Why is my timing way advanced sometimes?

Watching logs, I'm trying to figure out why my spark advance bounces off of the 41.8 degree limit I set, or just is much higher than it should be, fairly frequently.

Examples...

1700rpm, 37mph, 48 kpa, 11% TPS - should have about 25 SA, reports 40.4 SA

1850rpm, 37mph, 42 kpa, 10% TPS - should have about 25.5 SA, reports 41.8 SA

1925rpm, 17mph, 48 kpa, 15% TPS - should have about 26 SA, reports 40.4 SA

1625rpm, 35mph, 44 kpa, 9% TPS - should have about 24.4 SA, reports 41.8 SA

1400rpm, 43mph, 41 kpa, 5.5% TPS - should have about 23 SA, reports 38.7 SA

This is also the spot where I get up to 8 degrees retard. I've checked every Spark table I can find and cant find anything that would cause this weird spike. Anything oddball I should be looking for?

As far as other spots while driving, the timing lines right up with the table.

Ok, so I found something odd.

Under Derivative Spark/ Fuel RPM Control Parameters

Transient RPM Idle Spark Correction Bias = 0.0 degrees
Maximum Idle Spark Correction = 14.8 degrees (pretty close to how much my SA is over)
Maximum RPM for Derivative Term = 1650 rpm

I checked a bunch of other 5.7 bins and all them have

Transient RPM Idle Spark Correction Bias = 20.0 degrees
Maximum Idle Spark Correction = 3.9 degrees
Maximum RPM for Derivative Term = 1600 rpm

.....

I'm gonna look up what these are, and see if it may have something to do with whats going on.

So, messing with that just made it idle bad, took away a lot of the advance when I change 0 degrees to 20 degrees, so still looking for the problem.

For the moment I set Max SA to 36 degrees, since I dont need 41.8 at any point anyway, that I can think of.

Going to keep hunting for the problem.

spark bias settings normally are there to allow negative spark to be commanded as tables cant go under 0 but it depends who made the xdf and if they allowed for the bias or not changing the bias from 0-20 should remove 20 degrees of timing at the crank.normally no one touches the bias settings its just there for the ecu to see negative numbers if required.its allways good to check commanded timing to real timing at the crank before doing any tuning to make sure everythings inline.its also not uncommon to run upto nearly 50 degrees of timing in cruise situations depending on vehicle weight

is there a way to limit cruise advance on a 7427 $31 ?

Delcowizzid stated that over 40° timing advance is common, and it is true. WHy do you want to limit it?

peace
Hog

it seems that when it kicks in, in between 1600-1900 RPM I'm getting up to 6 degrees of knock retard. Obviously there may be another underlying problem, but I'd like to keep things under control until I can figure out what's going on.

With SA Maximum limited to 36 degrees I'm getting less knock in that area. Also, it seems that SA only goes above 30 degrees or so in that particular area. I dont hear any knock while driving, but putting in higher octane fuel helps keep the knock down. I have another thread where I was getting up to 8 degrees retard on 87 octane at the above mentioned RPM ranges at steady 20% throttle and 70 kpa. This is all part of working this issue out. Plugs are great (CR43TS, new .035), new C/R/Ws/Coil. I even turned down the EGR flow 25% to see what would happen.

Have you verified that this is actually happening at the crank? A few GM codes are known to have some glitches that show excessive values in the datastream, but is not actually happening at the engine.

Quote:

---

Originally Posted by Scrufdog

it seems that when it kicks in, in between 1600-1900 RPM I'm getting up to 6 degrees of knock retard. Obviously there may be another underlying problem, but I'd like to keep things under control until I can figure out what's going on.

With SA Maximum limited to 36 degrees I'm getting less knock in that area. Also, it seems that SA only goes above 30 degrees or so in that particular area. I dont hear any knock while driving, but putting in higher octane fuel helps keep the knock down. I have another thread where I was getting up to 8 degrees retard on 87 octane at the above mentioned RPM ranges at steady 20% throttle and 70 kpa. This is all part of working this issue out. Plugs are great (CR43TS, new .035), new C/R/Ws/Coil. I even turned down the EGR flow 25% to see what would happen.

---

Have you looked at the various adders in place that could be adding timing. Coolant compensation, EGR adder, etc? At the RPM and MAP KPA you state 38-42* of timing is about normal. My Vortec Express likes that much and would like even more if the distributor was not limiting the total amount of advance I can run.

I've checked all that I can find. I'm still going through everything again to make sure I didnt miss anything. I've been concentrating mostly on scalars, need to dig deeper into the tables.