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Discussion Starter · #1 ·
just wondering if anyone has tried a cat bypass pipe with the standard exhaust system? i should imagine noise would be up slightly and a slight increase in BHP, if someone has done this id love to no your experience.
 

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Yep, ive done it. mainly because my cat was shot and i didnt want to fork out for a new one. It does feel slightly more responsive and also sounds sportier. I had a stainless back box on it but it was too loud, so took it off and leftthe bypass on.



Another tip, very blue peter this one but good none the less, when my back box went i replaced it with a used backbox from another F. Poblem was tho, that the tail pipes were a wee bit rusty. So what i did was take to them with a hack saw and cut them off!!!!then went to the local auto store, shelled out on some good quality SS exhaust tail pipe finishers, possitioned these where the original tail pipes were, drilled a hole through the bottom of the valance and marked the tailpipes to be drilled. when id drilled them i simply bolted the new tail pipes to the valence with the actual exhaust exiting through these. Then bobs your mothers brother, new tail pipes on yor old exhaust! works a treat and looks as good as the real thing, beats the original (and rusty in my case ) pipes. You cant tell that these are just bolted on finishers as all you see is the tailpipes and yes the fumes do come through them.


Gary.
 

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Check out DR Bell`s site, there is a section on de-cats.

Robs tests were on an F, not the TF but would think they are the same.

What TF model do you have?.

Looking at the results other than noise there is very little to be gained from the de-cat, and if yours is the VVC, then you LOOSE power with a de-cat!!!. plus you have to re-fit your cat anyway come MOT time!.

http://www.mgf.ultimatemg.com/menu.htm
 

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Discussion Starter · #4 ·
yeh ive got a 160 vvc, so if a fit a decat i will LOOSE power?? ive never come across an engine that looses power when the cats removed?? is it something to do with the k-series? or VVC?
 

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cahzz said:
yeh ive got a 160 vvc, so if a fit a decat i will LOOSE power?? ive never come across an engine that looses power when the cats removed?? is it something to do with the k-series? or VVC?
Bear in mind those tests were done on an F. The TF breathing system is very different amongst other things.
 

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Discussion Starter · #6 ·
arhh its because the VVC need more backpressure according to that site very good reading so it turns out that the stock cat is the best option anyway, im still very supresied to see that it actually lost bhp and torque on the VVC very intresting, so does this mean that when the VVC kick in the loss would be more at top end and when the car is off cam it would have more power?
 

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I was there when these tests were carried out (indeed the MPi vehicle used was Scarlet). We were very puzzled by the results at the time and all sorts of theories were postulated. On the day the leading theory was that the tests we carried out were flawed. However, i know that similar results have been obtained when the tests were repeated - so there is definitely something going on here.

The current favourite theory is that the VVC MEMS is more sensitive to back pressure in the exhaust system (because it is primarily involved with the breathing / timing of the engine and therefore needs to be that much more sensitive to the way then engine is running than the MPi) and tries to compensate by altering the timing / fuel:air ratio.

The other thing to know is that the exhaust system changed with the introduction of the MY2K F in so far as a post cat sensor was installed. Therefore if you remove the cat from a MY2K F, or later (including all TFs) then you need to get around the sensor somehow, otherwise the MEMS will think there is a problem and could go into 'safe mode' which seriously limits the available power.
 

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straffs said:
Bear in mind those tests were done on an F. The TF breathing system is very different amongst other things.

Your right, the Tf has modified airbox, and a different exhaust, but the VVC, (Variable Valve Control, or Very Very Complicated) is pretty much the same.

If you check the tests Rob carried out, he thinks the VVC needs some `back pressure` to give the full benefit!, which is lost when removing the cat!.
 

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Discussion Starter · #11 ·
how intresting but mind boggling at the same time, so according to this then how does a performance exhaust benefit the VVC engine as they are 99.9% of the time claiming freer flowing = less back pressure? this is very confusing and leads to all sorts of oddball conclusions like a performance exhaust may not help at all.
 

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G.C said:
Yep, ive done it. mainly because my cat was shot and i didnt want to fork out for a new one. It does feel slightly more responsive and also sounds sportier. I had a stainless back box on it but it was too loud, so took it off and leftthe bypass on.



Another tip, very blue peter this one but good none the less, when my back box went i replaced it with a used backbox from another F. Poblem was tho, that the tail pipes were a wee bit rusty. So what i did was take to them with a hack saw and cut them off!!!!then went to the local auto store, shelled out on some good quality SS exhaust tail pipe finishers, possitioned these where the original tail pipes were, drilled a hole through the bottom of the valance and marked the tailpipes to be drilled. when id drilled them i simply bolted the new tail pipes to the valence with the actual exhaust exiting through these. Then bobs your mothers brother, new tail pipes on yor old exhaust! works a treat and looks as good as the real thing, beats the original (and rusty in my case ) pipes. You cant tell that these are just bolted on finishers as all you see is the tailpipes and yes the fumes do come through them.


Gary.

Hey Gary, pleased to know my trusty 'ol back box is still doing the bizz for you! :hyper:
 
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