On a 2000 Mazda MPV (V6), what happens if only the pipes are increased by 1/4 and maintaining stock muffler? Will there be any sort of increase in performance, if any?
Yes there will be a difference. Take the length of the pipe now the volume or the circumference that the pipe will be increased by, you're talking about more room for the exhaust fumes to travel in, just with a 1/4 inch increase.
There is a lot more to exhaust system design than just pipe diameter and muffler flow. Naturally aspirated engines create exhaust pulses. Exhaust systems must be tuned to allow these pulses to help pull the exhaust out of the cylinders. They are tuned to work the best over a large RPM and load range. Increasing pipe diameter *may* increase HP or torque over a small part of the RPM band, but usually at the risk of losing power in other parts. This is where the old idea of back-pressure more torque. Opening up the exhaust often moves the power band up higher in the RPM range, resulting in lower torque in the lower RPM's.
