unseenT/A

Swapped an L31 Crate engine in not too long ago, just confused setting up the timing.

I have it aftermarket computer controlled. Runs great, starts great. Initial timing 10 DEG, 32 Degrees total all in by 3500 12 degrees vacuum advance.

I guess what i'm confused about is ... In the factory trucks that GM had these in did they run a "vacuum" advance of sorts? Or is it just 32 degrees total timing with no vacuum advance.

I just basically need answered if i should be running any vacuum advance with this motor and how much, wasn't able to find much info on vacuum advance with the L31.

Thank you

sofakingdom

iTrader: (1)

Yes the factory ECM advances the timing further than "total" based on the vacuum.

Yes you should run vac adv if you drive it on the street. Doesn't too much matter for pure racing but is highly beneficial under cruise type conditions.

You should ask the MOTOR how much vac adv it wants, not the Internet. You do that by making a change and observing the engine's response. If it likes it, do the same thing, some more. If it doesn't, go back, and then a bit farther in the opposite direction. Repeat until no further improvement is possible by way of that particular adjustment.

Fast355

iTrader: (2)

I happen to have a factory PCM read from my 1997 G1500 Express van 5.7L 4L60E. At WOT the factory timing map starts at TDC @ 800 rpm, climbs to 18* by 2,600 rpm, the holds fairly flat rising to 26* by 4,600 rpm. At 3,000 rpm and high vacuum timing is 38* without EGR and 42* with EGR. Base Idle timing was 23* in drive and 18* in Park/Neutral with idle timing variation constantly modifying this value up to 5* each direction to control idle speed.. Keep in mind these timing values were for an un-aerodynamic vehicle, weighing close to 3 tons, with an overdrive transmission and taller gearing that had to run on 87 octane while meeting emissions.

GeneralDisorder

iTrader: (1)

I will add that the factory Vortec does not have a mechanical vacuum advance canister if that's part of what you are asking. It's all done in software based on pre-programmed timing maps and as mentioned by Fast355, they are not ideal for a performance application unless you plan to run the same cam, and regularly use low octane fuels.

If you are putting it into the car in your profile, and it still has TPI, then you can start off conservative like the Vortec mapping, and watch for knock sensor activity, etc while increasing the timing under load to see where it would like to be at with your preferred fuel choice.

Classic "vacuum" advance is largely for off-idle transitions and high manifold vacuum cruise. It will improve idle quality also but was generally "ported" for off-idle only operation to reduce idle emissions by increasing combustion temps (retarding the idle timing). At WOT the vacuum isn't present so it contributes little or nothing to the timing equation under WOT.

GD