To refresh the basic package, 95 LT1

383 with 224/236 cam on 114 lsa, 190cc AFR heads, 9.0 compression ratio. S-trim making 8 psi of boost.

Exhaust system is AS&M 1 3/4 shorty headers, high flow cat and then an SLP 2 on the left system.

When I first got the car back there was a bit of a drone or resonating at certain rpms and throttle positions. Not too bad, just a bit annoying, I would have lived with it.

We put it on the dyno for a couple of runs, and on one of the runs we blew some of the "stuffing" out of it, a great cloud of dust and debris came out, fibres and vermiculite types of particles filled the air. Looked kinda like a massive back fire, but it wasn't, it occured under full load, not while decelerating.

The droning/resonating got worse after this episode. So, it probably needs some exhaust work.

Any of you guys run up against this one?
The car definitely produces more noise than the stock engine.

Would eliminating the cat help?
What cat back systems are less likely to produce this resonating?

Any thoughts appreciated

LWM

 

You burned out the packing from your muffler. Probably too rich

 

Vermiculite? That's used in the mat that secures the catalyst bricks in place. I don't know which cat you have, but if you got some mat erosion, you might have damaged your catalyst. Is your cat one that has two halves pressed together and welded?

About the resonance. Is it an acoustical resonance (meaning that the exhaust noise gets louder in certain RPM ranges), or one related to vibration? If it's vibration, does it come in at about 2300 rpm, and continue to 3000 or so? On the stock system, there is an exhaust bracket behind the catalyst that connects the exhaust to a rubber isolator at the rear of the transmission? If you don't use that bracket, the ASM headers can set a nasty resonance at 2300 or so.

 

The air fuel mixture was a good solid 12:1 when on the dyno, I don't think a person would want to go any leaner. Any thoughts on going leaner. That being said, maybe 500 hp is enough to blow the dickens out of any exhaust system.

thx for the input

If it is the cat, then might I be just as well off with it gone, we don't have emmissions testing in Calgary Alberta.

The noise is more akin to a vibration than a harmonic type of sound going in and out of phase, and it does come in somewhere around 2,500 rpm and will be there for a while till I either give it more gas or let it upshift.

Is a resonator likely to help the situation? A resonator being one of those "Blue Bullet" type things, starts at regular diameter, then expands to 4 - 5 inches, goes for a coulple of feet, then necks back down to regular diameter??

Or ... should I say screw it and go to the 4" mufflex system, what are they like?? I'm still trying to keep some street manners.

Thx for the input

LWM

 

If it's a vibration problem, adding a resonator won't help, except that it changes the mass of the system and thus changes the natural frequency at which the system resonates. It would probably move it up very slightly. That's why I mentioned the bracket. If you make use of the bracket and attach it to the pipe and the brace at the back of the tranny, it should change the natural frequency a lot and stop the vibration at that RPM. A resonator helps the noise out the tailpipe. It won't help a mechanical vibration that's shaking your exhaust system.