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DISCUSSION => General Topics => Topic started by: itstheislander on March 03, 2009, 08:38:07 PM

Title: removing cat 91 e30 318is
Post by: itstheislander on March 03, 2009, 08:38:07 PM

im pulling my exhaust off tomorrow for some welding repairs, any gains in removing the cat converter? 91 318is e30

Title: removing cat 91 e30 318is
Post by: xwill112x on March 03, 2009, 09:13:41 PM

makes it run like hell up top.

Title: removing cat 91 e30 318is
Post by: papercutout on March 04, 2009, 03:37:29 PM

Just do it...


(nike - just did it :D)

Title: removing cat 91 e30 318is
Post by: Ricer Ennemi on April 18, 2010, 01:41:11 PM

I bought 4 into 1 headers,will shit through a 2.25 glass pack(no more cat N resonator)throug a 1.75 pipe(exit size of headers).2.25 pipe from GP to exhaust.Watcha think,to much?

Title: removing cat 91 e30 318is
Post by: BlueBMW on April 18, 2010, 06:41:15 PM

I replaced a previously removed cat on mine because it smelled bad.  I didn't notice a difference between not having the cat and having it.

Title: removing cat 91 e30 318is
Post by: dvmotorsports on April 18, 2010, 09:09:33 PM

It's louder. Less back pressure so you lose a tiny bit on the bottom, but you get more on the top.

Buy 6 flanges. Have them welded to the existing exhaust as well as the cat and a straight pipe so that you can swap them in and out when you need to.

Title: removing cat 91 e30 318is
Post by: carlos318is on April 19, 2010, 04:02:57 PM

I was thinking of doing the same but was not to sure on benefits from doing it so by the sounds of it there's not much point to a de cat

Title: removing cat 91 e30 318is
Post by: BlueBMW on April 19, 2010, 05:49:31 PM

I can see replacing an oem cat with a high flow aftermarket one, but I dont see any benefit to deleting it completely.  Besides its bad for the air :D

Title: removing cat 91 e30 318is
Post by: bmwman91 on April 19, 2010, 07:19:07 PM

If the stock one is broken/plugged, then replace it.  You can stick on a new aftermarket "high-flow" one for under $150US.  It will probably be exactly the same as running with no cat at all, it will sound better, you will pollute less and you won't risk a big fine (most countries have stiff penalties for driving without a cat).

Title: removing cat 91 e30 318is
Post by: BlueBMW on April 19, 2010, 10:07:46 PM

The cat I put on was about $70 for the 49 state legal version. ($150 for the CA legal one)  No change in power that I could tell.  And I feel like I'm super earth friendly!

Title: removing cat 91 e30 318is
Post by: carlos318is on April 20, 2010, 03:46:00 PM

Mine has a cat on it but in regards to the MOT (road safety test) it don't have to have one as they don't test it on a car that was built pre 1991

Title: removing cat 91 e30 318is
Post by: Ricer Ennemi on April 24, 2010, 04:53:46 PM

Yeah,I should buy a Hi-flow cat.

Title: removing cat 91 e30 318is
Post by: tony92ic on April 25, 2010, 04:42:39 PM

Unless your cat is damaged there is not much performance benefit to removing it. A modern cat isn't all that restrictive and a performance cat will flow as well as a test pipe.  Grassroots motorsports found about 1 hp on a miata between the stock cat and test pipe. UUC motorworks found about 5hp on a civic, but only 1hp difference between the test pipe and aftermarket cat. So it will likely vary depending on the quality of the oem cat. I am guessing BMW cats are pretty good.

I liked UUC's take on the final results: "With a difference of only 1hp between a straight pipe and a 49-state LEGAL cat, it seems foolish to flaunt the law and pollute the air that we all breathe. Hell, if you are only looking for those kinds of gains, just loose 10 pounds -that alone will make your car faster than adding 1 measly HP."

Also modern computerized/fuel injected cars need 0 backpressure. In fact backpressure hurts performance. The old backpressure/torque/HP tuning theories apply to carbureted engines.

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm