the idea came to me after researching Subaru flat 4's and what gave them their distinctive lope compared to conventional inline 4 motors, I discovered that was due to their unequal length headers. On a Subaru, Cylinder's 1 & 2 have shorter pipes than on Cylinder's 3 & 4, from my observations, cylinder's 1 & 2's pipes are roughly half as long as the ones on the opposing bank. The longer pipes cause a delay because the sound pulses have to travel further, however, the delay isn't long enough to sound like a v8, I reckon that if the pipes were four times as long as the short pipes, it would simulate the sound of a single bank of a v8 engine.
The only drawback would be heavily unequal exhaust pressure that could seriously weaken the longevity of the motor, but perhaps a larger diamater piping for the longer pipes would solve this potential issue.

It would be so simple to accomplish, you would simply need to collect cylinder's 1 & 2 together, have it going down wards, and have Cylinder's 3 & 4 collect into a really long pipe that travels behind and around the engine and merge with Cylinder's 1 & 2 like this



you don't actually need 8 cylinders to reproduce the distinctive v8 sound, you just need the pulses to fire in 90-45-90-135 degree sequences. On a subaru, though the firing order is 90-90-90-90, due to the unequal length of the pipes it sounds like it fires at 90-68-90-112, however if you double the length of the long pipes, you double the delay time, and it would most definitely bring you in or around the range of 90-45-90-135 degree rotational pulses. I've tried these angles on a wooden dowel with screws powered by a drill hitting a jug and re-produced each sound almost exactly.



I would like you all to hear this for yourself, Listen to the progression from Equal length pipes, to unequal length pipes, to a conventional 350 with a single open header.

Equal Length Pipes on a Subaru sound

Unequal Length pipes on a subaru sound

350 v8 single bank open header