my LT1 log onifold to a world castings 4150 single plane manifold. Quite an improvement in air velocity.
ATI also states that one can expect decrease in performance with timing advance being lower.
B.