July - August 1997 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Tubular exhaust headers are probably one of the "best buys" for your performance dollar. They deliver up to a 15% increase in horsepower over stock cast iron manifolds, and are often one of the first bolt-on items purchased by the auto performance enthusiast because of their relatively low cost and ease of installation.
All headers consist of three main components: the flange, which conforms to the exhaust port configuration of the cylinder head and actually bolts to the head itself; the primary tubes, which are welded to the flange and transport the exhaust gases; and the collector, which "gathers" the individual primary tubes into one large diameter tube.
The function of a conventional tube header is actually fairly simple, but header design is created through the application of some pretty involved principals of physics. Basically, the header's purpose is to dispose of the spent combustion gases in the most efficient and rapid manner. Street driven cars will benefit from the installation of any header, but when it comes to race cars, header design is critical to the overall performance. When exhaust valves open, gases are forced into the exhaust ports and then into the header's primary tubes. The gases began to lose velocity as they move through the primary tubes, but the gases pushed into the header by subsequent openings of the exhaust valves create a "pulse" which actually helps move the gases rapidly toward the collector.
As you might imagine, primary tube size is a very important aspect of header design. Smaller primary tubes increase velocity at low engine speeds and improve low speed torque and horsepower. Larger primary pipes yield lower exhaust gas velocity and, consequently, reduce efficiency and horsepower in the lower RPM ranges. Big primaries will work best at higher rpm's, in engines with higher compression ratios and longer durations. Keep this in mind when one of your street machine customers wants those killer competition headers for his stock "three-feefty". He just may be farther ahead with the smaller tube headers.
There are several basic header designs offered to best service the wide variety of performance and race applications.
The common bank-separated, FOUR (OR THREE)-INTO-ONE HEADER design is the most popular, particularly in drag racing. This design simply incorporates individual primary tubes from each cylinder bank to a V6 or V8 engine which join in the larger collector. This design is not nearly as sensitive to primary pipe length and much easier to fabricate.
The "180-DEGREE" HEADER design is used primarily in circle track racing applications connects some of the primary to the opposite bank of cylinders. This design creates an exhaust "pulse" in each collector every 180 degrees of the crankshaft rotation. Fabrication and installation of "180-degree" headers is much more involved than conventional headers.
An EQUAL PULSE HEADER is also used occasionally on circle track cars and joins all eight of the primary tubes into one collector. Exhaust pulses occur every 90 degrees of crankshaft rotation inside the single collector. This type of header design is often difficult to engineer and fabricate to fit the confines of most race cars, but can provide a considerable power increase over a conventional four-into-one header.
A header which has been proven to work well on street cars, as well as trucks and motorhomes is the "TRI-Y" HEADER design. Tri-Y headers divide four primary tubes per bank into two pairs of secondary tubes, each forming a "Y", roughly 1/3 the distance to the collector. These two paired tubes then combine to form the third "Y" just before entering the collector.
The "Tri-Y" design headers are not as sensitive to primary pipe length as other types of headers, but then they do not produce as much peak power as conventional four-into-one types either.
ANTI-REVERSION HEADERS are well suited to street applications and utilize a larger primary tube to increase peak horsepower. This increase is made possible through the use of an anti-reversionary cone installed in each primary tube. This cone is direction-sensitive to prevent exhaust gases from flowing back into the exhaust port of the cylinder head (a problem when too large a primary tube is selected).
Another common header design, seen in both drag and circle track cars, is the "STEP HEADER". Step header primary tubes increase in diameter in increments, or steps, and may begin with a 1-7/8" tube at the flange, increasing to 2" and then, finally to a 2-1/8" tube before entering a 3-1/2" collector. Step headers provide a happy medium for both low and high RPM operation, broadening the torque band and allowing quicker RPM recovery after upshifts or coming out of a turn.
Building ADJUSTABILITY into headers has become popular on drag cars. Extending the primary tube length will increase bottom end torque, thus reducing 60 ft. times and quarter mile E.T.'s. Adjustable headers may be quickly changed to deal with varying track conditions. A racer may want to extend the primaries on a "sticky" track for quicker E.T.'s or shorten the primaries to control wheel spin on a "loose" track.
Collectors may also be "TUNED" for the particular application and the type of racing being done. a couple of different types are available. The standard type involves four primary tubes welded into a formed collector.
Hooker offers a "Merge Collector" which features the primary tubes forming into a spear shape inside the collector, which they claim increases exhaust efficiency.
Both Flowmaster and Hooker offer Four-Into-One Collectors, which help to speed up the flow of gases within the primary tubes. Hooker and Headman also offer Collector Extensions which may be easily bolted on to alter both the torque and horsepower curves of an engine.
Appropriate header selection and tuning is an important consideration for any performance car - racing or street. Header catalogs often list several different primary tube and collector diameters for popular applications. Engine size and RPM range are probably the most significant factors determining which design is the "correct" header selection.
For example, a typical street/strip '69 Camaro with a 327 engine, which sees no more than 6000 RPM, would be well suited to a header with 1-5/8" primary tubes and a 3" collector.
If the 327 is replaced with a 454, built to run in excess of 7000 RPM, a header with 2-1/8" primary tubes and a 4" collector would be a good choice.
Track size is an important factor in circle track header selection, because a longer track will demand a larger primary tube. For a 350 Chevy engine , a 1-3/4" to 1-7/8" primary tube would be ideal for a shorter race track, while a 1-7/8" to 2-1/4" tube would work well on a longer track. The best overall collector diameter would be around 3-1/2" for either application.
Header research and development will continue to bring about new products to increase efficiency and horsepower. The new products we marvel over today will become antiquated in a few years!
When buying headers, keep in mind that the more you know about your specific application, the better you can fulfill your exhaust needs.
Have a question about what header is best for a particular application? We stock the current state-of-the-art headers, collectors and accessories, and we will be more than willing to assist you in selecting the correct header.