About a year ago I replaced my blown engine with one from a junkyard 85. The replacement engine had the stock intake, runners, etc. but the crank, cam, lifters, pushrods, and rocker arms had been replaced with COMP parts. I did not get a model number off the cam but I measured the valve lift & duration and they measured very close to the stock configuration (I don't remember the exact numbers). Before installing the new engine I replaced the lifters, distributor, spark plugs and wires as well as all the gaskets and head bolts. I also compression tested the block and it held pressure just fine.

The replacement engine idles just fine, usually in the 550-750 range, and cruises smoothly (40-70 mph), but under load (especially when taking off and turning) it has a bad spark knock and struggles to accelerate, very similar to what the FSM describes on page 6E3-70. I have replaced the fuel pump, fuel filter, fuel injectors, and knock sensor but only the fuel pump made a slight difference. Fuel pressure is fine and I have not had any SES codes show up. It's possible the aftermarket cam is too aggressive for the stock tune but I would love to narrow it down before I spend hundreds on a new ECM.

I could really use some help here, this car was a joy to drive before the old engine blew up and now a 5 minute drive feels like a chore.

 

Do you think the cam might be installed too advanced? Is the EGR system working correctly? Possible carbon build up on pistons.

 

You're saying you're getting audible detonation?

Possible causes...

Incorrect head gaskets - Didn't the Vette's come with .051 thick head gaskets? If you replaced the gaskets with the more typical .039-.041 thick gaskets, you increased you compression ratio by about 1/4 point.

Too hot of a heat range on the spark plugs

Too much spark advance - recommend re-checking your base spark timing (EST connector disconnected)

ex-x-fire mentioned EGR... the EGR will slow down the rate of combustion, which lowers combustion chamber temps

Incorrect fuel... you're sure you're running the high octane fuel?

I wouldn't think the cam (if it's relatively mild) wouldn't be the cause of this.

 

Don't know much about the cam unfortunately. EGR issue is possible but I vacuum-tested the valve before the swap and it worked fine, I also replaced the gasket later on. No EGR codes either. The cylinder walls looked pretty good when I had it apart.

 

The head gaskets came from an engine gasket kit ordered from Rockauto, Fel-Pro I think. I have never heard anything about a specific gasket size for C4s.

 

Ok so looks like the .051s weren't used until later.

https://www.corvetteforum.com/forums...thickness.html

What heads are you running?

 

Thanks for verifying that. They are the stock heads

 

So your compression may actually be lower than stock... looks like they were running ~.021 gaskets in 1985 according to that link... your felpro kit was probably .039 to .041...