Here is my dyno testing on a WAY milder motor.
First exhaust system (blue lines) consisted of 2.5" pipe to 3" FlowMasters, into 2.25" tailpipes. All pipe was compression bent.
2nd exhaust (red lines) was all 3". Compression bent from headers to mufflers, and with a 2.5" "H" pipe. Tailpipes are mandrel bent 3" (a gift from Steve C.). Mufflers are 3" Borla ProXS.
Impossible to say if the pipe size or the mufflers were responsible for the gains, or what % is attributable to each. Note that the gains increased as RPM increased - upper rpm gains were more substantial than those at "peak".
I had to install much thinner metering rods with the new exhaust, to correct the AFR.
7.8:1 compression 455 with unported pressed-stud heads, smallish cam, HEI, Q-jet.
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'73 T/A (clone). Low budget stock headed 8.3:1 455, 222/242 116lsa .443/.435 cam. FAST Sportsman EFI, 315rwhp/385rwtq on 87 octane. 13.12 @103.2, 1.91 60'.
'67 Firebird [sold], ; 11.27 @ 119.61, 7.167 @ 96.07, with UD 280/280 (108LSA/ 109 ICL)solid cam. [1.537, 7.233 @93.61, 11.46 @ 115.4 w/ old UD 288/296 108 hydraulic cam] Feb '05 HPP, home-ported "16" D-ports, dished pistons (pump gas only), 3.42 gears, 275/60 DR's, 750DP, T2, full exhaust
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