Decats Vs Catpipes

[5ky-lin3]

hey guys just wondering what is the better option. I know people say that there is minimal if any top end hp difference but ive also herd that cats also help with the exhaust flow through turbulence etc. Is this true? and if it were wouldnt a high flow cat be alot better for the car all round rather than a de-cat? Just wondering because ive just purchased a decat and wondering whether it was the right choice to go for a straight through 3" pipe (not counting the high flow muff before the cannon at the back)?

Cheers

[talky]

all i kno is a mate has a e cat pipe on his sr and goes excatly the same as wene it didnt

[PM-R33]

So your asking what is better, a cat or no cat? Obviously performance wise no cat is better, legality wise you have to run a cat so there is your answer. Want to be legal, run a cat.

[Harey]

Remember exhaust characteristics are different for an N/A and a turbo car. A dump pipe on a turbo car should theoretically be as big as possible, you want the best flow possible period. However, on an N/A car it is different, it is more complex, an exhaust restriction can help with down low power but reduce top end and vice versa.

Whereas a nice large dump pipe on a turbo will give better response and top end, its a win win. Up to a point obviously where any increase in flow doesnt make any difference.

In summary to your question, the decat pipe will be better performance wise on your turbo car.

[08yannch]

so explain why my car runs like shit when i run open dump pipe but runs tops when i put a cat on?

soz 4 highjacking ur thread

[Munkyb0y]

probably coz it hasnt been tuned for that set up.

[Blitz]

probably coz it hasnt been tuned for that set up.

Or the cat itself is stuffed/blocked/melted.

All common.

[Harey]

Your cat must be very restrictive to cause such a change, most cars would barely notice a difference. When you remove it, the exhaust is flowing a lot better meaning more boost, meaning more air for the engine causing it to run lean.

Define running like shit? loss of power? If its extremely lean you will loose power.

Edited April 21, 2010 by Harey

[Brezza]

so explain why my car runs like shit when i run open dump pipe but runs tops when i put a cat on?

soz 4 highjacking ur thread

probably coz it hasnt been tuned for that set up.

yeah exactly right,

when i had my cat taken off and a decat put on the tune was very lean and the car made 260rwkw on the same boost, which was down from 285rwkw with the cat, but with a little tweaking of the Nistune to correct the mixtures. the car was able to make 320rwkw with an extra pound of boost.

i went to a decat because my cat was restricting flow because the guts of it had come loose and was moving around in there.

if you got an aftermarket ECU get it retuned, if its stock. then maybe the extra flow is just too much for the ecu to handle?

[bozodos]

also if you're running an ECU with no closed loop mode, the cat might not heat up properly, and therefore block up

[krzysiu]

My car made 5rwkw in the top end and 30rwkw more in the mid range all by replacing the cat with a decat. Making 280rwkw

[Parag0n]

A good high flow cat will be quite expensive, around the $500~+ range. Metalcat comes to mind but the postives outweigh the negatives, probably a small 10-15rwhp possibly to be made by running straight through but the legality of being found out with no cat and having your car slapped with a major defect and towed away makes up for that.

[Parag0n]

http://www.horsepowerinabox.com/HPIAB2/category85_1.htm

Just realised HPIA is having a special on their 3inch Magnaflow Hi-Flo cat... $225 very cheap price.

[greensa14]

I was running a decat. I put my cat back in, and did feel the car seemed to run better or soother with it on. It was down a little in power....but the car felt better to drive.

[Harey]

I was running a decat. I put my cat back in, and did feel the car seemed to run better or soother with it on. It was down a little in power....but the car felt better to drive.

Yup it could do that. Are you running the standard ecu?

All you are doing is improving your exhaust flow, which has to be a good thing, it just means sometimes the ecu is not ready for it and needs to be adjusted.

[RBceffy25]

Had my car around three years now and when I first got it I used a holesaw with extension to core out my cat and the whole insides disintegrated and just simply fell out (alot easier than trying to bash it out. that’s if it’s in pretty good condition like mine was). I found that when I first did it my exhaust note changed ever so slightly (more aggressive but probably more due to gaskets constantly blowing out at the time) and boost response is slightly better and went to rev limiter fine on stock boost. HOWEVER since I have now fit FMIC and played with the boost a bit it definitely starts to lean out around 5000-5500 and above.

(I’m only running 8-8.5psi due to R&R as I have a massive CAI which was causing compressor surge and R&R. Its now partially blocked until I get Haltech and tune. And yes I have cleaned my AFM, gasket gooed, taped and cleaned my coils and re-gapped to .8.)