While I was walking home from town at 3 in the morning I started spec'ing up my next 300bhp Coupe and had an idea that I'd like to try...

Basically, I'd want to put a lever in the driver footwell that switches between cat and decat, so I can choose if I want it quiet or loud and change it instantly.

So here's how I'd do it:

Run a straight through exhaust with decat pipe, put a butterfly valve inside the decat pipe attached to a cable running through to the cockpit. stick a cat converter in parallel so when the butterfly valve is closed the exhaust gases are forced to go through the cat, and when it's open they'll just go straight through (mostly).

What do you think?

How about buying a throttle body and using that as the valve? Obviously, would require some custom welding and fabrication, maybe modification to the floor to make the 2 exhaust routes fit side by side?

sounds like it could work although Im not sure on what effect it would have on the actual running of the car.
instead of a cable you could use a solenoid and just have a switch in the dash?

Could do, if someone told me how to do it

I think it might be a bit more restrictive than usual running it through the cat due to forcing it round a crude diversion but that wouldnt matter, that's just motorway cruising mode...

Or how about this:

Have the butterfly valve in the exhaust also controlled by the throttle pedal, so have both cables coming off the pedal then going separate ways. the further you put your foot down the louder it gets, and when you release the pedal you can hardly hear it...

there a system that is boost operated.
basically there's a T pipe (and screamer pipe) just before the cat which has a actuator attached to a butterfly valve when you turn the boost up the actuator will open and you then have a straight through system.

I think this is easier said than done it's all well and good just saying put a valve in the middle but then you have the problem with alot of back pressure. The car would need to be mapped with the valve on i'm not sure if it can be done on the standard Bosch ecu you'll have to wait for flea to comment. Also if it's done by the gas pedal and opens with the gas pedal I'd suspect you'll get alot more lag as at low rpm's the valve will be closed. I'm not sure bout this but don't some of the more expensive cars with active exhausts have another sort of bypass of the valve for vacume purposes or am I talking rubish?

http://www.atpturbo.com/root/releases/release091004.htm

There you go

Ben

If it was controlled by the throttle pedal then at low rpms as soon as you put your foot down it would be wide open, so there would be no extra lag.

Mapping wouldnt even come into it, it wouldnt make a difference. You dont have to get it remapped if you swap the cat over!

well the man has spoken he has made a valid point there

Bet that's VERY loud, I'd still want it going through the back box!

Would using a butterfly valve on the front pipe not also have implications as far as the MOT's concerned? I would have thought so, seeing as the exhaust gasses are being diverted, this would give a false reading at the exhaust tip.
Just seems like you would be leaving yourself a lot of work to do come mot time, if so.

Iain.

Depends which MOT station you go to I guess...

if this works i would probably want one because at 2500-3000 rpm on motorway my car is headache loud.

thats a neat bit of kit, and as its controlled by boost pressure ( i assume it s actuator is t eed into turbo actuator pipe )it could further be controlled by a solenoid valve so you could effectively have it working or not at the flick of a switch.... for mot time

Good idea Bosco, that could work.

I still think I would be a touch worried about how loud this would be, and also the effects on the turbo, being that you would be running with virtually no back pressure. Would step gap rings be enough to aleviate this???

Dynoflow.co.uk used to do an electrically operated butterfly for that same reason a few years ago. It's not advertised any more but he might still sell them.