If you hang out at any racetrack, car show, or dyno shop for long enough, you’ll hear all kinds of stories. Old wives tales about this thing or that thing making big power, cars that guys used to own, and tuning tricks seem to be at the top of the list. Well this weeks BangShift Tech story was pulled directly from one of those old wives tales meets tuning trick. Each and every one of us that has been around racing or hot rods for very long has heard the theory that the correct exhaust collector length is key to making power, and that you can figure out that perfect length by painting a stripe along the length of your collectors, and then cut them where the color of the paint changes from getting hot. This is supposed to be the perfect length, due to heat concentrations and vortechs, or flux capacitation, or something like that. Well, we’re here to tell you it’s not.
Our testing began at Westech Performance Group where we bolted our venerable Dart SHP 372 to the dyno for a day of thrashing. You’ll remember this engine from several of our other tests, although in this configuration we had a smaller, 650 cfm, Holley Street HP carburetor, and Hooker Super Competition 1 3/4 inch Camaro headers instead of the normal dyno headers. Once we were ready to run, we sprayed a stripe of white paint down the sie of our Hooker 18″ x 3″ collector extensions and proceeded to run the engine for a few minutes to let the paint “bake on”. We then ran the engine through three baseline pulls to see if we could get the paint to change color and show us where to cut our collectors for optimum length and therefore optimum power. Baseline testing had us at 520 horsepower and 473 pound feet of torque.
The color changing paint trick showed us that we needed to cut 2 inches off the end of each of our collectors, as you’ll see in the video below. So after a quick trip to the chop saw, we bolted the collectors back on and were ready to make another set of pulls. Three pulls later, and we could see that the 2 inches had made some impact to our power curve, but nothing dramatic. At 521 horsepower and 474 pound feet of torque, we were up so little it’s hard to say it wasn’t dyno variation. Below peak torque we saw a slight dip, but again, not enough to claim the cut was a horrible idea…yet.
Since the “optimal” collector length didn’t seem to make a difference in our little 372’s power curve, we decided we needed to make a more drastic change in order to make something happen. To make the collector half as long as the delivered 18″ length, we cut 7″ off of our already 2″ shorter collectors. The collectors were bolted back up and testing resumed. The 9″ collector length made a much more dramatic change and confirmed that the small torque dip in our previous test was in fact a sign of a trend with the shorter collector. With the 9″ collector we saw a serious dip in the horsepower and torque curves between 3100 and 4500 rpm.
For a final kicker, we yanked the collector extensions off all together and went with the headers only. To say this a bad idea for your hot rod or racecar would be a serious understatement. Just like the other tests, peak power was virtually unchanged, but below peak torque we lost a lot of power. More than even we expected. Between 3600 and 4300 rpm we lost 50 lb ft. or more and nearly 30 horsepower. The strange thing is that we saw almost 50 pound feet of torque INCREASE at 3000 rpm. It would need to make 10 times that to make up for the rest of the power loss.
Check out the video below for our conclusions!