I will clarify that zero or no back pressure as in my case. With my specific comment, it was personally related and motor specific. Perhaps I should have clarified.

We spent better part of a weekend on a test and tune. Not that I wanted to keep a substandard 400 in my track car, we went through everything..from timing to the carb. I had 4 Pontiac guys, with probably 70 years combined working on this car. Two of them consistantly run in the 11's and Pete runs in the low 9's, so there is no question that the motor was tuned potentially.

I have had maybe 2 or 3 different exhausts in my 67. Am I an exhaust expert as in Jim Hand? no. Do I have the money and sponsorship to try 20 different systems? no as well..

I changed two things on the car..switching the motor to 3" and changing the mufflers from 3" to 2.5. The motor ran better with the mufflers installed. These mufflers were slowing John's car down and sped my car up. Unless I am totally missing something untechnical, restrictive mufflers, car ran better.

A less restrictive exhaust is great for a motor that needs it. I do not think a 300 HP motor, as in my case, needed or required 3" exhaust.

Without getting technical, a 428 lets say has heads which flow a max of 300 cfm per intake. That is potentially 2400 cfm total. What exhaust requirements are needed for this motor? Lets assume correct carb, cam, headers and intake.

Then you have a stock 400 which usually runs in the 170 cfm per intake for 1360 cfm potential. Lets assume the same as above regarding the cam, intake etc.

The exhaust requirements are dictated by complete potential of the motor at "X" lift, rpm, shift points...etc.

By allowing a stock motor free room to expel, there is no consequence to performance without changing any motor parameters?

Sorry if I fragment..I am between cases at work and needed a breather.