Originally posted by Mbirt:
To eliminate the "American vs. European" issue, is it safe to say that this,
http://www.youtube.com/watch?v=pdi8tHbmAdc, is the sound that you do not desire?
As you eluded to earlier, you cannot create frequency content that the engine does not produce. For 3000-7000 rpm, the firing frequency, the 4th order sound, will be 200-467 Hz. The exhaust sound will also contain frequencies higher and lower than this. Higher frequencies will come from the exhaust blowdown event and greater pressures will generally create more high-order noise. The most significant will likely be the 8th order, or double the firing frequency.
Lower frequencies, the ones you are trying to avoid, come from uneven mixing of pulses due to geometrical discontinuities in the exhaust ducts cylinder-to-cylinder and odd firing orders. For a cross-plane v8, this will usually be the 1.5 and 2.5 orders.
This video,
http://www.youtube.com/watch?v=DnzxuS1HHpQ, has an interesting comparison between a C63 AMG and an M3 but with similar exhaust systems. The M3 has a higher-piched, raspy tone, while the C63 has more of the broad-range sound with more low-frequency content. The biggest difference I see between the cars is the log-style manifolds that come on the C63. Its uneven mixing of pulses creates odd-order sound and steals energy from the 4th order to do so.
My advice would be this:
-Don't skimp on the headers. Equal length will yield the least half-order sound.
-Use an X-pipe or merge into a Y.
-Keep it symmetrical
-Long, uninterrupted pipes to the muffler(s) at the end of the system. The Innotec M3 system gets its rasp from this.
-Corsa RSC (sounds great on E92 M3's) or one of the short-length Dynomax VT mufflers. The Corsa muffler kills the lowest frequencies with quarter-wave tuning, while the VT has a passive flapper valve for low frequency control. The flapper snaps open quickly at medium flow rates. The longer-length versions will contain more fiberglass, which might kill the character you desire to create.
-Early exhaust valve opening to allow the sound wave to steep-front and create the raspiness that seems to help the M3 (with the right exhaust) sound like your ideal cross-plane v8.
I've got a few FFT and sound analysis apps on my phone I like to play with. It was interesting comparing the C63 to the M3 in the video above when they switched from car to car. You might get a kick out of doing some quick analysis of exhaust sounds that you do and do not like. Being able to quantify the sounds with frequencies will help you decide which tuning elements to use.