First, the critical distance is from your exhaust port on your engine to the beginning of the expansion chamber, the muffler has nothing to do with this. Our pipes are designed to work at around 12 inches plus or minus an inch or so for tuning. Altering it 4 inches, shorter, or 12 inches longer, making the 24 inches I noted earlier, will be detrimental to the proper tuning of the pipe. It is all about timing and the speed of sound. 24 inches doesn't make if harder for the sound waves to travel it just takes longer which is why it destroys the tune of the pipe. For the sake of example, if you went to 24 inches. The timing of the sound pulse would be delayed to the point that your exhaust port would be closed by the piston when the sound wave is supposed to close it. This will defeat the purpose of the sound wave closing the exhaust port when it is open keeping the unburnt intake charge from leaving the combustion chamber creating a better more powerful combustion. So say you go with the 8 inches you are looking at. At 8 inches the pulse would hit the exhaust port too early and do approximately the same thing. A 1/4 inch difference in header length can make a difference in power band so I hope this explains that 4 inches from the recommended 12 inches will be well passed the effective length to make the most power out of your engine. Will it run better than a stock pipe? Yes, but are you really taking advantage of what a tuned pipe is supposed to do? No