Article By Jim Kaekel, Jr.
It's common knowledge that one of the most cost effective methods of improving a car's performance is to install a set of free-flowing, full length exhaust headers. Headers not only increase horsepower, but improve throttle response and fuel efficiency. As it is with carburetors, camshafts or cylinder heads, bigger is not always better when selecting headers. Correctly sized headers are paramount to achieving peak performance.
Joe Beyea of Beyea Headers (Genoa, N.Y.) states that one of the most common mistakes is selecting headers with primary tubes too large for the application.
"Most people have it drilled into them that an engine is just an air pump, and that they can install the largest headers they can get," says Beyea. "In reality, they would probably be better off using a header with smaller primary tubes with a broader torque curve". An engine with larger tube headers may actually make more peak horsepower on a dyno, but at the sacrifice of low- and mid-range power, a loss particularly noticeable on a circle track car exiting a corner, or on a drag car at launch and after gear shifts.
The type of car/chassis, engine type/displacement, cylinder head/valve sizing and operating RPM range must all be taken into consideration before investigating the primary tube and collector dimensions of available headers. Dyno and track tests have proven that a header that is even an 1/8" too large will diminish low- and mid-range horsepower. The reason for this is that the primary tube diameter establishes the velocity of the exhaust gases as they exit the pipes. The tubes need to be sized so that they are big enough to not restrict the gases, but not so large that velocity is insufficient to scavenge the cylinders of all gases.
For popular engines, header catalogs often include several part numbers with varied primary tube and collector dimensions. Small, 1-1/2", 1-5/8" or 1-3/4" diameter primary tube headers are ideal for small displacement (283-383) engines, while larger, 1-7/8", 2" and 2-1/8" diameter headers work best with larger (396-454) cubic inch engines. Primary tubes with 2-1/4" and larger diameters are designed for 500+ cubic inch engines in drag race-only applications.
Once proper diameter tubes are selected, primary tube length, measured from the flange to the collector, is another important consideration. Longer primary tubes increase bottom end torque while shorter tubes increase top end power. Adjustable headers use slip-on collectors and primary tube length can be adjusted using varying lengths of slip-on tubing, usually in 2", 4" and 6" lengths. An existing set of headers may also be modified with an adjustable header kit. After the collectors are cut off, the desired length of tubing can be slipped on. Using weld-on tabs and related fasteners, the collectors can be re-installed and are now removable. Experimenting with different lengths of primary tubing during "test and tune" is now much easier.
Another primary tube variation involves "step" headers. Used by drag and circle track racers alike to extract that last bit of horsepower, stepped primary tubes progressively increase in diameter, usually in 1/8" increments, from the flange to the collector. The "stepped" tubes help increase exhaust velocity for that last bit of added horsepower. Stepped headers are typically offered in single- and double-step versions; a single step header may have 1-7/8" to 2" primary tubes. A double step primary tube header may go from 1-7/8" to 2" to 2-1/8" between flange and collector.
"Tri-Y" headers, where the primary tubes are configured per the engine's firing order prior to exit into a four-into-two-into-one collector. According to Beyea, the design uses the engine's firing order to "fatten up the torque curve", improving bottom- and mid-range horsepower when compared to traditional four-into-one headers. Because of their added cost, "Tri-Y" headers are typically seen only on high end race cars.
Recent advancement in header design has yielded the merge collector. Similar to four-into-one collectors, merge collectors have two distinct differences. A conical spear extends beyond where the primary tubes meet inside the collector to smooth the transition. A "choke" or reduction in the collector's diameter, roughly 6" before the outlet, minimally improves scavenging and is used only by racers wanting every last bit of horsepower. (30)
Header construction is also undergoing a revolution of such. Not so long ago, nearly all street or race headers were constructed from mild steel, however, stainless steel has been steadily gaining popularity. Although nearly twice the cost when compared to mild steel headers, stainless steel units offer virtual lifetime durability due to eliminated corrosion. Beyea says that due to the fact that corrosion is eliminated, stainless headers are not plagued with rust scale build-up inside the tubes and the associated loss of performance like conventional mild steel headers. Engine life is also improved since corrosion can no longer be introduced into the engine when the engine is turned-over.
Regardless of the vehicle or it's intended usage, it is highly recommended that reputable technical personnel should be consulted prior to the purchase of a set of headers.