All the headers for this test...
All the headers for this test were supplied by Schoenfeld Headers. Our baseline was a 13/4- to 17/8-inch stepped design with unequal-length primaries and a four-into-one collector. It produced better numbers than our old 15/8 headers, but Schoenfeld had designs that proved even better.
Finally, for the third test we switched to part number 1024VY. This is Schoenfeld's Tri-Y collector design. And again, both the baseline and test headers had stepped, unequal-length primaries.
The Engine During each round of dyno tests, we've improved our dyno mule slightly. It began as a stock Chevy crate long block with a cast crank, cast pistons, 8.50:1 compression and four-bolt main caps. It was outfitted with a Weiand Action Plus dual-plane intake, a Holley 650 cfm four-barrel carb, a one-inch open carb spacer, and a CV products water pump and pulley set. We kept the seven quart Champ Pans competition pan from that test and averaged 355 pounds of torque at 3,600 rpm and 275 horsepower at 4,400 rpm.
The next test involved swapping out different rocker designs by Comp Cams. In order to better stress the rockers, we ditched the stock cast-iron cylinder heads for a pair of as-cast heads from RHS that featured 64cc chambers, 2.020 intake valves, 1.600 exhaust valves, and 200cc intake runners which flowed significantly better. They were assembled with beehive springs that tested around 180 pounds on the seat. We also went with Comp for a solid roller camshaft that gave this engine quite a bit more snort than the mild stock cam. It is ground with 0.400 lobe lift (for 0.600 total lift when matched with our 1.5:1 rockers), and the solid roller lifters are also from Comp. We tested six different rocker arms during that two-day session, and for this go-round we kept the Comp Cams steel Pro Magnum 1.5:1 rockers which produced 401.7 pounds of torque at 4,600 rpm and 401.1 horsepower at 5,700 rpm because they are most typical of what can be found on a high-end race motor.
The straight-tube headers...
The straight-tube headers had a tube diameter of 13/4 inches all the way to the collectors.
Because we are striving for the most consistency possible from one dyno pull to the next, we didn't tune the engine for maximum power. Timing was set at 38 degrees with the advance locked out. The Holley carburetor was set up with No. 70 jets at all four corners, and seven quarts of synthetic racing oil was used for lubrication. Although different headers may perform better with different amounts of jetting in the carburetor or different timing settings, those factors did not change throughout the tests. Also, when each pull was made, lead instructor Doug Wolfe made sure that the water and oil temperatures were as consistent as possible. Our targets for the beginning of each pull were 190 degrees F for the oil and 180 for the water. For each header, we made two back-to-back pulls to make sure we were getting consistent numbers. We've printed the output numbers for the second of each of the pulls for this article, starting on p. 78.
The Baseline In previous tests we used a borrowed set of headers because NTI had them handy and they fit the dyno. We're not even sure of the brand, but they used a 15/8-inch primary with no steps. Using those headers with Comp's Pro Magnum rockers (the same used throughout these tests), peak power was 401.7 lb-ft of torque at 4,600 rpm and 401.1 hp at 5,700 rpm. Since our last test, NTI had also recalibrated its dyno and pulling back up the old tests with the new calibration, the peak numbers for both horsepower and torque increased by two (403.7 and 403.1, respectively).
After giving Doug Schoenfeld our engine's specifics, he chose a larger, 13/4-inch primary tube for our headers, and the choice was a good one. Our new baseline headers dropped a bit in peak torque with 386.1 at 4,600 rpm, but it significantly opened up the torque curve. Torque stayed higher in the upper rpm ranges which bumped peak horsepower up to 416.4 at 5,900 rpm. An improvement of 13.3 ponies! For each test we also recorded average torque and horsepower numbers in an rpm range most racers typically see in short-track racing. Our rpm range was between 4,300 and 6,300, and for our baseline headers the averages were 375.36 lb-ft of torque and 377.73 hp.