[Lee]

For giggles, here is a comparison of two exhaust systems as tested on my '73 T/A clone. At the time of this testing, the motor had "66" Dport heads (stock other than springs) on a 455, giving 7.8:1 cr. HEI, Qjet, Performer, mild cam, and a set of Tri-Y headers.

First test was with the exhaust that came on the car. 2.5" pipe to 3" FlowMasters (not sure of the model), into 2.25" tailpipes. Rather ugly junctions where the sizes were mated, all crush bent. Carb tuned to give ~12.8:1 AFR at WOT.

Planning on future upgrades, I went a bit overkill (for the motor at that time, at least) on the replacement exhaust.

The 2nd exhaust was 3" right off the headers, H-pipe, into 3" Borla ProXS mufflers, then into 3" mandrel tailpipes (thanks to Steve Coombes!). The pipes from headers to mufflers were crush-bent, but the guy has 40+ experience and does a pretty decent job (see photo). I was shocked that the AFR leaned out noticeably, yet the power was still better than with the old system. After a couple of metering rod changes, I got the results shown on the graph.

If only looking at peaks, the new system gained 14hp/9tq at the wheels. But notice the old system nosed over at 4,400rpm, while the new system extended the powerband. By 4700rpm, the new system was 21hp/25tq higher.