mohammed_k81

have an automatic 6speed corvette c6 with the below modifications and I’m facing knocking issues Air Fuel Gauge is reading 12.6 at full throttle timing was set lower about 18 I even lost HP on Dyno just to try to reduce the knock

Piston 4.070 Crank Stroke 4.125 LS3 heads PRC aftermarket 250cc 68cc pistons flat -3 Gasket thickness 0.051
Displacement 429 Compression Ratio 11.733 Fuel used is nothing less than 95 Octane

Heads long 1 7/8 Zo6 cutback VR intake system Stock ls3 manifold

Cam Spec is Gt19 227/239 lift 678/688 on 1.8 rocker im on 1.7 – Some have suggested changing the cam which could be my issue.

What do you guys recommend should I change the heads to 72cc which will give me a compression ratio of 11.23 which would be good?

G Engines

what block is it

mohammed_k81

Its ls3 Block

G Engines

I have seen engines knock from a lack of timing, this could be the case how much timing have you had in it so far?

mohammed_k81

The tuner put the timimg down to 18 and I have noticed the cars response is not as good at all.

Do you think the cam is not working with this combo?

allblowdup

I have almost the same engine but with a carb. I run 29 deg total timing no issues on 91 at 2000 ft elevation. What kind of power are you making?

mohammed_k81

I made 520 RWHP on 18 deg timing which is really good since I’m an automatic A6.
Some friend recon these number ant so good but I think they are because if my car was manual I recon I’d be hitting 550 RWHP plus.

hellbents10

iTrader: (10)

Not enough cam for the setup it will make it hard to get timing in there.

Sales4@Texas-Speed

iTrader: (4)

I think a cam change would definitely help but if you are going through an automatic I think those are some decent numbers to start with.

litle88

iTrader: (20)

Just get rid of it and get a Bugatti if you don't have one already lol.

Ellesone

Its a small cam for the cubes but your compression is kind of high but you have 95 octane...LOL
how is the quench area on those heads? I had prc heads before and an issue with detonation.... same bottom end swicthed with AFR heads milled to the same chamber and never an issue..... Not saying this is the issue but I've seen this before in my car .. AFR heads have a bigger quench area I think is what I was told. Could this be an issue in this case? Combination of things on the edge and it just doesnt work good ?

OR is it simple intake air being too hot and high compression and the way you guys measure octane over there is different than we do over here and that is the real reason? Guessing out loud here..LOL

Ellesone

I asked.. quench would help a little but its more likely the compression and fuel... lack of octane for the high compression and probably IAT being high as well isnt helping.
I would drop compression

mohammed_k81

whats the best cheapest why to drop compression ?

Ellesone

heads would be the easiest way at this point but some would use the cam to hold the valve open to bleed off that way... not a good way to do it ... heads and if you cant drop enough then pistons...

mikey

iTrader: (6)

Can you actually hear it knocking or just seeing it with a scanner?

Inssan

Try race gas and see if the knock stops. Then you will know if it is detonation.

Might also check for a lean cylinder... uneven flow injectors? Do the left and rihht banks have similar a/f ratios?

Ellesone

All good points .... make sure of the issue first... Are you seeing it on data and can you feel the car go into the low octane table and pull timing when it does..? Felt like someone pulled a parachute on my car everytime it happened. I could tell right away...

The race gas should help if thats the issue.. did someone pour the heads to make sure of what you have before buttonning it all up?

FlamingTA

iTrader: (8)

Change to a bigger cam. With that high CR and small cam your DCR has got to be sky high. Changing to a bigger cam will help bleed off the cylinder pressure a bit. I run 11.7cr and run 26 degrees of timing on 93 in Texas heat, not a problem.

ND400

What needs to be established is if its detonation, or mechanical knocking. You do know an ls3 with a 4.125 stroke comes within a bees dick of exposing the oil ring, is your windbags tray spaced properley. What are your egts like? Pm me I'd be happy to help you resolve your issue