Bolting on a set of headers has long been one of the first moves a hot rodder makes in search of more performance. Traditionally, factory iron log exhaust manifolds are set aside in favor of a pair of tube headers, typically of the four-into-one configuration. Most enthusiasts know that the headers will provide an increase in power, generally attributing the gain to a reduction in backpressure. Sure, reduced backpressure is a factor, but the real secret to a set of headers is the tune, determined by the length and diameter of the primary tubes and collector.
We hit the dyno with a stout 427ci big-block Chevy to put a variety of Hooker header confi
At each combustion event a pulse of high-velocity exhaust gas makes its way down the primary tube, and upon reaching the collector at the end of the primary tube, a low-pressure wave is reflected back up the pipe. Now if that low-pressure wave reaches the exhaust valve while it is open during the overlap period, the pressure differential will actually scavenge the cylinder of exhaust gases. Since the intake valve is also open during overlap, the effect is to actually draw on the induction side. This scavenging effect actually aids in filling the cylinders with a greater charge for higher volumetric efficiency and, of course, more power. The cam plays a vital role here, since the overlap provides the window for the scavenging effect to occur. Obviously, a very short duration cam with virtually no overlap will represent a closed door to the exhaust header scavenging. Fortunately, most hot rodded engines are also equipped with bigger cams, which go hand in hand with headers.
Since this scavenging effect is dependent upon the low-pressure wave communicating with the cylinder through an open exhaust valve, the arrival of the low pressure wave needs to be correctly timed. To achieve this, the tube length and diameter are the main tuning variables. Luckily, a high-performance engine with a healthy cam represents a pretty big target as far as the tuning window, so the tuned length is not super critical. Even headers compromised for fit with moderately varying tube lengths will provide most of the benefit of the "perfect" tuned length.
Our first setup was a set of Hooker Block Hugger shorties with 1⅞-inch primary pipes. The
Primary tube diameter also affects the tuning of the header, but to a lesser degree than length. The primary tube needs to be of sufficient capacity for the volume of exhaust gases. There are many ways to estimate the required primary tube diameter, from simple formulas to elaborate and complex computer programs. Engine horsepower is an accurate predictor of required primary tube diameter, with the rule of thumb being a higher-powered engine will require a larger primary diameter.
Often the final header configuration is more a matter of what is available and what will fit the car than a result of number crunching theory. In the real world, cataloged header configurations are what most guys have to choose from, and the usual compromise is the bigger the header gets, the harder they are going to be to install and the more intrusive they become in the engine bay. The key piece of information that is usually missing is just how much of a difference in performance is gained or lost with a given header choice. If a 1⅞-inch header will put you within a few odd horsepower of a 2⅛-inch header, most guys would see the smaller header as the sensible choice. If on the other hand, there is 10, 20, 30, or even more horsepower dependent upon stepping up the header size, at some point it is going to be worth the effort to get the bigger pipes under the hood.