Backpressures of around 0.6 psi ahead of the cat at full power are regarded as acceptable by our engine people (they aren't happy about it). I think about 1/3 of that is in the cat, and 2/3 in the mufflers. Most of the muffler loss is required to get good subjective acoustic performance, rather than meeting the noise regs.

Cheers

Greg Locock

Hai Greglocock,
  Thanks.

  I have concern about the reply. When I converted the 0.6psi in to bar, it came around 0.04bar(I have taken it as absolute pressure).The atomospheric pressure itself is approximately 1 bar. How the pressure in exhaust muffler can be less than atomspheric pressure. If it is less it may act as suction.
  Please excuse me, If i understood things wrongly and then asking a question in completely wrong way.

  I can't get cat. What is that cat?


With Regards,
Amit

I'm pretty sure Greg meant 0.6 psi gauge, which is atmospheric plus 0.6 psi or 15.3 absolute.

Blacksmith

Hai blacksmith,

 Thanks for the clarification.

Cheers,
amit

The "cat" he's talking about is the Catalytic Converter,. a series of ionizing screens that convert gases. Mandated for all cars to meet emissions regulations.