A lot of people look at exhaust mods as one of the first modifications to their 4 strokes. Usually for power, sometimes just anti-socially for more noise.

Given that exhausts are tuned for any engine at some stage in the original design at least, and measured and tested, I wonder how many of the modifications people rate after time has passed as a true improvement?

I can see that removing a cat convertor or other restrictions may improve exhaust flow but wouldn't this be at the loss of either bottom or top-end power depending on the original design? If the loss is of the bottom-end power which most dirt-bike types are aimed at then it may improve top end a bit. Is this actually the case, does one necessarily suggest the other? I know that shortening the pipe from header back is said to improve top-end power but I do not want to shorten pipe!

What effect does an overall increase in pipe diameter have?

I know also that a megaphone type, or even a funnel type tip is said to improve final exhaust speed. But this goes against what I know of venturi effects from restricting flow!

Is the 'bomb' type bulge helpful on anything but a properly tuned and emission measured exhaust system? I see them on 4 strokes now when they used to be just on performance 2 strokes.

Beyond subjective imagination and noise levels which only kids really want to make louder, has anyone really rated an after-market exhaust? I am especially interested in 200cc.

So if any engineers or really experienced modders and tuners read this - please explain!