Quiet exhaust

[diesel des]

Anyone ever try putting a valve on one of the 2 pipes on a twin system. I'm thinking one of the valves people use as a vent to atmosphere cutout but piped inline. So at cruse your running on 1 2.5 inch system then on boost the other valve opens and both systems run.

[Tzummie]

@ Philj : where did you get the Helmholtz solution from? I've been thinking about that for the RS2, as I've done it with a Jeep V8. Only difference was with the V8 I could put the resonating pipe at 90-degrees on the main exhaust pipe. So the principle works really good with 90-degree-placed pipe, but am not sure how well it works with such bent pipe (less cancellation or maybe a shift in frequency occurs?).

Any info welcome!
thnx.

[philj]

Considered the valve, but most of the aftermarket ones don't look too hot, and when you look into it, alot seem to have reliability problems. Are there not OE solutions? The 63 AMGs all sound the ballocks with no drone, assume its either a helmholtz solution inside the box, or possibly a valve. I vaguely remember seeing something similar on a 6n2 Polo Gti exhaust.

Helmholtz I just found on google. You need to record the frequency at which the drone occurs, then add it into a formula and it'll give you a pipe length to cancel the drone. Apparently, as it is sealed it's a high pressure area which exhaust gases don't try to enter as they can't come out, but soundwaves do, and with the right length tube it cancels out the frequency which is causing the problem.

Starts with working out what frequency is causing the problem, then it'd be interesting to see how people setups impact that. Ie whether the frequency is different between exhaust/downpipe/turbo size differences.

[Error404]

I would have thought for a 5 pot the resonant frequency in Hz will be (rpm x 5) / 60 at the rpm at which it happens.
So for example, 2200rpm = 183Hz

[Kieron20V]

Only 1 exhaust stroke per cylinder per 2 revolutions though, so will it be half that?

[Error404]

good point. half that then

[Tzummie]

the drone in my RS2 (BN pipes) occurs between 2000 and 3000 rpm. Roughly +40dB @ 100Hz.

Calculating : 100Hz divided by 343m/sec. (speed of sound) = length 3,4m > quarter wave length = 85,75cm. This is the length needed to cancel out the 100Hz drone.

The funny thing is, that this guy http://www.audizine.com/forum/showth...eing-installed (with 2008 Audi A4 B7 6mt) measured a length of 34" (which is 86,36cm) to cancel the drone at 2.000 rpm.

Which is almost exactly the same length as mine. And he doesn't have the 5 cyl?
it seems that the 2000rpm drone is independent of engine type: flat-6, V8, straight-5.

T.