Lots of knock, stock 2005 C6... help!

[JustinC]

Hey guys,

I have a 2005 C6 Corvette (manual) here and we are having some problems with knock, which may or may not be false. The car made

362 Stock
364 Axle-back
373 Axle-back and Intake
365 Axle-back, intake and handheld tuner

Returned the tune back to stock with the handheld and we are still back down in the 365-range. Broke out the laptop and started scanning, seeing 4-6-8 degrees of knock from 3000-6500.

KR seems to be "random" but I mainly see it around 3800-4200, at least that is the large spike. Changed Max Valid from 12 to 1.3 and doubled the decay... didn't do anything. Added some fuel (it was a touch lean, like 13.4 at 4000) and it got worse. Changed plugs/wires, same thing.

I checked the tune file vs. another stock from the repository, verified that it is stock.

Now, I have heard these things like to have some false knock, but we have already checked the usual suspects (exhaust) for rubbing, doesn't seem to be the case. Also, it does this on the street, so it can't be a dyno thing.

I am a noobie, so any help would be appreciated.

Thanks,

Justin

[Sherminator]

What is the mileage on the car? Any odd sounds from the engine? To determine false or actual knock, cut the timing down by a sufficient amount like 10 degrees in the suspect areas, confirm good premium fuel(no additives as most off the shelf ones are junk and your car is mostly stock so not needed). If you still have it then it's a mechanical problem ie plugged cats, bad sensors(knock, MAF), bad fuel injector(s), mechanical noise(valvetrain, loose exhaust, loose drivetrain items)

Try one thing at a time and see the results.

[eficalibrator]

KR seems to be "random" but I mainly see it around 3800-4200, at least that is the large spike.

This is generally where peak torque and cylinder pressures are coming in. If it's going to knock anywhere, this is usually the first place and makes sense.

Added some fuel (it was a touch lean, like 13.4 at 4000) and it got worse.

You ADDED fuel to get to 13.4:1? How lean were you trying to run and why?

You're basically right on top of peak flame speed at this ratio, so you're going to have a very rapid burn event that isn't going to be tolerant of fuel octane deficiencies. This might work on a dedicated race car, but I'd want more safety margin and cooling. I would suggest trying again down around lambda=0.87 (12.7ish) and see how that goes.

[Redline MS]

What is the mileage on the car? Any odd sounds from the engine? To determine false or actual knock, cut the timing down by a sufficient amount like 10 degrees in the suspect areas, confirm good premium fuel(no additives as most off the shelf ones are junk and your car is mostly stock so not needed). If you still have it then it's a mechanical problem ie plugged cats, bad sensors(knock, MAF), bad fuel injector(s), mechanical noise(valvetrain, loose exhaust, loose drivetrain items)

Try one thing at a time and see the results.

Valid statement but false knock can also come from noises you can't hear by ear...The LS2 knock function is very aggressive and sensitive. I agree with Sherminator to pull a decent of spark and see if it still exists..could be bad fuel as well..

Big question is how much spark is commanded and how much is the PCM pulling?

Also try adding some fuel down there if its lean...my experience with LS2 engines is that like fuel down low..you can also knock if your too rich as well.I would also run an lambda of .85 at peak power.

Then if it is real knock then you can move to another direction...

Howard

[Sherminator]

Valid statement but false knock can also come from noises you can't hear by ear

Very true!

[JustinC]

Thanks for all of the response so far.

Sherminator - The engine doesn't make any strange noise or audible knock and it has ~48k on the odometer.

You ADDED fuel to get to 13.4:1? How lean were you trying to run and why?

Greg,

No, I added fuel FROM 13.4, bringing it down to 12.5-12.8 and it got worse. The stock tune was running around 13.4 from 2500-4500), which was lean and I was hoping that some extra fuel would make it better... no dice.

Howard,

I have attached a datalog showing Advance and KR, please let me know what you think. Again, this is with the factory tune, no changes. You can ignore the WB02, it wasn't hooked up for this run. The KR you see spikes to 6.2 and 5.2-degrees.

[Sherminator]

Do what we suggested and see what happens.