Exhaust flaps don't work like the internet would make you be

[BlueIn2Red]

Fascinating. i've plugged them under the boot floor, and the increase in noise is very modest, almost bugger all really. And I can't say I've noticed a drop in mpg as a result of this.

I found the same, no difference in noise (I always run in dynamic/sport anyway) for me so I reverted to default.

EDIT - sorry, missed Deano's subsequent post explaining this.

[BlueIn2Red]

HMS is top of my list so far. No way a milltek and the akro is expensive when you could create the same sort of effect for 1/4 of the outlay. + i don't like the tips

Do you mean their "x-pipe" or full system (or both?) please?

[DeanoC]

HMS is top of my list so far. No way a milltek and the akro is expensive when you could create the same sort of effect for 1/4 of the outlay. + i don't like the tips

Do you mean their "x-pipe" or full system (or both?) please?

Great question, i want to try the x pipe first. I'm not after ridiculous noise. Removing the front resonators and replacing with an x-pipe should give you a decent enough gain in volume, a more raw italian v8 type note and decent enough gains from the bank to bank scavenging.

I'm awaiting the internal and external dimensions from them. Also with tax and shipping etc. it maybe cheaper and easier just to go get the same thing done custom

[Sammyb349]

DeanoC - legend. I've been puzzled by this too. Thanks for doing the hard work. I figured having the car in individual with engine noise set to dynamic and everything else in comfort/auto would get me the same as flaps open and seems like you've confirmed this.

I too want a bit more though. I'm at a loss as to whether a x-pipe/H-pipe will actually cause similar scavenging issues to the highly engineerd standard set-up. Is it not best to play with the boxes at the back?

[DeanoC]

DeanoC - legend. I've been puzzled by this too. Thanks for doing the hard work. I figured having the car in individual with engine noise set to dynamic and everything else in comfort/auto would get me the same as flaps open and seems like you've confirmed this.

I too want a bit more though. I'm at a loss as to whether a x-pipe/H-pipe will actually cause similar scavenging issues to the highly engineerd standard set-up. Is it not best to play with the boxes at the back?

Pleasure. Yea with individual and engine sound on dynamic you valves will only close on a cold start. To open them simply cycle the modes, bang it in S then back to D or rev it in gear over 3000rpm

On the exhaust the advice i have taken from those that know more than me is predominantly:

Linking the banks is a good idea. The difference here over manifold flow and scavenging design is that the gas will take the path it wants. It's not forced.

X vs. H - An X simply copes better at high RPMs and we have revvy motors so that is easy

Where to stick it - as close to the collectors as possible seems to be the consensus.

I think just replacing the front silencers with the x will give a nice volume increase and a more exotic v8 sound - only one way to find out right. Im awaiting some answers from HMS.

[MikeFish]

If adding a cross pipe improves anything why didn't Audi make the front / centre resonator an X pipe? Both pipes go into the same box anyway so surely if there was any benefit they would have done it already? I suspect the difference in performance is negligible or adds at the high end but robs at the low end etc. Difference to sound might be different though but not as big as the difference front getting rid of the resonator box itself.

[Ian_C]

HMS is top of my list so far. No way a milltek and the akro is expensive when you could create the same sort of effect for 1/4 of the outlay. + i don't like the tips

I remember dribbling over the HMS Youtube vids of the B8 RS4, C7 S6 + RS6, wow, what a noise!!!!

[DeanoC]

If adding a cross pipe improves anything why didn't Audi make the front / centre resonator an X pipe? Both pipes go into the same box anyway so surely if there was any benefit they would have done it already? I suspect the difference in performance is negligible or adds at the high end but robs at the low end etc. Difference to sound might be different though but not as big as the difference front getting rid of the resonator box itself.

All fair points and something you have to take seriously considering we are talking top end RS cars built for performance.

Similarly you have to ask why so many reputable exhaust manufacturers and custom fabricators add them to v6 and v8 front engined cars.

[Ian_C]

https://m.youtube.com/watch?v=99IWLxWhzSY

[MikeFish]

It's a fair point and my post has no technical evidence and I know very little about scavenging. My opinion is purely from experience. When I added a H pipe to my rs4 it was combined with a lot of other mods at the same time so can't say what difference it made. But when I later removed it I didn't notice any difference from a power point of view. Another member on here that tried it said the car revved better without the H pipe (and aftermarket exhaust).
I suspect the aftermarket exhaust manufacturers use them to improve sound rather than performance.

[DeanoC]

It's a fair point and my post has no technical evidence and I know very little about scavenging. My opinion is purely from experience. When I added a H pipe to my rs4 it was combined with a lot of other mods at the same time so can't say what difference it made. But when I later removed it I didn't notice any difference from a power point of view. Another member on here that tried it said the car revved better without the H pipe (and aftermarket exhaust).
I suspect the aftermarket exhaust manufacturers use them to improve sound rather than performance.

Yea a H pipe is never going to be ideal on a high reving cross plane cranked V8. That will just be for a deeper sound in my opinion.

Cross plane is the key here. As it fires 90. not 180. like a ferarri it is impossible to balance the pulses without swapping across banks so an x pipe up near the collectors is the best chance you have of increasing scavenging on the relevant exhaust valves and therefore performance.

If you don't go too large on the diameter you shouldn't impact low end torque.

[Ian_C]

I always thought the high revving Ferraris were flat plane crank? They certainly were in the era of the 355/360

[DeanoC]

Yes 180 degree flat plane

[HDPete]

Very interesting read! Thanks for the effort. Your post explains beautifully how the various modes operate. Thanks!