possibley a great modifcation to HHO vehicles!

[Creativity]

huh i reread my post and it seems it may give some bad impression that backpressure is a widely desired effect.
It is not!Especially in small every day cars an exhaust system is the first place to look for economy gain (with power as a nice present).
After installing of a less restrictive exhaust a figures of  8% fuel savings are not uncommon on a 1.2 liter engines with +5HP at the high end of rpm's.
Simple fact that most of the road cars have short exhaust times(non-race type cars) makes it clear that the benefits of backpressure in the first phase of the exhaust are very small and only in the lowest rpm range.For the remaining part of the rpm spectrum,backpressure steals economy and power.
When the car get modified cam's with longer exhaust times(more of a racer type),the backpressure can recuperate some of the lost power at a bit wider low rpm's range,yet stealing hp at higher rpm's.
Where we speak about racer types,here comes the EXUP as a good response for the needs of an engine(but variable valve timing is even better).

Further one more thing about the backpressure curve.It is proportional to the square of the engine airflow.