is there a hp diffrence with tailpipes vs. turndowns

[Ron1]

During a chassis Dyno session last year, one of the things measured was exhaust back pressure. With a full exhaust system in place it measured 0.018. The object (as far as I understand it)is to get as low a number as possible. I have had a number of people tell me that a little bit of back pressure is good, but I have yet to find an explanation as to why it is good. Let me qoute from an article on exhaust technologies.."During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger."
And from an article by Dr.Gas on back pressure.."Re-synchronizing the exhaust pulses reduces back pressure, cancels sound, and provides additional scavaging." again talking about improving power by reducing back pressure.

Ron