Here is an important thing to remember in any form of racing, including circle track, every track is a horsepower track. You can never go wrong with adding horsepower, since you can always use your right foot to dial in the power, but you can never push down any farther if you don’t have enough.
The one complaint drivers always have is not enough power.
In our quest for power, however, we often look for the big peak numbers, as those are what sell engines. Obviously it is better to have 350 hp than 300 hp, but there is much more to the performance of an engine on the race track than simple peak values. After all, we don’t race dynos, we race cars, and those cars have to accelerate up and down, again and again, running through a very specific portion of the power curve. Success on the track (especially circle track) depends less on the peak power a motor makes and more on the average power production over the usable range.
This header test was designed to illustrate how you can greatly improve acceleration without significantly altering the peak power output of your motor.
How is that possible?
Well, headers provide much more than just exhaust flow. They actually provide a tuning effect that bolsters power production in specific rpm ranges. Much like an intake manifold, the design criteria of the headers determine the effective operating range.
Alter things like primary tubing diameter and length, collector size and style and even the length of the collector extensions, and you can significantly alter power production of the motor. More importantly, you can alter power production in specific rpm ranges, where it might be most useful.
In the case of this Street Stock motor, the desire was to improve power production to help get the car off the corner and motoring down the straight. The header test shows that choosing the right header for the combination achieved just that.
For many racers, headers fall into the same category as cam timing and induction theory, meaning they are one of the least understood aspects of the entire exhaust system. Most racers understand exhaust, as in more flow can equate to more power, but unlike mufflers and exhaust tubing, headers do much more than just flow exhaust.
Proper headers actually provide a tuning effect on the power band. This tuning effect is not unlike that offered by the design of the intake manifold on the induction side. The power gains offered by true, long-tube headers come not so much from absolute flow, but rather from the scavenging effect offered by things like the primary (and collector) length. Though this scavenging effect helps get exhaust out of the combustion chamber, it also helps improve airflow into the chamber during overlap.
The timing of this scavenging effect – basically at what rpm it is most effective – is a function of things like the length and diameter of the primary tubing and collector along with the length of the collector extension. Dial in these design criteria to your combination and you can maximize power production.
To illustrate the gains offered by changes in header design, we set up a test on a mild 350 Chevy Street-Stock motor. The small block featured Vortec heads, a COMP circle track cam and guided roller-tip rockers. The motor was equipped with a dual-plane, Eliminator intake from Speedmaster, which we ran both with a 2-barrel Holley, 4412XP carb (using a Wilson adapter) and a 650 XP 4-barrel carb. To illustrate that the changes in the power curve were not specific to the 2-barrel combination, we also ran the back-to-back header test with the 4-barrel.
The two headers we elected to test differed in both primary tubing and collector diameter.
The first sprint car style headers tested, featured 1 ¾-inch primary tubing and 3 ½-inch collectors. Both headers were run with collector extensions that extended their respective collector sizes. The 1 ¾-inch headers were tested against a set of 1 5/8-inch chassis headers from Speedmaster. The 1 5/8-inch primary tubing was combined with 3-inch collectors feeding collector extensions. We often see a trade off in power, as the larger headers make more peak but often lose power down low, but such was not the case in this test.
For our first test, we configured the small block with the 500-cfm Holley 4412XP carburetor and 2-4-barrel adapter from Wilson Manifolds. The motor was run first with the 1 ¾-inch sprint-car style headers and 3.5-inch collector extensions. Equipped as such, the 350 produced 382 hp and 399 lb-ft of torque. After installation of the 1 5/8-inch headers from Speedmaster and 3-inch collector extensions, the peak numbers stood at 383 hp and 409 lb-ft of torque. The peak power output changed very little, but the smaller headers offered significant torque gains, from 3,000 rpm all the way to 5,400 rpm. Obviously the combination responded well to the smaller headers, as the torque output changed by over 30 lb-ft.
To illustrate that these gains were not specific to the 2-barrel combination, we ran the same test after installation of a Holley 650 XP carburetor. Run with the 1 ¾-inch headers, the 4-barrel 350 produced 418 hp and 421 lb-ft of torque. Run with the 1 5/8-inch headers, the 350 produced 417 lb-ft and 424 lb-ft. Once again, the smaller headers significantly improved torque production down low, with no penalty in peak power production up top, meaning more average power production where it is needed most. More average power equals better acceleration, which in turns equals more wins.
Boy, this testing stuff is exhausting!
The results of the header test were somewhat surprising, as the smaller 1 5/8-inch headers not only improved torque production down low, but lost no power at the top of the rev range.
Having extra torque production can really help get the car off the corner, as acceleration is less about peak, and more about average power production. The right headers for the combination will offer the highest average power production over the usable rpm range. The 1 5/8-inch headers from Speedmaster improved torque production significantly from 3,000 rpm all the way to 5,400 rpm.
To illustrate that the gains offered by header design were not specific to the 2-barrel, small-block combination, we also ran the motor with a Holley 650 XP 4-barrel.
The differences in the power curves were similar to the 2-barrel combination, as the smaller 1 5/8-inch Speedmaster headers improved power production down low with no penalty in peak power production.
Having an extra 30-35 lb-ft of torque will really help get the car up to speed.
Comp Cams, www.compcams.com
Wilson Manifolds, wilsonmanifolds.net