hey guys i have a 94Z A4 with stock manifolds. i deleted the cat cause it wa clogged. i put a strait pipe in place of it and i have the LM1. performance wise it took ALOT of my power down and didn't even gain alot up top. the only reason i took off the cat was cause it was clogged. has anyone ever done this and if you have what are your opinions on it? i think its not worth any gain. it just hurt my performance. i don't like it. i think i'm gonna put a high flow cat for a while till i get my LT's.

 

Removing the cat shouldn't create a huge power loss. You may have a problem elsewhere. If you plan on doing LTs soon, it would probably not be worth adding a new cat considering you won't be able to use the y-pipe anyhow.

 

i can get a cat real cheap and get it installed real cheap as well. the car felt real weak after i did it though. i don't know if this has happend ot any other people out there.

 

Get some headers on there and you'll like it.

 

lol. yeah i'm gonna wait til the summer to get some LT's .

 

sorry, what is LM1?

 

SLP loud mouth exhaust 1 there is also a LME2

 

so you guys with the LM's don't do this cause it's sorry.

 

i have the LM just installed with LTs and ORY.. my car runs great

 

yeah but you have LT's. that's a world of difference when compared to stock Manifolds like the way i have. i will be doing your exact same setup in the summer. though.

 

I figure you would gain power if the cat was clogged and you took it out.

 

If it truly was his yes he gained power. Theres no way removing the cat would reduce the power on a modern engine assuming you put a test pipe in place. If you just hollowed it out increased turbulence might cause you to lose a little on the top end but no way he lost power on the bottom end, clogged cat or not. If back pressure had a positive effect on power then modern fuel injected computer controlled engines would come with valves in the exhaust to create back pressure at low rpms.

 

The Yamaha R1 engine DOES have a valve in the exhaust to add back pressure. I don't know why, maybe for resonance tuning and not backpressure, but that's a very highly tuned engine so I guess it works. To the tune of about 150 HP/liter NA, anyway.

 

The EXUP valve is not just a vale that "adds back pressure". It is about resonance tuning and creating pressure waves on the intake stroke to keep fuel from flowing out of the cylinder and into the exhaust. Backpressure is pressure in the exhaust system which is there at all times and actually pushes AGAINST the piston during the exhaust stroke thereby decreasing the power of the engine. Backpressure DECREASES power. There is no gain to be had from sticking an exhaust restriction into your exhaust. Now using tuned exhaust where the pipe is properly sized for the rpm range of the engine is an entirely different story. While using a smaller pipe can increase backpressure if its properly sized it won't and instead it will increase the velocity of the exhaust gas thereby helping the exhaust to flow faster, not slower as backpressure causes.

 

Thanks for the explanation. I think there must still be increased backpressure as an unwanted side effect when the valve is closed, which must be more than offset by the resonance benefit, then.

So, that means it the Exup valve must close at higher RPMs, since the resonant frequency is higher with the shorter effective length to the valve? I can't see mine when I'm driving around, but I always thought it was closed at idle. Hmmm.

It seems to reduce noise a bit when closed, too.