Burst Knock Reduction or Just Too Much Timing Advance

[Digital Corvette]

2000 C5 Corvette M6, 228/232 111 +3 cam, LT headers, FAST 36 lb injectors. I think I am getting close with the tune but am still battling some KR at WOT.

See attached log at around 13:30 and again at 27:30. Does this look like burst KR or do I simply have too much spark advance, or maybe there is something else going on? I have set Burst KR spark reduction to 0, but maybe this doesn't disable?

Thanks for your help.

[Matt Vardaman]

Your timing is way too advanced. You had 7 degrees of spark retard in the high rpm pull. I made a quick new timing map.200621 Final Timing fix.hpt I would suggest using it, then make timing adjustments slowly from there. When you make table adjustments, make them in small increments. Also make sure you view the 3d map as a split view, it makes it really easy to see big problems ahead of time.

[Digital Corvette]

Thanks Matt. Most interesting is that in the peak torque cells (4400 - 4800 rpm, max cyl airmass) you have increased the timing from 18 to 22 degrees. I am trying to understand... In viewing in the split screen your timing table has smoother transitions than mine. You will notice in the log that the KR happens at the 3-4 gear change as I get back into the throttle. In this gear change timing goes from WOT 25.4 at 6200 rpm and 0.80g cyl airmass, to closed throttle 37.9 at 4800 rpm and 0.03 g cyl airmass (KR starts at about this time), and back to WOT 21.2 at 4800 rpm and 0.84 g cyl airmass. Is the rapid transition the issue? I also have seen that the stock timing tables have a "valley" at peak torque rpm, I was trying to preserve this in my tables as a recognition of max cyl pressures. Let me know what you think.

[Digital Corvette]

I should add that in the second KR sequence (27:30) the timing is at 19.1 degrees when the KR kicks in, this is not during a shift, just a WOT pull in 3rd. In Matt's revised timing tables this would have been commanding 22 degrees of timing so KR would have been higher. I am reluctant to just flash it a go! All the reading I have done suggests these engines should be able to carry at least 18 to 20 degrees of timing at WOT + peak torque when NA and the fueling is correct. Running 243 heads that have been lightly shaved (0.003") and Cometic 0.040" gaskets, which yields a static compression ratio of about 10.8:1. The pistons measured 0.006" out of the hole so the the gaskets provide a narrow squish clearance of about 0.035" when warm, this should help with knock (so says the theory). Comments?

Rob

[Digital Corvette]

Anyone?

[Matt Vardaman]

Every vehicle is different, what I made was just a base map. A starting point with 5 degrees removed from .72 cyl airmass down then smoothed the entire table. A timing map that works great for first gear wot can be too much for higher gears. Timing is load based and temperature sensitive. You have a major spark knock problem. If it was a boosted engine, it could have been the end of the engine. The map I made is more than safe, I removed timing. Also double check your timing modifiers. The general consensus is to start with low conserative spark and gradually work up. If you want to be ultra safe, remove two degrees from the entire table I made and go from there, or talk to a local dyno tuner.

[Digital Corvette]

Thanks Matt. I will have a go at it and post results back. Will be about 10 days until then, on the the road at the moment.

[Matt Vardaman]

Thanks Matt. I will have a go at it and post results back. Will be about 10 days until then, on the the road at the moment.

You're welcome, and good luck!