1995 4wd Suburban:
TBI L05
Crane cam [part # 113931] LSA is 114°
intake lift at valve .440" and duration at .050" is 210°
exhaust lift at valve .454" and duration at .050" is 216°
Trick Flow heads with 1:5 to 1 rockers and K&N air filter
1 5/8" Flow Tech shorty headers with O2 sensor
2 1/2" dual exhaust with dual cats, mufflers and no crossover tube.

I'm working with a BJYL 7427 and a $Od def file.
This is a 2WD and 4WD, 1/2 and 3/4 ton truck bin for '93 to '95 models.

Factory timing setting for this motor is 0° with the dst wire dissconnected.
With the mods I've done to the motor, it won't run at 0° so I bumped it up to 8° btdc with the dst diconected. Should I tell the PCM that my base timing is 8° or leave it set at 0°?

With the PCM initial spark advance set at 0°, the distributor timing set at 8°btdc and the high value in the SA table set at 30°. This means that my total advance would be 38°btdc (although the PCM would believe it to be 30°btdc).
If I set the distributor timing to 0° (as the factory specs say) and added 8° throughout the SA table then the motor would fire the same way as in the first example?

The ECM CONSTANTS setting for Initial Spark Advance is 0°and the setting for Main Spark Bias is 9.8°.
I think I understand the Initial Spark Advance function, but can someone tell me what the Main Spark Bias setting does?

Can somebody explain to me what changes I should expect to see on ma SA tables with the mods I've made (cam, aluminium heads)


Thanks for any help in advance!

Tedd

 

The cam is the brains of the engine, the intake closing point is critical retarding it even slightly makes a huge difference. By changing that you've upset the *cranking comression*. So to get away from the engine just windmilling you've had to move the timing up, ie normal.

Al heads generally like more tiiming in cruise, and less at WOT. Some do great with as little as 26d. Remember less MUCH better at WOT if you can make the same HP.

The self ERG'ing of the cam will dictate, your timing and fuel at idle.

 

Thanks,


Now if the ECM thinks the timing is at 0° and it's really at 8° btdc, then won't I need to put in a reading of -8.7° for 400 RPM and 100Kpa for quick starts?

I assumed this truck bin would be very conservative with regards to timing. I was hoping that getting the SA tables right would find the power and smoothness I'm missing now. I'm having a little trouble getting the routine of modding the SA tables. Has anyone found a good method that they have refined? Right now I'm just doing one SA value per run, y'all never said it was quick but I'm sure there has to be a quicker way. Maybe burning multiple chips before a run? Anyone have a good system?

Is there a sticky post that goes into detail about the SA timing aspect of tuning?

Tedd

 

No. During cranking the ign module determines SA until a certain RPM (which I can't remember) is reached. Once that RPM is reached the PCM controls timing.

It seems to me that you should set initial so it starts up easily (8° in your case) and then set the Initial Spark Advance in the prom to the same number. Its really a matter of not confusing yourself and keeping the SA numbers readable/meaningful.

The Main Spark Bias value is subtracted from the timing calculation to allow for negative numbers in the main spark table. If you set MSB to 0°, then timing would increase by 9.8° all across the board.