2.5 Inch exhaust to small?

[FETorino]

Hot gas needs more room than cool gas. Go 3" to the mufflers for sure. The restriction from the muffler back created by the 2.5" pipe won't be that bad. Of course if your 3" muffler's cfm rating (if it exists) is less that 750 each then that is your cork anyway.

From one exhaust article I read and saved a while back.

Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent.

Knowing that 2.2 cfm per open-pipe hp means zero loss from backpressure allows us to determine how much muffler flow your engine needs. Just make a reasonable estimate of its open exhaust power potential and multiply by 2.2. For instance, a V-8 making 700 horsepower on open exhaust will require 700 x 2.2 = 1540 cfm.

A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (10.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe. Houston we have a problem.



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