Tri-Y headers use a more elaborate...
Tri-Y headers use a more elaborate collector design that pairs different tubes together to create a stronger scavenging effect. The headers are more expensive, but in the right situation they are worth it.
Collector design is important because it can greatly affect scavenging. When the exhaust valve opens at the end of the power stroke, the exhaust flow isn't a steady stream out of the exhaust port and into the header tube. Instead, it is a strong pulse of high-pressure gas. Right behind that high-pressure pulse is a pulse of low pressure. It is this low pressure pulse that can help pull additional exhaust gasses out of the port ahead of the piston rising. The measure of how well an engine removes burnt exhaust gasses and replaces it with a fresh air/fuel charge is called its volumetric efficiency (VE). At low rpms, it is easy for an engine to have a VE very close to 100 percent, but as the rpms rise, it becomes more difficult to get complete cylinder filling in most engines and VE drops. But a good header design with a collector that best matches the engine package can significantly improve VE. In fact, in NASCAR Sprint Cup engines that have a well-matched intake and exhaust system, VE can actually be up over 100 percent. That's impossible in our engine and unlikely in all other race engines where rules limit carburetor size and the types of cylinder heads allowed, but good collector design can help improve the VE in any engine.
The trick in Tri-Y headers is that because the tubes are matched up in pairs, the exhaust pulse can pull more strongly on the mating tube, having a better scavenging effect in that cylinder. This means that if the collector lengths are right for that engine package and the tube pairings match up well with the firing order, Tri-Y headers can produce improved power over similar four-into-one headers. But if the package isn't optimized things can go bad quickly. In fact, Schoenfeld Headers specifically says that these Tri-Y headers we are testing do not work well on an engine with a 4-7 cam swap.
Note the difference in the...
Note the difference in the torque curve. We were surprised when the straight tubes performed better than the stepped ones.
With our package, at least, Schoenfeld's Tri-Y design worked wonders. Peak torque was 18 better than our baseline at 404.1 lb-ft and horsepower was 2.7 better at 419.1. Average torque jumped all the way up to 386.98 (an improvement of 11.62 across the range) and power was up to 388.91 (11.18 better). The Tri-Y headers, in fact, produced the best average torque and horsepower numbers of any header combination we tested. This definitely is the way to go with our engine.
Conclusion Here comes the tough part. Unlike the previous tests where we were able to separate oil pan and rocker arm performance from the rest of the engine package, header performance is critically dependent upon the design and performance of the rest of your intake and exhaust system. For example, had we used a single-plane intake manifold instead of our Weiand dual-plane intake, the results could have been completely different. The same holds true if we had used a different set of cylinder heads, camshaft, and even the length of the exhaust pipe after the header collectors.
With our particular engine...
With our particular engine combination, the data shows that Tri-Y headers are definitely the way to go.
So, while we cannot unequivocally say that Tri-Y headers are the best design for your race engine, we can say that small changes in the way a set of headers are constructed can have a significant effect on your engine's performance. In fact, finding the set of the headers that best compliments your engine package can significantly improve power.
And, as NTI instructor Rick Touchette points out. Any power gains made by improving your header's performance is "free power." The power isn't made by bolting on ultra-light (but weaker) rocker arms that may break unexpectedly or lighter oil pressure that may require you to go through more frequent rebuilds. This power is made by helping pull more fuel into the combustion chambers, and as long as your rods and pistons are strong enough to withstand the extra horsepower generated, additional power causes no more stress on your race engine.