right now it's fairly stock. I was going to hollow out the cat or or get a high flow keep the current 2 1/2 pipe I have on it and put an old original 15 year old cherry bomb on it. so what should I do with the cat? my body put a straight pipe in place of the cat 2 1/2 all the way to a high flow dual outlet muffler and it bogs out in the bottom end and peps up on the top. he's got a 92 3.1 and I got a 84 2.8

 

His buttom end sucks becuase he droped his back presure to much with the strait pipe. If he had some sort of forced induction it would be the other way around.

I would go with the high flow cat. Remember Tips can play a big role in flow as well. Don't go to large or low end will suffer. No more then 3", but stay 2 and 1/2" on the pipes.

Here's a link to an article on Flowmaster's website that talks a little bit about it.

http://www.flowmastermufflers.com/cg...r/muffsys.html

 

I doubt it has anything to do with backpressure that's too low! The guy's motor probably just needs a tuneup, or he's got some holes in his exhaust somewhere.

I'd say to stick with a cat for "legal" issues, high flow will be cheaper than aftermarket "stock", and will bolt right in. (Some parts stores sell a cat for $100 but you need to buy their $20 bolt-in kit! And GM will want $300 for a cat.)

Don't use a cherry bomb. It'll sound like crap.

My tips are 4", and I haven't noticed any drop in low-end, either. (Megs chrome-rolled-edge straight-cut, 4010X, $45/ea. Nuthin' like adding some hot-rod history to my hot rod! http://www.coneengineering.com/megs/megshome.htm Unfortunately Summit stopped carrying them about 8 years ago, gotta deal with the jerks at Jegs to get 'em.)

 

I had the same problem when i was running 3 inch pipes from the cat --> muffler and 3 inch tips. When I went down to 2 and 1/2 inch pipes with the 3 inch tips I gained my low end power back. I made sure to test the theory out by not doing any other work on the car before going down to the 2 and 1/2 inch pipes.

When running large pipes go put your hand in front of the exhuast tip and feel how low the presure is. You can blow harder then it is pushing. Same reason behind using 1 3/4 - 2 inch pipes when running ture duals on V6. Keeps just enough presure in the system.

 

Never never never hollow the cat. It serves no purpose unless the existing one is clogged beyond salvation. When you hollow it, yes it's louder and might pick up a BIT of power over a clogged cat, but think about flow dynamics. The air is packed into a 2 1/2 inch tube, then hits a large empty chamber. Naturally the air will rapidly expand to fill the empty cat, THEN it has to SQUEEZE back into the 2 1/2 inch pipe again. This creates flow turbulence and unwanted backpressure. Either put a high flow cat on, or replace the cat with a straight pipe.

 

With too large of exhaust pipes, the exhaust doesn't "flow" through the pipes. The fumes just hang out in the pipes and don't move! The larger pipes wind up just being a "container" for exhaust instead of a "pipe". So you wind up "choking" your engine... it can inhale all the fresh air it wants, but when it exhales, the exhaust has nowhere to go except to pack itself into those large pipes. (If that made any sense!) I'd definately argue against putting a 3" exhaust on any v6 motor; hell, our 2.5" dynomax system is originally spec'd for v8 motors! And if a V8 motor is fine with 2.5" piping, our v6's are more then happy.

It's the same idea of running a long pipe to the back of the car without a muffler. The muffler increases the scavenging effect of the exhaust gasses; without the muffler, exhaust fills the pipes and doesn't exit as fast as it could.

But since Sparkchicken84's friend has 2.5 inch piping with a 3.1 and "lost power", I think it's just due to a tuneup problem. I'd imagine the car was able to run near stoich before his friend's mod, but after the mod, the engine's running too rich or too lean, and can't handle that bit of extra flow. Or it might be something simple like an exhaust leak (which would, again, interrupt the "flow" of exhaust thru the pipes, and cause the pipes to fill with exhaust that isn't moving).

One of my passenger side exhaust manifold's studs has worn-out threads. This cause the tight join between the y-pipe and manifold to loosen up, and I get an exhaust leak (that i can smell and hear). As soon as I tighten that nut up (and get rid of the leak), the car runs a lot better. One of these years I'll either put a new stud in, or put my headers on; but I've been super-lazy lately.

 

Thats why I thought that the gutted cat may be causing the problem, but I didn';t think about it causing the engine to run way to lean or rich. The ECm would still be reading the O2 sensor and trying to adjust the amount of air sent to the cat and the spark advance to adjsted for the "malfunctioning" cat. In turn causing the loss of power. Ok my brian finally put it all together.

 

I personally doubt the screwed up cat will cause a problem along those lines, because the O2 sensor is situated in the Y pipe, BEFORE the cat. Therefore the O2 picks up ONLY exhaust. NOT filtered exhaust or anything of that nature. And since there are no sensors in the pipes whatsoever past the O2, I don't see how the ECM can try to compensate for a cat problem when the ECM, really, should have no idea whether the cat is working or not.

I think what Tom was saying was, if the engine is not tuned up properly, the extra exhaust flow will naturally increase the efficiency of the motor and the engine may not be able to compensate by adjusting the A/F ratio properly, causing power loss.

 

I'm planning on going 3" mandrel bent exhaust....

 

I recently had my stock exhaust replaced at a local shop (After weeks of drivin around w/ "marbles" in my muffler). When they pulled the cat off, it was COMPLETELY hollow, not one piece of catalyst was left. Most of it was in the stock muffler which was brand new when I bought the car (couple months ago?). I had it replaced with all aluminized steel 2 1/4" from the Y-pipe back, and a dual outlet turbo muffler. The cat was replaced with a high-flow model(half the size of the stock one). I never had any "bogging" before or after the swap, and that's two completely diff setups??? I did lose a slight bit of off idle torque though, but torque in the 1500-3000 range is paramount! the car is pulling much harder at higher Rpms. I agree with the other guys-KEEP the CAT! Oh yeah, complete exhaust cost me 265

 

ok I'm back and read it all. we did a tuneup on his car and all the exhuast was new so no holes/leaks. still boggy. BUT he has stock intake duct work and a black azz filter. we just got him the formula cowl hood. I'm thinking of turning it into a working cowl or just open it and put a (maybe) ford or corvette cone onto the tb itself. well I did see an idea at work......just take a chicks stocking/nylon/kneehigh........and rubber band it over the tb......must make alot of hp huh? lol.

but then there's my car. the 350 at my house is looking more and more appealing. starting with my ignition switch. it only cranks when it wants. so I might put a push button. then the dam 2.8 only fires when it wants. must of made the car mad when I stripped it to paint and clean up.