Flap in TT exhaust, what's it for ?

[steve-v8s]

OK all you TT people I was following a TT recently and noticed there was flappy thing in the RH exhaust. What on earth is that for ?

[up-the-dubs]

Damn, thats got me curious. I just know I'm going to rear-end the next TT I see, trying to have a look up his chuff!

[catso]

The 3.2 TT and A3 have a 'butterfly' valve in one of the tailpipes, I believe it has something to do with meeting noise emissions (or maybe back pressure?). I have looked at the one on my A3 and it has a linkage to a small motor/actuator to close it but I've not managed to see it close whilst the engine is running (car not moving) so maybe it only closes at certain RPM/road speed combinations.

[Fattb]

I believe it opens at higher rpm to emit a nice exhaust note and stays low at cruising/lower rpm so as not to become to boomy...or something like that, I think BMW used something similar on some of their earlier 6 cyl cars,

[steve-v8s]

I was following one for a few miles in very slow traffic on the M25 and had plenty of time to try to work out what was going on. As far as I could see it was not related to road speed or RPM. My best guess is it works on engine temperature which may have been going up and down based on the fans. I know some of the Red Italian things have a hatch that opens at high revs but this example was little more than idling. Come on someone must know.

[Fattb]

This may help have'nt read the whole thread though

www.tt-forum.co.uk/ttforumbbs/viewtopic.php?t=38069

[steve-v8s]

That solves that one then, thanks for the link.

[fortjefferson]

steve-v8s said:

That solves that one then, thanks for the link.

That just can't be worth all that f@@king about, can it?

[Hughesie2]

Aston have a similar thing, suction valve type arrangement which if you disconnect makes the car louder through the whole rev range, not just noisier above 4k rpm.

[catso]

Well reading that article, it seems that the only purpose for the flap is to reduce noise slightly but if it really makes "7hp & 5ft-lbs" difference when open then it should stay open all the time, the car is by no means excessively loud. I think all that faffing around with switches is a bit silly, why not just take it off

[fattb]

catso said:

Well reading that article, it seems that the only purpose for the flap is to reduce noise slightly but if it really makes "7hp & 5ft-lbs" difference when open then it should stay open all the time, the car is by no means excessively loud. I think all that faffing around with switches is a bit silly, why not just take it off

I think thats what a lot of people with the 3.2 TT have done, rather than going the long route