Quote:
Originally Posted by Vman455
My old car had a straight-through aftermarket exhaust on it when I bought it; I put up with it for a few years and then replaced it with stock. Until someone can point to credible evidence that a straight-through design will increase fuel economy, I'll stick with stock and enjoy my quiet car  |
They don't. In fact they (and single fat pipes) can seriously damage economy. The only benefit they give is at maxpower/rpm (Wide open throttle)
Otto engines need back pressure at low speeds or they run badly/stall. You'e be better off working out what rpms the engine runs and and tuning the extract pipes so the valve opening impulse is reflected back to the exhaust port out of phase, for the next opening at the most common speed (It provides a suction effect). That's what tuned exhausts on 2 strokes do.
You can tune for multiple engine speeds by using paralleled varying lengths, but space and weight become issues.
As the Prius uses Miller cycle (effectively a short stroked inlet stroke via extremely late inlet valve closure), even that might badly affect the economy. This is the kind of thing which requires decent computational power and a lot of dyno testing to verify any changes.
I'm a little surprised that Toyota isn't taking an interest in what hypermilers do in order to roll changes that work into their halo cars.