Cat bypass => what will happen?

[erics]

posted the above on boxta.net but wanted more advice...

What will it change: sound? performance? will i get a light on the dashboard?

Has someone done it???

[MNBoxster]

posted the above on boxta.net but wanted more advice...

What will it change: sound? performance? will i get a light on the dashboard?

Has someone done it???

<{POST_SNAPBACK}>

Hi,

Sound will deepen, performance gains are moderate to non-existent with a potential drop in low-end performance.

You will have operating issues if the 0² sensors aren't dealt with. The DME compares the voltage readings of the pair of 0² sensors dedicated to each bank both pre and post-Cat and between each pair (OBDII). I know that for the Boxster, the distance between the two is also significant and there s/b a voltage difference as well. If the DME does not read this, it will run/start poorly.

I know that for the Lotus Esprits (I own one too), the fix is to add a certain value resistor on the 0² wire(s) which tricks the ECU into sensing a proper set-up. I don't know if this is the fix for the Boxster.

Happy Motoring!...Jim'99

[Amfibius]

Thanks for your response, Jim. That seems a bit counter-intuitive - removing a cat will result in "moderate to non-existent" gain in performance? Your post seems to imply that the reason for this is because of the engine management. Would it be possible to regain the HP by reprogramming engine management? After all, you now have a freer flowing exhaust ...

[MNBoxster]

Hi,

For NA cars, going Cat-Free will have minimal gains, if any. The reason is that some Backpressure is desirable to maximize the scavenge effect of the exhaust. Reducing the scavenge effect (because the exhaust pulses are elongated) will rob some HP and these effects are most seen on the Low-End.

Add to that, the Cats today are much more Free-Flowing than even a few years ago, meaning that removing them simply doesn't yield that much of a Net gain.

Then, as you mention, the ECU (DME in Porsche parlence) will go into a default MAP if it sees things which are too screwy, resulting in a lessened performance, especially in the Mid to Upper power band.

On a Forced Car, on the other hand, especially a Turbo, going Cat-Free can have significant gains. The Turbo usually provides sufficient restriction on it's own and the increase in EGP (Exhaust Gas Pressure) can be used to either make more Boost, and/or reduce Lag, both performance enhancing things.

Happy Motoring!...Jim'99