vertC6

I just recently completed my build of a 2013 427 (LS7) convertible, here is the link of the work that I did.

https://www.corvetteforum.com/forums...e-details.html

I installed Pfadt 1 3/4 headers, FAST intake (ported), Halltech CF112, Yella Terra RR, baby cam, and heads done by AHP with .030 milled to put my CR at 11.6:1.

Once I got everything back together I took it to Alex Pietz who is a professional tuner from the tuning school. Upon tuning he immediately noticed KR retard upwards of 7 degrees at 20 degrees of timing. We brought it back down to 18 degrees with at 12.4 AFR. Still I was getting KR which at that point he believed was false, so he turned the sensitivity of the knock sensors down 20% and still we saw 2 to 3 degrees of retard at 5000 rpm.

I drove the car for 3 more weeks and only saw KR on my HUD a few times mostly under load at low rpm (normal). I decided to buy some torco and bump my octane up to 102 to confirm once and for all if the KR was real. Once we put it back on the dyno sure enough at 5k it retarded 2 degrees and 3 degrees at 6200 rpm.

Now that I know if is a false knock how do I find it! I rechecked the header clearance and nothing is touching. I re-torqued all the header bolts, I checked the belt tensioner and anything I could think of to cause a false knock and I found nothing! Any ideas on what to do next?

Unfortunately, this is not my first experience with this, I had a 97 eclipse GST turbo that would nearly stall because the computer was picking up a knock that wasn't there. After replacing the ECU, knock sensor and a few other parts I finally moved the sensor from the block to the base of the intake manifold and it was gone! Drove it for 5 more years like that.

Thanks for the help

69427

Any piston slap issues going on? Piston slap excites the cylinder walls at the same frequency as actual detonation does.

Frans96ss

Put stock rockers back on it

vertC6

I don't think the rockers are causing it because I know other guys running them with no issues.

madmatt9471

It has happened before and it's not unheard of - I had a very similar issue on my C5 with the aftermarket valve train (Harland Sharp RR's)

It's worth giving it some thought and maybe in the long run a try "If" you have exhausted all other avenues

Thanks,Matt

schpenxel

Asks for advice

Gets advice from very well respected shop

Doesn't accept advice

Frans96ss

Dont you love the internet?

schpenxel

We need to come up with an askhole meme

vertC6

What about the balancer? I noticed with the naked eye it seems to wobble just ever so slightly. Part of it is the bolt is not perfectly symmetrical so it gives the illusion of it wobbling. Could an unbalanced dampener cause a false knock?

Frans96ss

No....... how do you know it's false knock and not real knock?

vertC6

Because we ran 16 degrees of timing at 12.1 AFR and 102 octane, impossible to have KR with that.

Frans96ss

ok .

vertC6

Anything that I could be missing?

KB9GKC

Hello,

If the above is true then your knock sensor is responding to an actual mechanically induced engine noise that it mistakes for pre-ignition.

Douglas in Green Bay

vertC6

Agreed, Now I just need to find out what is causing it.

rabrooks

How is your oil pressure?
IS it low for a new build.
Is this LS7 one of the LS7's that came in the 427 that went bad in less than 7000 miles? I ask because most of those engines lost bearings. If you have a reconditioned crank or reconditioned rods or mains. The clearance may not be as perfect as it should be. What was your bearing oil clearance and you rod side clearance. Those are the things that saw the most damage on those engines. The mains can have large oil clearances and never make a knock audible to the human ear.

Just Curious.

other than that, my first thought was piston slap as was mentioned. And I would have to try the rocker change. Only takes a little while.

vertC6

Pressure dropped 2 psi with the Johnson SLR lifters, but psi is still good, 50 psi at 1700 rpm

69427

Is there some reluctance to answer my question?

vertC6

Not at all but for a 2013 with only 10k miles it has very little wear and my oil analysis came out perfect, so my question is why would my LS7 be any different from the thousands of other ones that don't have this issue?

RamAir972003

Change your plugs and wires see if that helps...