I know this is an older thread but I would like to add some info. About 30 year ago Motorhome life changed their tech writer. I don't remember his name but he had a small shop in LA and a dyno. His recommendation was to have a 3 inch dual exhaust system with crossover pipe that was also 3 inches. I wish I had saved the info on the correct muffler but it was a school bus muffler. very inexpensive with good flow and low noise. We had a Vogue with a 440 and followed his advice. The M600 chassis gives lots of ground clearance and I was able to build the exhaust pipes using straight pipe. very simple. The bonus was that this system with Hedmann headers reduced the heat in the engine compartment and eliminated the dreaded vapor lock!!!

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Originally posted by Moisheh:
I know this is an older thread but I would like to add some info. About 30 year ago Motorhome life changed their tech writer. I don't remember his name but he had a small shop in LA and a dyno.

That was John Geraghty. I was a regular customer. My money there was always well spent. He had a chassis dyno that would accept motor homes.

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His recommendation was to have a 3 inch dual exhaust system with crossover pipe that was also 3 inches. I wish I had saved the info on the correct muffler but it was a school bus muffler. very inexpensive with good flow and low noise.

With 2 1/2 inch Thorleys, I chose to stay with a 2 1/2 system all the way back. I favored the tri-wye design for RVs and tow cars, anyway. My memory is that he recommended 2 1/4 or 2 1/2 as the ideal pipe diameter. I have the info, but it is in AZ, and I am in CA right now. I will try to remember to check it and post here when I get back there and find it.

Bill: Thank you for remembering his name. I have been trying for 10 years!!! I have been around a long time! I also remember being at an FMCA convention in the early 80's. GM had some engineers that gave seminars for owners of the P30 chassis. They gave away that GM chassis manual to every seminar attendee. They were very smart guys! I was impressed. They mostly came from the School bus division. Didn't you work for GM?

Moisheh

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Originally posted by Moisheh:
Bill: Thank you for remembering his name. I have been trying for 10 years!!!

Didn't you work for GM?

Moisheh

Nope. That was Jake. RIP