Detonation
 

I recently had a cam done with milled heads (about 20), lifters, push rods and roller rocker arms and also added a catch can. I already had, heads reworked, headers, intake and cold air box. I got the car back and it is detonating horribly at 4,000rpms and over. I verified springs (went with a stiffer spring) and replaced the cam sensor, as well as verified by multiple tuners that the tune is not causing the detonation. Any ideas what this could be?

Step one, buy a scanner, monitor to see if it is pulling timing and by how much. Step two, make sure your tuner didn't detune or disable the knock sensors.

Step three, triple check the catch can routing - there have been a few folks that have mis-routed the PCV lines, causing excess oil to pull into the intake, so check for that, as well as vacuum leaks.

Also, what roller rockers? Pretty much anything besides stock requires a spacer or taller valve covers. And if they are YT and you are using the rubber spacer they provide, I can tell you that is a marginal solution; you can have the rockers tapping on the cover with any slight mis-alignment of the cover.

What octane are you running? :cheers:

Why not take it back to who did the cam install if this problem was not an issue prior to the cam install?

I am not sure what detonation feels like, but if the vehicle is sputtering like it is hitting redline at 4,000 and over.... for me it wound up being a spark plug with a hairline crack in it and was arcing under high load.

Good luck

I had a high load high speed (4K+ rpm) mis-fire and it was two cracked spark plug wires that I did not see when I did my checking.

Check physical cam timing. If the cam was installed a tooth advanced or retarded it may be causing detonation under load, even early in the RPM.

If the cam was installed a tooth off in advanced, it will bring way too much pressure and will cause detonation even worse if the cam was already grounded/cut with 3, 4, 5 deg advanced timing.

Cam timing and ignition timing are two completely different things.

Check cam timing.

Careful, careful, careful...detonation is no joke on these cars. Were the heads milled twice by mistake, or only milled once along with the recent cam. In any event...take it back to the guys that did the work...something isn't right.

When you say "detonating horribly"...can you hear detonation at high-rpm/high-load?

If you took .020 off the heads, you've increased the compression ratio somewhat and, depending on the octane of the fuel, the spark calibration and the WOT fueling, that can cause detonation. It's likely that the camshaft is increasing cylinder pressure at high rpm.

You say "mutiple tuners" say the calibration is ok, but can you tell us something about 1) the spark tables and 2) the AFR at WOT?

Great response....It pulls a ton of timing out, up to 12 degrees or more; sometimes negative timing. I don't believe that the knock sensors are disabled because the scanner picks up the knock. I am mildly skeptical of the catch can and have not yet verified that since the car is at the mechanic that did some of the work. I went with Harland Sharpe rockers and Holley 1/2" tall valve covers. I run 93 octane pump fuel, but ruled this out as being a possible issue since we added race fuel.

Its back there now. Ive changed spark plugs at least twice and wires once.

Good question but this was verified. Its going back to the guy, who by the way is very reputable.

Their aluminum bodied roller rockers? If so, those have been proven to be not such a good idea on our cars...those nose over pretty bad at high rpm. Probably not your issue at hand, but I'd get rid of those.

12 deg sounds like something is hitting causing false knock. Check your exhaust.

or an aftermarket intake manifold hitting the firewall

That is interesting that you say this about these roller rockers. These are always debated on the forums because some have had success but others have not. Many feel that a stock trunion-upgraded rocker is the way to go and is proven. I ran with that with modded heads and bolt ons and the car ran great. One of the builders involved in putting the car together when I did the cam cold me on the roller rockers. I am a bit suspect and still have my upgraded stock rockers. I think that after verifying the catch can I will try the other rocker arms and see if this might be the culprit.

Yes, we verified this and all is correct.

You can hear the detonation/pre-detonation. Its bad and the engine fall all over itself. Under normal driving, drives perfectly ironically. As far as the spark tables, you can pull out a bunch of timing and it really does not make any difference.

We did - see nothing....

Would that cause the engine to fall all over itself and detonation?

Are you using NGK plugs by any chance?

Yes....All NGK

That's your problem right there - I came to the conclusion that NGK plugs were crap in an LS7 when I once had engine knock at friggin idle.

Put stock plugs back in and you'll be fine, but if I'm wrong I'll buy them from you and pay for shipping.

I will try anything at this point. Thanks for the advice!

Good luck.

Check to see that the rockers are not hitting the valve cover or the head valve cover rail. It may not be a height issue but more of a hitting the side or head issue. The problem with aluminum is it can take up a lot of room versus steel.

plenty of us utilize ngk plugs. they, like any plug, have to be gapped appropriately for the setup. 0.040 gap on ngk tr6 plugs does not work on my car, but 0.045 sure does

Did you measure the push rods?