Yeah I hear ya, I still stand by 2 1/2 inch exhaust for street strip 300-400 cubic inch engines. Flathead maybe 2 1/4. If you got something wild bump up to 3 inch. For a car that is driven there's so many tradeoffs. If ya got a ride that has a set powerband and a ch***is that allows unlimited tube routing you can get a lot more technical and dial it in, use the science. It just doesn't apply to most of the rides here that are driven thru off idle to 6k. Do the best exhaust you can with the space you've got, size the header pipe and collector length with the space you got. I still like talking theory, what's on your mind.
It is more based on volumetric efficiency in regards to supporting a particular horsepower level, than it is about a particular cubic inch displacement. My 300-hp 200-inch six, and a 300-hp SBC 400 would need the same size exhaust system, which would be dual 2-1/4" (rounding up to common size, and to account for inefficient bends), or a single 3". A 90-hp flathead would need a single 1-3/4" system (rounding up to common size, and to account for inefficient bends).
That Harley pipe story has a lot of really good info that applies to most any 4 stroke engine. 1 snippet from it that makes huge sense, and something I learned a long time ago: "...15 PSI at the inlet and exhaust..." and then it goes on to discuss the pressure differentials at each end created by a running engine. At the end of the day there's not much real tech we can use building our exhaust systems much past fit dilemmas. Maximized for 1 range of power then other points of performance will suffer. I frankly believe in a single large diameter tube running under the car that can exit through a dual outlet muffler. Enough volume to make a broad powerband, easier to retain heat, easy to package, with the right muffler a pleasant sound with no obscenities. My last O/T DD had such a system and would routinely knock off 25 MPG at 75-80 MPH and the A/C running. I also liked the OEM sound from the crossflow muffler, having just enough noise to say "hot rod" minus the dreaded interior drone. Not romantic, nor an exhibitionist (excessive noise), but it served the car well.
I am a big fan of the single exhause, for maximum effeciency, but then again, I am also a big fan of inline sixes.
Had a 2 barrel 283 SBC and single 2-1/2" exhaust on my 37 Chevy Two Door Sedan. The car ran amazingly good.
The Jalopy Journal
