For that RPM I believe that 18-19" would work. However, you can always fab up the elbow and the tube seperately, test a length of pipe, clamp it down and see if it worked. If not, trim the pipe down, reclamp and test, kind of like tuning a tuba. Then whatever works, weld it up permanently.
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2013 Ford Mustang GT
374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd
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