jjwalker

I seem to have more power with a cat installed!

I took my cats off a year ago and replaced them with straight pipes. Cut the precat off because it was hollow, and the main cat still worked, but it was clogged up with the precat's debris.

The cat delete pipes I installed where of the same size as the stock pipes.

I had to smog test my car and I threw on a magnaflow cat specifically for RX7's, but the pipe is smaller. I threw it on anyway to pass inspection but yet, I dont get it...

My car is MUCH more responsive. It was better when I got rid of the clogged cat, but it is even better with the aftermarket, wrong pipe sized cat. I am well versed when it comes to exhaust theory and it just doesn't make sense. Of course this is with the butt dyno, but it is so noticeable I am willing to believe the butt dyno.

Any ideas on what is going on here?

Brigdh

iTrader: (9)

The butt dyno is almost always wrong

flipkc

Honestly, I heard those Magnaflow cat converters are pretty much straight through so I doubt the lack of power you were expecting is now just "newly" felt. But you could be right, possibly it's providing some backpressure? Although I thought lack thereof was preferred.

AGreen

iTrader: (11)

Non-turbo? Your aux ports are now working. Enjoy.

J5sense

iTrader: (3)

^^^agreed

jackhild59

iTrader: (7)

Nope. S5. Ports are electrically operated.

jjwalker

Yup, S5.

tweaked

iTrader: (4)

I noticed it too. I had the RB silencers, bought a cat when i had to start smog testing and noticed a bit more pep. Aux ports still don't work as they should. I cooked up some valves and nipples and things from the air pump. Still working on the electric port mod.

jjwalker

My car is an S5, not an S4.

Derekcat

iTrader: (8)

I've been told that NAs like a tiny bit of backpressure... Perhaps the mufflers are a bit too free flowing?

Not that that makes any sense to me...

jjwalker

Doesn't make sense to me either, thus why I said the whole thing was stupid.

j9fd3s

iTrader: (3)

i noticed this too. i pulled the cat off and replaced it with a straight pipe, and under 4000rpm's it made a lot of noise but didn't really go anywhere. over 4000rpms no cat made a teeny bit more power.

i put the cat back on, and its better under 4000rpm, and i lost a little up top.

i've encountered this before, and its a tuning thing, the stock ecu just isn't right under 4k when you pull the cat out.

jjwalker

I was coming here to post the same thing.

As you said, it seems to shave some off the top and adds some to the bottom.

I was looking online for resonators, but didn't find anything that was free flowing enough. I really hate the sound of a decatted RX7 to be honest. Every resonator I have found online that could handle being in the cat location is either way too thin/cheap steel or incredibly expensive. I am not willing to throw money at exhaust components like that, without knowing how its going to sound.

ugh, is all I got to say.

I may just do the math and redesign the whole damn exhaust based on my numbers, but god that is expensive.

j9fd3s

iTrader: (3)

mine has an HKS cat back, and without the cat its too loud.

Derekcat

iTrader: (8)

And here comes the magic of Racing Beat mufflers...
Even with a Header and Presilencer it sounds great at every RPM and is very quiet at cruise.
[The weight does NOT matter unless it's a flat out race car, the dual tips look better, and it certainly has more power than it did with a stock catback]

jackhild59

iTrader: (7)

So just put on a correctly sized cat and be done. Quiet, more than acceptable performance and NOT stinky. That's what I did years ago. My generic cat is coming up on 5 years now.