After each test, NTI's students swarmed the engine to make the header swap and get things
Stepped versus Straight Tubes In racing applications, stepped-tube headers are preferred because they can help improve scavenging and broaden the torque curve. That's what we were looking for here, and this is also where we got our first surprise.
In our tests the straight-tube headers with a consistent 13/4-inch diameter outperformed our 13/4 to 17/8 stepped-tube baseline headers all around. Peak torque was 394.0 at 4,600 rpm and horsepower was 419.3 and 6,000 rpm. Those are improvements of 7.9 lb-ft of torque and 2.9 hp. Average numbers also improved to 381.5 for torque (an improvement 6.14) and 383.71 for horsepower (an improvement of 5.98).
The most likely reason for this improvement is that the 13/4 to 17/8 combination is just slightly too large for this engine package and it responded better to the slightly smaller average diameter of the 13/4-inch straight-tube pipes. It definitely was scavenging better in the upper rpm range. By looking at the brake specific fuel consumption (bsfc), which is the pounds of fuel the engine burns in one hour to produce one horsepower, you can see how efficiently an engine is working. More horsepower at a lower bsfc means that the engine is burning more of the available fuel droplets during the combustion cycle. But when the combustion chamber does not change, as in our case, a higher bsfc usually equates to more horsepower. At peak rpm the straight-tube headers produced a bsfc of 0.538 while the baseline at the same rpm was 0.535. It is a small amount, but it does show that the straight tube headers were scavenging better in the upper-rpm range and allowing more fuel to be pulled into the combustion chambers to be burned on the power stroke. Also notice that just when the dyno pull ended at 6,500 rpm the stepped-tube headers were taking over with more power. If this engine were able to breathe a little better and rev higher, the stepped-tube headers would begin stacking up better.
There wasn't much difference between Schoenfeld's equal-length design (shown here) and the
Unequal- versus Equal-Length Primaries Unequal-length headers are popular in classes where the available traction is limited. The thinking is that unequal-length tubes, while they can create less peak power than equal-length tubes, spread out the torque curve and help make a race car more driveable.
In our second test we pitted a set of Schoenfeld's equal-length headers against our unequal-length baseline set. To be honest, however, the difference between the two is minor. But the results on the dyno were noteworthy. At the low end of the range, the unequal-length baseline headers were better, but the equal-length pipes started producing more power at 4,400 rpm. After that, the numbers were higher for the equal-length pipes all the way through the rest of the pull. Peak torque moved up a notch and showed up at 4,700 rpm (389.3 lb-ft) and horsepower peak also moved 100 rpm higher where it was 419.1 at 6,000 rpm. Surprisingly, both peak numbers were better than our baseline-3.2 and 2.7, respectively-and the average numbers were also better.
Four-Into-One versus Tri-Y Collectors Our third and final test moved away from the primary tubes and pitted the two most popular collector designs against each other. Four-into-one collectors-where all four primary tubes on each side dump into the same collector at the same spot-have been the standard for years, but Tri-Y collectors-which are a set of three two-into-one collectors arranged in sequence-are gaining ground. They were developed by high-end race teams, and their added complexity usually makes Tri-Y headers cost a little bit more, but they are catching on among Saturday night race teams.