rx007racer

My question is can a replace the pre cats on an 88 with a straight pipe from manifold to main cat, cuz mine is rusted and i think it is where the exhaust leak is coming from and is quite loud now. But in doing this will it affect anything. PLEASE DO NOT TELL ME TO JUST REPLACE ALL CATS, I HAVE AN S4 SO I NEED THE MAIN CAT FOR THE PORTS TO OPERATE AND DON'T HAVE THE MONEY TO CONVERT TO ELECTRICAL PORT OPERATION. Also will it make it louder at all that would be sweet if it did. If any body has done this let me know the bend required or anything alse useful. Thanks All!

spot_skater

You're gonna need some way to open up the auxilary ports. Look into a RB presilencer to throw in the middle. If you run a straight through exist there probably won't be enough backpressure to open up the aux. ports, and then you'll lose a lot of top end power.

It will be louder though...

rx007racer

ya i know but i said i was going to keep the main cat and get rid of the precats.

spot_skater

Yes, you can do it. The main cat should be sufficient, unless it's blown out, in opening the ports. Since there will be less restriction if you get rid of the precats, it will probably be louder, too... but probably not as loud as an exhaust leak...

mrslysly

i have an 86 n/a and was looking at replacing the pre cat and the cat with straight thru pipes. I thought that all you needed to operate the aux ports was to keep the acv and the air pump?

Or is there any place to get an electrical control for the aux ports?

sar

iTrader: (1)

86-88s (s4s) use exhaust back pressure to actviate the 5 and 6th ports while 89-91s (s5s) use the acv and airpump.

To the poster: Tha main cat will be enough backpressure, there shoud'nt be any problem welding in a straight pipe with either emissions or for 5/6th port activation. There may be a very slight noise increae but in actuality I dount your precats are actually still in there. If the section didnt have a leak and you could just break the precat if it's still in there.

On a final note, I doubt that removing the precat will give you any significant power increase through the negligible back pressure decrease - the pre cats should flow at about the same rate as the stock main one.

You could consider switching to air pump activation instead of backpressure - it's relatively simpler and cheaper than the electronic version since you'll be keeping the airpump anyway. That or there is somebody talking somewhere about vacuum actuation suinge ngine vaccuum and the release hole for the actuators.

Node

iTrader: (1)

absolutely. precat=death incarnate

mrslysly

So I could remove both pre cats and the cat and switch over to using the air pump to activate to Aux. ports? Is there a previous thread covering this? (not trying to be lazy, but if anyone knows off hand a good one to check out, thanks) Would the setup off a s5 fit on an s4 engine for the Aux ports?

sar

iTrader: (1)

http://www.rotaryresurrection.com/ro...r/airpump.html

The UIM for the s5 includes some ecu stuff for vdi and 6pi i belive, so you would need to trigger that somehow. I think this airpump method is best in anycase.

rx007racer

Here is a link to a write up on the electronic activation of the 5th and 6th ports for s4 NA engines. It s a good detailed write up with pics I found a while pack. Also thanks for all the help guys but actually i got up under my car and found that it is leakin at the flanges that connect the y pipe to the main cat so i while have to install a new exhaust gasket there so it wont leak anymore. Also it make a strange sound when it has been sitting and when you get on the gas hard it does it sounds kinda like a air horn only not as loud, thats the only way to describe it. Im thinking that it does this because the exhaust is cooled and that the gap in between the flanges cause this noise, due to the vibrations of the metal. Tell me what you guys think. Thanks again, rx007racer

harley71105

On my 88 S/4 NA I used a straight pipe from RX7.com and it worked fine. It was the model with the "presilencer" which was nothing more than a swell in the pipe with some holes inside and does not restrict the flow. I think I paid around $120 bucks for it and they even had the gaskets...its prefit and bolts right in...took me a couple of hours. No problem with the ports even though this particular pipe replaced my main cat too...on the back I have two stainless steel glass packs w/straight through design...sounds great....a little loud but hasn't got me a ticket yet.

KeloidJonesJr.

iTrader: (8)

RB ftw. Dude, if I were you I'd just get the whole system. 4 peices which includes: downpipe>presilencer>y pipe>muffler.