I've been debating getting a down pipe for a while because I don't need all that attention from The Man. So, I've been considering keeping one cat by replacing one with a custom pipe or the Stromung elimitator, etc.

But just how good is one cat versus two cats (and versus no cats). To me having two cats not only clogs things but seems like it would create this really oddly pressured chamber in the exaust path that would sorely mess things up. One cat would clear that up. But as for flow, how much would one less cat increase it? They are pretty dense.

OK enough musing... anyone else have thoughts about this?

jason

 

I really don't think you will see that much difference between having 1 and having 0 cats, that difference will become bigger with the amount of power you are putting out, but I'd say the power difference, on a stock turbo'd car isn't much more than 5HP. Although I really have nothing to prove this, just from reading around the boards...so take it for what it's worth.


Green Answer: The difference btwn 1 and 0 cats is not worth the damage that you are going to do to my already ****ty lungs (and other peoples). Even with one cat there you are cutting down significantly on emissions, and will still be well w/in standards, while not having a big effect on HP. Personally, I would never go to 0 cats, the extra power is not worth the possible consequences ie. getting busted, or in general extra pollution.

 

Remove 1 and get a high flow for the other.

 

what high flo do you recommend?

 

I had just the third cat for a while (gutted up, bosal dp, cobb catback), and removed the 3rd one with the Scoobysport eliminator. No real discernible power differences, a much louder exhaust note, and very much stank coming out of it. I did modify the 3rd cat heatshields to fit on the eliminator pipe so I had something to point to as a cat. The stink was too bad, so the cat went back on.

In contrast my vishnu with one high flow doesn't stink at all. Someone on NASIOC even passed the CA smog sniffer with only one cat (MRT hiflo). Keep one cat...it's not really worth it.

 

So anyway...my recommendation would be the bosal DP (~$200 with gaskets) or the scoobysport bellmouth. Both mount to the stock 3rd cat, and will definitely give palpable gains.

 

and it's the midpipe cat you guys are keeping?

 

There really isnt a differnent between no cats and 1 cat power wise that i've noticed i've driven cars with each and pretty much the same. So if it's a opinion keep your rear cat and make sure it's a high flow one. not getting a high flow one would be the reason you would lose power.

 

The downpipe cat is much bigger. Lose that one.

 

I had the following set up: uppipe, stock dp, third cat pipe, stock rear exhaust pipe, and aftermarket axle back. All in stock size (2.5" dia. I think). Was hitting about 190whp.

Then I changed to the following: 3" full turbo back. Was only hitting about 193.8whp on the same dyno with a tune.

And on a different street dyno tested in 2nd gear about 203whp.

Only difference was the 1st dyno run was summer and the last was winter (I've been told this can make a difference).

So for the money and time I could have just used my 1st set up and be just as happy. Now with no cats the car is very loud at WOT and has a low rumble at normal driving below 3k rpm. To really get the benefits from a larger 3" exhaust and no cats (from what I've read) you need a larger turbo and the supporting mods.

Please correct if I don't know what I'm saying. Still new to all the turbo stuff.

Thx,

Lowball

P.S. here is a quote from SPD Tuning who is on the conservative side of tuning from what I've been told and from what I've read on his site. I'm sure some would dissagree with this statement.

"Since we live in the clean air era, SPD Tuning Service does not recommend removing the large down pipe cat on road car. First, the down pipe cat is actually the same unit used on the STI 280hp cars. This main down pipe cat is known to run a clear 300-320hp before is considered a power restriction. It is just not a significant restriction at the power levels we can achieve on pump gas. Yes, if we eliminate the main cat there is a mild improvement in spool-up of the turbo, but not huge gains that justify turning you car into a pollution machine. Just do not worry about it. My 260hp WRX runs a down pipe cat. It runs just fine. For a mild tuning of the engine please keep your main cat. The power gains will be quite similar with or without, if other details are taken care of."