Dementia

I just bought a used rb setup off cl locally. My 88 s4 gtu is all stock and in good condition, and don't want ruin anything. The setup was run on an 88 s4 gxl that had an overheating/blown motor. The exhaust is rb headers, and rb high flow cat, and a custom magnaflow dual catback. The catback utilizes the connector to cat (from a rb catback) into stock pipes into 2 magnaflow mufflers. I'm worried about the cat because I can't find any info on it on the rb site. Will it make much difference to buy a straight) pipe to run instead of) cat? Also, I'm going to smog before I swap out exhaust, so passing is not an issue. And any thoughts on this catback assembly are appreciated. Thanks ! -Hogg

fcmazdarx7

If the cat is a highflow design(straight through hollow core) then you wont see much of a difference between it and a straight pipe. A factory cat is very restricive due to all the honeycomb material inside, but high flow cats are much less restrictive.

Dementia

Thnx, any thoughts on this catack setup? I'm still learning about backpressure.. Not sure how to post a pic on here either

Casual_John

iTrader: (8)

You don't want back pressure with a rotary. (You want vacuum, actually.)

Sounds like you got a Racing Beat exhaust system, except for the Magnaflow mufflers. Keep all the Racing Beat stuff, and if you like the sound of the Magnaflows...keep'em. Changing the catback mufflers just changes the sound effect; not the performance. The hi-flow cat helps performance; a straight pipe would flow a bit better, but would definitely be loud and may burn up your mufflers prematurely. Let the Cat take the brunt of the heat.

GSLSE-YA

You might want to consider a RB silencer if you remove the cat. It will still be less back pressure and will help keep the sound resonable.

How big is the piping coming off the cat? What does the "Y" pipe look like and size? Is it the stock Y pipe?

Dementia

ill get some pics up in the next few days thanx for your thoughts..

Dementia

here are the pics. upon closer inspection, the stock piping has some ugly bends that look like they are restricting the flow a little

Jeff20B

iTrader: (13)

Those bends are known as "crush bent" and they are a bit of a restriction, which leads to the dreaded backpressure we rotorheads are always looking to minimize/reduce.

Be on the lookout for mandrel bends. There's no reduction of inside diameter with just a slight bit of wall thinning on the outside (apex) of the curve due to stretching.

If possible, choose one wall thickness more for your u-bends if purchasing tubing to make a custom exhaust. For instance, if you're going with 16 gauge straight tubing, get 14 gauge u-bends. The outside of the curve thins down to around 16 gauge.

Rotaries are loud so the thicker the wall, the better. Infact I'd recommend 11 gauge for everything near the engine and only drop it down to 14 gauge near the end, with 16 at the very end where less muffling is required.

GSLSE-YA

Your cat is probably the most restrictive part of your current exhaust. Not sure what diameter it is, but it sounds like you have a collected header into a single cat, then "Y's" into dual exhaust. You may get hp gains by getting rid of the cat.

I don't think your after cat section is much of a problem even though it's not mandrel bent. It would be more of a problem with a ported motor.

Dementia

I'm a newb to rotaries and am wondering about the 5th and 6th ports. They are controlled by the exhaust backpressure right?
The racingbeat website says that the road race header has a spot to install the backpressure pickup tube, but with a high flow exhaust setup like mine, isn't backpressure going to be minimal? How will this exhaust effect the that intake process??