Plus and minus points for removing cat

[NTY]

Re: De-cat Performance

[Hmmm interesting... I was just wondering would (a) & (c) improve bhp and torque if applied to a force inducted motor compared to a NA model?

I did some experiments in my ex E7:

1) Gut cat: No improvement

2) Replace mid pipe to bigger diameter: Slight gain.

3) Modified exhaust valve in rear muffler: Slight gain.

4) Full exhaust: Substantial gain but increase lag

5) Change aftermarket front pipe (Down pipe): No gain

 

[Racebred]

wow you have extensive experience. but really strange that for E7 your exhaust mods didnt amount to anything substantial. i thought it should benefit greatly from it

 

[NTY]

wow you have extensive experience. but really strange that for E7 your exhaust mods didnt amount to anything substantial. i thought it should benefit greatly from it

It did. The highest gain comes from a full set of after cat fujitsubo exhaust. It cut my quarter mile timing by 0.6s. I subsequently changed a bigger diameter front pipe but no gain.

Any other mods to the exhaust are not worth the performance gain. You can see that all my recent and current cars exhaust are left un-touched. The only exception is the MR-S. The front cat was gutted out because of some engine reliability issues. It is highly recommended in overseas' MR-S forums.

 

[rex7_vtec]

Re: De-cat Performance

[Hmmm interesting... I was just wondering would (a) & (c) improve bhp and torque if applied to a force inducted motor compared to a NA model?

I did some experiments in my ex E7:

1) Gut cat: No improvement

2) Replace mid pipe to bigger diameter: Slight gain.

3) Modified exhaust valve in rear muffler: Slight gain.

4) Full exhaust: Substantial gain but increase lag

5) Change aftermarket front pipe (Down pipe): No gain

This is extremely interesting...I've always heard other Evo owners say how much better their car feels after the front pipe... Even I myself thought it would have made a difference....

 

[NTY]

Re: De-cat Performance

This is extremely interesting...I've always heard other Evo owners say how much better their car feels after the front pipe... Even I myself thought it would have made a difference

Feeling is deceiving and figures don't lie. That's why I always measure car performance with timings.

I had the after cat exhaust fitted before I put in the sheepdog front pipe. No gain to timings and terminal trap speed. But front pipe mod to WRX did yield some gain.

When I first took delivery of my M coupe, I was very disappointed because it feels "slow" in contrast to E7. But after timing it, I realised it matches the E7 in 400m timing and even got a higher trap speed.

 

[Shaun]

Hi again TY. Of all those cars you listed, how many run twin O2S per bank?

If many, it would negate the results without logs of afr and/or ign timing as compared to stock since mode of operation would not be known. O2S potentially trips mode shift.

If few, it would suggest that the restriction lay in the muffler, or pipes, or both.

The NA cars in the list might even run worse times with a freed up exhaust system and no tune to compensate for esp in the cam timing dept.

FWIW, my experience with a low power turbo VW, a high power turbo Audi, an even higher power twin turbo LS1, has been the opposite yours. I attribute it to different ratios of cat to muffler/piping restriction ratios in different makes of cars. Different emphasis on sound vs air pollution maybe.

 

[Shaun]

Hi Shaun,

I read some forums, some saying that the earlier we merged the twin-pipe to one, you will get very good low-end torque ? true ?

BMW 6 pots has two 3to2 header and the 2 pipes will go into cat, mixed in there and than out 2 pipes again to center box, mixed again, 2 pipes to rear box.

I believe that would be generally true since if you merged the runners earlier, you would be able to design a header that uses the active runner to scavenge the ones not in use. Then again at low RPM there is not a whole lot of exhaust gas velocity, so the effect is probably limited. A study of the BMW inline 6 firing order, header design and pipe diameters, is required for a definite answer. A good test process will also tell.

What is for sure though, is that all the joining and seperation of the exhaust gasses in the stock BMW system is not good for flow. Anytime gasses have to split, there is unnecessary backpressure. Twin systems also increase boundary layer area. I believe they did it for packaging, looks, noise control. A twin system is also heavier.

 

[NTY]

O2S

Hi again TY. Of all those cars you listed, how many run twin O2S per bank?

I think only the 190E has a before and after cat O2S. Most Japanese cars have only one O2S per bank, usually before-cat.

 

[k3nny]

Just pop your head under your car.

For my 4pot valvetronic, its 2 off the headers into 1 pipe.

That goes into 1 box, i can assume its the cat.
Then further into 1 more box, the resonator.

Then the muffler.


For my gf's 2.2l 320i, it has 3 pipes down to 2 pipes off the headers.

Then into 1 box with 2 inlets which i can assume is the cat.

And then 2 "seperate" resonators. Like a hotdog on each pipe.

Then the muffler.