Also called four-into-one headers, this style consists of four long primary pipes feeding into a single collector. The majority of aftermarket full-length headers available range in primary length from about 27 to 32 inches. The length of the primaries makes this style the most efficient in terms of performance, for the reasons described above.
Unfortunately, long, curving primaries might be impractical--if not impossible--for a lot of hot rod applications. The best headers in the world won't work if you can't fit them in your engine compartment, so some compromise--such as shorties--is necessary.
Also called block huggers and other names, this popular style consists of four short primary pipes ending with a usually shorter collector. They were developed for applications where full-length headers won't fit, such as on a closed-hood roadster or a lowered car. Although their primary advantage is practicality, tubular shorties provide better port separation and free flow than a restrictive cast-iron manifold and are a perfectly good choice for ordinary street applications.
Since you're giving up the advantages of pipe length, it is important to make sure you select the proper diameter, to keep torque where you can use it. Also, since the primaries are shorter, proper valve timing is a little more critical than with full-length headers.
Also called dual-purpose, megaphone headers, cutout headers, or by brand names such as Sanderson's Lime Fires, these headers are styled after the headers used in dry lakes racing and are very popular on traditional fenderless hot rods. They feature primaries feeding into an outside-the-chassis flared pipe that can be run open or closed with an end plate to route gases through an under-chassis exhaust system. In addition to this option, advantages include ease of fit and longer pipes than a shorty header. And let's be honest, they look and sound great and complete the style of a nostalgic rod.
The collector is the part of the header that funnels the four primary pipes into a single exhaust pipe--or in the case of a tri-Y, the part that connects the primaries to the secondaries and the secondaries to a single pipe. However, that's not all it does. Remember those gases moving through the pipe away from the heads? As high pressure forces that gas toward an area of lower atmospheric pressure at the end of the headers, it creates an expanding vacuum in the primary pipes. At the collector, the gases suddenly hit an area of much larger volume, which causes them to slow down. At the same time, the vacuum draws a refracted wave back up the primary back toward the engine. Since all four pipes meet at the collector, a wave might move back up one or more of the other three primaries, which enhances the scavenging in the correlating cylinders. Collector size is as important as primary size, and just like with primaries, exhaust moves faster through smaller-diameter collectors. The length of the collector affects the torque curve in the same way as primary length--a longer collector improves low-rpm torque.
A few header companies have developed collectors that use the tri-Y design. Instead of joining the four primaries at a single point, these collectors run the primary pipes into two slightly larger pipes, which are then connected into a larger single pipe. The advantage is that gases are able to maintain a higher speed for a longer time in the collector. Those low-pressure pulses that create the scavenging effect are more isolated and better controlled, just as they are in tri-Y headers.
Tri-Ys consist of a four-into-two-into-one design, with four primary pipes paired into two slightly larger-diameter secondary pipes before reaching a single collector. One advantage of this design is that the increasing pipe diameter broadens the torque curve, providing higher torque throughout the low- and mid-rpm range than a similar four-into-one style header. Doug Thorley Headers specializes in tri-Ys, specifically for the RV and truck markets in which lots of low-rpm torque is important.
The principle behind tri-Y design is that the most effective port separation occurs by pairing the cylinders that are firing farthest apart, in other words 180 distributor degrees (or a complete crankshaft revolution) apart. Unfortunately, the firing order on a Chevy engine would require pairing cylinders (1 & 6, 8 & 5, 4 & 7, and 3 & 2) from opposite sides of the block. By pairing cylinders 1 & 5, 3 & 7, 2 & 4, and 6 & 8, tri-Ys come as close as possible.