High comp motor...how much timing needs to be pulled?

[MrDrezzUp]

2005 Corvette
418 CI, 11.87 SCR, 8.52 DCR
L92 heads
Comp Cam 230 XFI (.612) / 234 XER (.598), 117 LSA
94 octane

For the last few weeks I have been trying to build the 0-3000 RPM area of a timing table for my car. After each logging session I review the knock histogram and then subtract timing (copy the knock histo --> High Octane table --> Paste Special --> Subtract) from the High Octane table.

When starting from a stop, I seem to continually encounter knock. I have been removing timing after each logging session that encounters knock.

Should I continue to log and remove timing wherever knock is encountered? Is it just that not enough timing has yet to be removed?

[Mep_q8]

is the pinging actually audible? It might be something else.. lighter loads and less airflow needs more spark to ignite..

[LSxpwrdZ]

Can't forget burst knock also! Quick throttle transitions will cause it... It's usually false knock or predicted knock. I alway's disable that feature while tuning to get a real idea of how the timing is effecting the motor... I alway's re-enable it though. My personal cars, it's disabled.

To me you don't have enough timing in that map at lower RPM's. Your compression isn't too bad high either. Looking at the tune, your fueling is a little jagged. I would put a semi stock timing map back in the thing with 3-4* pulled from it and get that fueling lined out to the "T" before you start messing with tuning your timing tables. Once you get the fuel inline it'll make timing a little easier. Also you may have to go back and adjust fueling a tad after significant timing changes cause timing will change fueling some too.

[MrDrezzUp]

I do not know if I am experienced enough to know what true ping would sound like? The exhaust I have on my car is also quite loud (Corsa Extreme) which makes distinguishing any sort of engine noise difficult. :-(

[MrDrezzUp]

I was suspecting/hoping that it was burst knock. For testing purposes I have zeroed out "Spark Control" --> "Spark Retard" --> "Base vs. Cylairdelta" table. This does not seem to have made much of a difference (although the histogram I have for burst knock does not show any burst knock anymore).

[LSxpwrdZ]

I kept editing my post as I looked more in depth on the tune you posted. Try to get your VE and MAF tables smoother and dialed in and then play with your timing some more.

[MrDrezzUp]

I have the VE disabled, but the MAF can certainly be tuned/smoothed out a bit more.

Perhaps enabling the VE (and dialing it in) might make a difference? It certainly would not hurt to try.

[murphinator]

the knock info you are applying from your histograms is it set to + ( max value) or A (average value) ?

[MrDrezzUp]

Set to "Max".

A tuning guide I have stated it should be set to "Max". Is that not the case?

[murphinator]

Set to "Max".

A tuning guide I have stated it should be set to "Max". Is that not the case?

you are good , I was concerned you may have the histo set to avg and you were doing paste special with average values and thats why you were having trouble reducing timing to get rid of kr

[MrDrezzUp]

Another logging session and a bit of knock (same area) shows up. I adjust the High Octane table accordingly. It feels like the car might be a bit happier now when starting from a dead stop? There used to be a bit of shudder sometimes when I was starting from a dead stop, and now it feels like the shuddering might be lessened?

Is it possible that a high comp motor might actually need the timing to be that much lower (in that area) than a stock motor?

[LSX378]

You have forged pistons in this stroker?

Mainly light/part throttle knock?

If you cant get to any knock phones, a piece of 1/4" ID plastic pipe from your ear to the top of the engine somewhere will help you hear real knock.

[MrDrezzUp]

Forged pistons.

More timing was pulled today, and after that (for the first time) I had a logging session without knock. I have pulled as much as 20* of timing in some cells.

[LSX378]

Forged pistons.

More timing was pulled today, and after that (for the first time) I had a logging session without knock. I have pulled as much as 20* of timing in some cells.

Forged pistons and high comp tend to make the engine more noisy, especially at lower rpm light load. This often causes knock retard when it is simply mechanical noise. Plus the pistons will tend to produce a little more noise when they are at the bottom of their stroke as well unless you have longer liners.

A different plan of attack would be to desensitize knock on a per cylinder basis in relation to lower load airflow. You will see how the factory does this across the lower airflows.

If you increase the mulitplier you should see the KR drop off.

Then the trick is to listen for real knock. Which depending on the exh noise can often be heard easily.

Again assuming you are talking non WOT stuff as this is where your tune appears to have timing pulled out.

[MrDrezzUp]

I am indeed referring to non-WOT timing.

When real knock occurs, how often can it be heard? I.e. if the knock sensors are desensitized a bit (for testing purposes) and I am testing at less than 3000 RPMs is the engine in danger of being harmed from real knock that I might not hear?

[Racecar]

I had a forged LS6 with a 233/239 cam and 8.9 DCR. I could only run 25 degrees timing and I had to run half pump gas and half 100 octane race fuel. The knock only occured at WOT usually after a gear shift. With as much compression as you have, I would set the timing table to 25 for anything over 3000, then concentrate on getting your VE corrected. A good VE table is the heart of a good tune.

[BBA]

With as much compression as you have, I would set the timing table to 25 for anything over 3000, then concentrate on getting your VE corrected. A good VE table is the heart of a good tune.

I have a hard time believing that...or your tune did not work well.

To make sense, you may have meant to say set timing at 25º at anything BELOW 3000 RPM instead, that would smooth out the cam effect on low RPM timing.


If you have timing set at 25º above 3000 RPM at WOT on ANY LS motor with high compression, you are driving with knock retard.

[Racecar]

My LS6 had 8.9 Dynamic Compression. That's about .9 too high to run pump gas so I had to limit the timing to 25 max to keep it from knocking at WOT. That's with a good tune 12.7 A/F at WOT. At lower rpm <3000 I taper the timing off, less at lower rpm.

BBA, would you run 25 degrees for all cells <3000, even at idle? Does that work, or will it knock and stall at low rpm?

[BBA]

My LS6 had 8.9 Dynamic Compression. That's about .9 too high to run pump gas so I had to limit the timing to 25 max to keep it from knocking at WOT. That's with a good tune 12.7 A/F at WOT. At lower rpm <3000 I taper the timing off, less at lower rpm.

BBA, would you run 25 degrees for all cells <3000, even at idle? Does that work, or will it knock and stall at low rpm?

It works for cams with a lot of overlap. The timing will be controlled by the base timing map and other variables, the timing will not go to 25, it will stay steady up to about 19º at 2000 RPM or below and really smooths out the engine, not to mention it cures the hot restart issues.

As for high RPM, I am surprised you need 25º anywhere except under coastdown airflow regions. Have you logged to see if timing is actually that high under WOT? My guess is it's not.

Remember, the timing tables set the limit to timing at different points, not the minimum. You will most likely always be below what the timing table allows.

[Racecar]

Yes, on my other motor, the timing tables were set at flat 25 over 3000, and that is what I get at WOT. I had to make one change when I would push in the clutch to shift, it would advance the timing, that was causing knock when the clutch was back out. The new motor with flat top pistons is set at 29. That is what it reads when I scan.

I have have not tried very many settings for timing under 3000. The car has great power, but it could be smoother for street driving. I will try your formula and let you know if it works for me.