Put your hat down and wet your whistle pardner, because this story's about a bunch of hot air. We all know the airflow in and out of an engine is what the horsepower game is all about. Taller intake manifolds, larger cylinder heads, and radical camshafts are all players that generally increase the amount of air an engine will move. However, without the help of a properly flowing exhaust system, the engine's potential power will fall short. Big-tube headers are capable of moving large volumes of air with minimal restriction, but bigger isn't always better. A high-velocity small-tube design can be much better at promoting a strong power curve. To test this, we'll compare four popular small-block headers with different primary pipe sizes on a stout street/strip crate engine.
Our crate mule is a Smeding Performance-built 383ci small-block that has proved itself worthy during several CHP dyno-test sessions. Smeding conservatively rates it at 440 hp and 440 lb-ft of torque right out of the box. We tested this engine last month in our "Size Matters" carburetor shootout and it produced well beyond its advertised power ratings. This header shootout called for intermediate-length small-tube Flowtech headers measuring 1 ½ inches in diameter, as well as three full-length large-tube Hooker headers measuring 1 5/8, 1 ¾, and 1 7/8 inches. We would have used a long-tube header design for the 1 ½-inch tube diameter size, but no off-the-shelf piece was available.
Before we begin, let's review the important aspects of header design and the theories behind them. When mass-producing a product of this nature, it's nearly impossible to ensure an exact manufacturing tolerance from one piece to the next. The fact that header companies are able to pre-manufacture pieces that will fit on a variety of cars is impressive, but if you want more, investigate a custom-built set.
A mass-produced shelf header typically starts at $99; the custom-built header usually begins around $1,000. The builder of a properly tuned header must account for the many engine variables responsible for altering the power curve, e.g., camshaft timing, compression ratio, engine size and speed, intake design, and so on. Engine exhaust airflow tuning correlates to header-tube design, for which the most critical factors are the length and diameter of the tube and the size of the collector. All of these variables will shift the engine's power curve around peak-torque rpm, at which point the exhaust gas moves fastest.
The first dimension to consider is the diameter of the tubing. A large-diameter header tube is capable of moving huge volumes of air to promote peaky torque numbers and horsepower numbers at the upper end. A small-diameter header tube increases the exhaust-gas velocity (air speed) to help scavenge the exhaust port efficiently at low engine speeds. This promotes a strong low- to mid-range torque curve but has a tendency to restrict top-end horsepower potential once the exhaust port begins to move more air than the tube diameter is capable of discharging. The balance is to choose a tube diameter that will provide the best power curve and acceptable peak numbers.
Length plays an equally important role in tuning the power curve, as does the diameter of the tubing. Long-tube designs (typically 30-plus-inches) create a strong exhaust-gas pulse signal throughout the pipe that enhances low- to mid-range torque and horsepower. On the other hand, as the tubes become shorter, the tuning effect has less time to enhance the engine's low-speed benefits. A short header tube will allow the low- to mid-range power to fall off while increasing the upper-end potential of the engine. Many companies offer headers in intermediate and short-tube lengths mainly for clearance reasons, but these same applications can be used to alter the power curve of the engine.
Once tube diameter and length are balanced to deliver the best power curve, collector size becomes the next concern. Collectors join several primary tubes into one mutual area where all the gases are forced to combine and vacate the exhaust system. A long collector tricks the primary tubes into thinking that they are longer than they actually are. This is a way to lower the power curve to produce more bottom-end torque and horsepower without having to sacrifice top-end power. When 1 7/8-inch-or-larger primaries are ordered, it's common for them to come without collectors, because engines using tubes this big are typically intended for upper-rpm operation. So before ordering headers, the engine builder should always consider the engine's operating range and choose tubing dimensions accordingly. What you want is usable power. A dedicated drag-race engine takes little advantage of its low-rpm power, but a road-race engine lives or dies by it. We, however, want a motor that runs strong across the entire power curve, and that's why we found out which header would make the best overall power on our Smeding-built 383.