1992 LT1 with LT4 heads and Vortech S/C. 1.6 RRs, hardened pushrods, LS7 lifters.

The problem occurred over a period of a couple months, during a period when the car was on airport duty and rarely saw above 3000 RPM.

The problem showed itself as a loud tapping noise at idle that got really loud at around 2000RPM - seemed to be located at the rear of the engine on the passenger side. Performance was also pretty bad - I figured the tapping was causing knock retard but (see below) there was probably a different reason.

Removed the intake and valvetrain on the passenger side and found #6 cam intake lobe all torn up:

Click > Cam lobe

I pulled the passenger side head, and found a lot of carbon build up on the valves and a few carbon flakes 'loose' in the cylinder:

Click > #6 plus carbon flakes

The piston looks pretty good (the pistons are forged).

My working theory is that the carbon flakes were causing a pre-ignition event at full intake valve lift on each cycle. The pre-ignited mixture was forcing the valve closed and putting extra pressure on the cam lobe hence causing the damage and worsening tapping.

When I removed the intake it was absolutely covered in oil - I believe it was not sealing properly. (All the valves had oil sitting on top of them. In addition, the engine has low tension oil rings. Oil consumption was horrible (1qt / 100 miles).

two big questions:

1. Is my preignition theory even feasible
2. If I get the intake milled so it properly seals to the heads, AND get the motor re-ringed for standard rings, and replace the cam, springs, pushrods, rockers, guides etc (which will cost me between $2.5k and $3k in parts and head work) - what else can I check for damage, and how do I minimize the risk of this happening again?

Thanks for reading this far!
Andy

Your pre-ignition theory is not feasible as to damaging the cam. It will cause the bad performance.

It looks to me like that cam is ground on a cast "Everwear" core. IMO this is 100% what caused your cam problems.

The cast Everwear cores are JUNK and well known to fail, sometimes for no reason. I can also see the chatter marks from when the core was ground. That rough surface will cause the roller to skid across the lobe.

Look to replace the cam/rod/main brgs because the metal from the failed cam will damage them. Take the oil pump apart and check the gears for debris damage. Std tension oil rings should take care of the oil consumption problems as long as the cyls are still straight.

To minimize the chance of this happening again:

1. I would suggest you get your new cam ground on a billet core. The LS7 lifters your using are pretty good and should hold up fine.

2. Since you have a supercharger you must be very careful with your spring pressure. MORE is better. Too little will cause the valve to bounce on the closing ramp and eat up any cam. You can run up to 155lbs seat and 400lbs open spring pressure on a hyd roller.
Will

Good recommendation on the Billet core. I plan to use one too on the new engine I'm building for my son's 96 Vette.

I haven't priced the difference in cost yet though. Any idea on how much more a Billet core is? - generally speaking, of course.

Thanks,

Jake

Quote:

Jake-
I've been getting custom grinds on billet cores for right at $300. That's for a late model small nose with the long dowl pin and a pressed on melonited dizzy gear... The most expensive one there is.

A roller ground on a cast core use to cost me about half that...
Will

Thanks for that info. That's a great price because I've been paying $300 for cast core HR cams.

I probably won't be able to get the prices you do, but at least, now, I have an idea.

Thanks again,

Jake

I remember years back, faulty cam lobes was rather common... I know that Pontiac had some issues and so did Toyota.

My first Vette was a 77. Got it cheap when I was young because it wasn't running right. Got it home and started pulling it apart and the factory cam had worn one each of the lobes in front of the bearings to almost perfectly round. The lifters stopped spinning on these lobes and the cam had cut a groove in them. Real pain to remove those lifters. Replaced with a cheap Sig cam and lifters from the speed shop and never had another problem. I believe it was a problem with the metal of the casting of the original.

I know that metalurgy was a major issue with the Toyota cams. The mid-80s Supra, with the twincam I6 was burning off lobes like crazy. Toyota had a "secret" warrantee replacing them as cars came in for other service.