Most Car engines are designed, taking into account the back pressure of the exhaust to make the spent gases flow at the correct rate , so that the proper inlet breathing can be attained
By modifying your exhaust system your car might not go any faster, and it might use a lot more fuel,
expelled in the form of un burnt mixture out the exhaust, and not to mention the Noise, the Smell
Old American proverb: Lots of beat, but no speed
From my V8 Days
Johan
