[Aussie Mike]

I've built a super quiet under body system for mine. No problems with starting and it seems to rev OK. I've got my own noise meter and it registers 87dB at about 4K RPM.

There's a bit of a theory I followed when I built it.

According to Boyle's Law The volume of a fixed mass of gas is inversely proportional to the pressure at a constant temperature PxV=K. That basically means that if you increas the temperature of a gas it's volume increases. If there's no room for it to expand then it's pressure increases.

So with that in mind the exhaust gasses are hottest when they leave the combustion chamber and cool as they make their way rheough the system before exiting at the exhaust tip. So with Boyle's law in mind the exhaust gasses are at their larges volume (size not noise) as they leave the motor. The larger the pipe we used the lower the gas pressure will be. We are trying to quieten the system down so it makes sense to try and slow the gas down.

To make mine run quiet I used 2.5" pipes out of the factory manifolds into the cat converters. I used 2 hi flo cats as these can be quite restrictive. I want to muffle not restrict the system. From the Cats I reduced the pipes size to 2" as the gasses are cooling and taking up less volume. These two 2" pipes run for about a meter before going into a Y and coming out as a single 2" pipe. This pipe then runs into a HQ Holden V8 oval section mufler and then on into another 2" round muffler.

Using small pipes and mufflers to quieten will increase the gas pressure and therefore the velocity giving that high pitched whistle.

I'll be in Sydney in a couple of weeks. I can throw my noise meter in my bag and bring it up and you can check your system.

Cheers