A little background :The car is a 99 FRC vette. Its a 346ci with eagle forged crank/rods, and diamond pistons. Pistons are stock cr units that have been flycut .080". Heads are prc 227's with 52cc chambers. Cam is a pat G custom grind 236/244 .609/.612 114lsa. The shortblock was in the owners previous car 317 heads a a 224/230 futral cam it made around 600/650 on 11#. He got the vette and we swapped heads and cam on the motor and stuck it in. The car has been running great for about 2500 miles with that combo. The car put down 605rwhp with a 125 shot on a safe tune. About 2 weeks ago we started having a rattle/vibration which we though was coming from the tq tube or the clutch. We went over everything and it checked out ok. About 5 days ago the noise got really loud. We could deffinately hear it coming from the motor now. We pulled the plugs and found # 3,5,7 had cracked/broken porcelin and 3 & 7 had broked/melted straps on the plugs. We pulled the driver side head off, the vale train, head and pistons looked fine and were tight in the cylinders, the walls still looked freahly machined. We spun the motor over with the head off and the noise was gone. We slapped the head back on and put the valve train back together. Its now making the same noise as before even just spinning it with the starter. Im at the point where we are out of ideas. It has a sound like something is hitting the windage tray but only with the head off. I dont see any bent valves or signs of them being bent but im running out of ideas. Oil pressure is still good at aroud 40psi during crank/idle. Cut the filter open and drained the oil, no metal in either. Any ideas of what it could be? Heres a pic of the plugs (these are tr6 and we've swapped to br7's now)


And here is the dyno video from about a month ago.

http://youtu.be/eYUaYF36sRk


Ill get a video tonight of what it sounds like but it just doesnt make any sense.

 

I am no engine guru but I think the nitrous was run somehow improperly be it low fuel pressure, too much spark something like that and detonated it to death.

My guess as to why you only hear the noise with the head on is the bearings aren't loud without the compression/vacuum applied to the pistons when the heads are on.

 

I figured it was te compression that made the difference. There are no signs of detonation on the cyl head chambers or the piston surfaces, i think it may just be a spun rod bearing. He turns the motor pretty high at aroud 70-7200 rpms but aside from the plugs everything else looks good. Guess we'll just have to drop the drivetrain/motor and check out the internals.

 

The rest of the plugs looked great though and there is no bearing material in te oil/filter, its just a mystery as to whats causing the knocking sound.

 

Nitrous and compression get along just fine if the fuel and spark are right.
How high is the compression? I know 52cc is small.

The plugs being destroyed says it was detonation, that could contribute to a spun rod bearing but a spun rod bearing wont do that to the plugs.

 

Theres no doubt that there was at least a little bit of detonation but everything looks fine on the inside it was just the plugs and it has only been sprayed may be 2 times on the dyno

 

Cr is around 11.8-11.9

 

Detonation was the problem a tr6 is to hot of a plug for that kind of compression with nitrous, imo this seems like a nitrous setup from the specs described above so your question would be answered better in that section & are you sure his compression is that low because with the specs given I would put his compression about a point higher???

 

Well we pulled the motor out, its mint, cyl walls are good no broken ring lands and all bearing are fine. Pulled the bellhousing off and found the vibration/rattle.... Te "driveshaft" and slave are fubared. Still trying to determine what caused this, everything was loc-tited when installed. Anyone have any guess?