Exhaust pipe diameters are not guessed but determined. Piping of too large a diameter can cause gasses to exit too quickly and it was found to warp exhaust valves in some instances!What is the diameter of the exhaust pipe on the coupe? Is it 3 inch, 4 inch? I am sure a larger diameter will help to relieve back pressure so I am wondering what it already has versus what it should have... Or a better question is what is the optimal size? Is the Larini a different size from stock?
Anyone here know?
The rule of thumb was that the ID of the pipe never exceeded the OD of the largest valve in the cylinder head.
You are not going to relieve pressure just by going to larger diameter pipe. You have to look at the internal construction of the resonators and mufflers in the system, and then you have to balance the system collectively.
I have never designed a system with tubing larger than @ 50 mm which is right around 2" and that included systems with large 12 cylinder engines.
Look at race car photos from the 60's and 70's and see the tuned exhausts with tubing about 1.5" diameter!