Spark advance / Ignition timing...

[Keitha]

I am working on fine tuning the PCM in my 1991 Suburban and could use some advise on ignition timing. Motor is a mild 383 with stock heads, full roller cam w/ a LT1 grind. I am looking for some baseline spark advance numbers. I have my base timing set correctly at 0 degrees w/ brown wire unplugged etc and the motor is running fairly well but it really runs out of steam quickly at WOT. Spark timing seems to back off at WOT levels and I can feel power reduce. When I back off the throttle a bit it seems to increase power.
What spark advance do you recomend for the various throttle/load positions:
Idle =
Cruise at 2,100 rpm =
Moderate load at 2,000 rpm =
Moderate load at 3,000 rpm =
WOT 2,100 rpm =
WOT 4,500 rpm =

This will help me check/program my spark curve.

thanks,

 

[dyeager535]

Realistically with idle, run it high. 20*+ is common stock. Depending on the motor, more might be better/needed. But I'm guessing your cam doesn't need a ton more. I've played around and don't seem to notice idle getting any better after about 24*.

Are you hitting knock at WOT? I've had that happen when timing was too much, and the ECM would pull timing due to knock. If so, back timing off a bit (if it's not lack of fuel causing the issue) in the areas just before knock occurs. From my experience a stock tune will work (not optimally, but will work) on a modified motor in regards to timing, but if/when fueling is off, knock is a/the real problem.

As I've seen it written before, give the engine what it wants for timing, not what you think it needs. I understand where you are going with wanting a baseline, but do you have fueling straight?

 

[black dawg]

x2 on the fueling, sounds like you could be going lean.

 

[Keitha]

Fueling is good. My WBO2 is reading 12.3 - 11.? when in PE mode (WOT and/or high loads). In theory, WOT would back timing out due to low vacume advance - correct? I wonder if my curve has too much advance removed or too soon.

 

[dyeager535]

I noticed my setup was falling flat on it's face in a VERY specific RPM/MPH range, and found by looking at the logs, that timing was going to or near 0* at those instances.

Timing is completely controlled by what is programmed in, I will say "regardless" of vacuum, but obviously MAP is vacuum...however since the values are whatever you (or GM) put in there, it will only pull timing if that's how it's programmed.

As I recall from my bin, PE actually added spark in some ranges, and subtracted in others. Kind of weird, but what GM determined to be good for whatever reason. I believe some folks use JUST the timing tables to control spark, and zero out PE, so that you don't run into those conflicting situations where timing table shows 30*, but actual advance is showing as 24* or whatever.

I think that approach makes sense, as long as your PE doesn't enable if you are doing something like towing a trailer on flat ground, where load has increased over unloaded, but you are not into the throttle heavy. I haven't dug into MAP really in-depth, I just know it's possible.

You know as well as I do, Mark's got a better handle on this than I do.

 

[Keitha]

I spent some time last night studying the spark tables. The PE is an adder and is set to add 4.95* at all RPM ranges. The Main spark table really pulls timing out of the mid to higher MAP ranges. I will experiment a bit by adding 10% to the 60+ MAP range cells. That will still give it about a 10* reduction in advance. Then if I do not see any knock, I will start to push the PE adder up a bit.

 

[Keitha]

Update: I have only made the 1st round of spark advance per the previous post and it made a good difference. When I get a bit of time, I will push it a couple more degrees and see how it responds. Basically, the spark table that I was using just pulled too much advance at higher throttle positions.

 

[dyeager535]

It is amazing when you have this amount of control, how much difference even a couple of degrees of timing in one particular area will make, isn't it?