Rob, we don't need any formulas for a basic understanding.
So what you are saying is that the pressure of the exhaust gases increases as does the
volume?
Not exactly. The volume itself causes that 'more' gas that has to pass the pipe.
More gas at the same time > increased speed > more drag and more work for
the engine to spill the gases out.
Why are aftermarked downpipes better? They are bigger. Larger diameter means
slower gas speed. Irritated? More diameter means that the same amount of gas
passes easier and less forced > lower speed required to let the same amount of
gas passing through it.
Even more - this is irony of fate - larger diameter means (much) larger surface and
larger surface provides increased cooling. That's (one reason) why Volkswagen
uses smaller diameters: they want to force cat heat up after start.
With a tyre, there is no where for the gases to escape to so its going to gain pressure in a
different way to an exhaust with an open end. So surely the higher pressure will force the
exhaust gases out faster?
Alex, my example was just to explain that heating up gas (air) will gain an icrease in
volume (and pressure). It doesn't matter if there's something closed or if there's leak.
