RACEON

The timing and performance on a friends Vette is slipping at top end/WOT runs. He noticed one of his catalytic converters was making a rattling noise on his 96 LT1.
Can a cat cause this kind of problem making the ECM think there is spark knock?
ALso, someone told him its normal for SLP metal matrix cats(similiar to his) to rattle as they expand and contract so they will be loose in there casing.

ACE1252

In my experience, if a cat rattles, it's not a good thing.

Wild Willy

If the cat gets partially clogged, it can cause excess backpressure at higher rpm's and loads, and even cause real knocking-

Injuneer

How did you determine "timing..... is slipping"? Do you mean you put a scanner on it and it is getting knock retard?

RACEON

Yes, it is pulling 4-5 degrees at above 5000 rpm from what is programmed in for timing.
The sensors are put in with teflon tape and torqued to 14ft/lbs? and there is an LT4 knock module in there.

Injuneer

Is it possible you have a fuel pump problem? Have you checked the pressure?

RACEON

He has a Racetronix fuel pump in the Vette. That should be fine and he is running a 43.5 psi with vacuum off. TR55 plugs and a new opti, GM Opti.

Injuneer

What is the pressure at WOT when the engine starts to drop power and knock?

Apparently Walbros never go bad????

RACEON

43.5 on the dyno when intially tuned.

Injuneer

But its not on the dyno, and you are indicating the engine is losing power at WOT. Have you rechecked the fuel pressure?

RACEON

I will have him check it. thanks