I always remember my Dad talking about 'Back pressure' on engines.

Just a thought though, in your research Keithy2, was that rebound in the exhaust study with Catalitic converters in place or not? Would they also effect things before the pulse hits the baffle plate?

It would also be interesting to know how engines would be peforming now if manufactures didn't have to comply to all sorts of legislation but instead produce the motors to run how is best for them.