Exhaust size

I used the calculator here to evaluate not only what the engine might need but what the pipe I had would like. The first box is optimized for engine size and RPM while the second box is more generalized and considers engine HP.

First is what the engine would like @3600 RPMs which is 60MPH in the car and the upper end of speeds I routinely travel. As you can see the 1" ID conduit I chose is still over sized because it recommends 1" OD with 0.0625 wall thickness (16 Ga), which would be 7/8" ID.

Second is what RPM level the 1.75" ID pipe would support with the same size engine. This is about 10,000 RPMs higher than I ever get to.

Third is the HP level that the 1.75" ID pipe would support. This is about 100 HP more than I have.

Finally the engine displacement that the 1.75" pipe would be optimized for at 3600 RPMs. The engine is a 1.0 liter 3 cylinder, I could run 4 of them through that size pipe for what I am doing.

So you can see I've been running a pipe that was 3 to 4+ times too large for my engine and operating conditions. I don't have any numbers yet but the Instantaneous FE reading on my MPGuino are at least as good as or slightly better with the smaller pipe, so I certainly haven't lost anything.