Muscle Mustangs & Fast Fords
Ford Aluminum 5.4L Header Swap - All-American Header Test Part 2
The Supercharged Sequel
Having run our header test in the last issue on the normally aspirated version of the GT1000 motor, we were anxious to retest the motor with some boost. You will remember that the 5.4L GT1000 was built to officially reach the 1,000hp mark in supercharged trim--no easy feat, but one simplified by creating a much stronger normally aspirated combination to which boost would then be applied.
Knowing that the best supercharged combinations start out as powerful normally aspirated combinations, the 5.4L was built using an all-aluminum Ford GT block, augmented with forged internals. Wanting to maximize power, the static compression was purposely increased over the traditional 8:1 numbers run on the factory GT500 and Ford GT motor. The 10:1 short-block was topped with a set of CNC-ported GT500 heads and custom Comp cams. It was the cam timing that played havoc with our header test run on the normally aspirated combination, allowing us to improve the power output by over 100 hp with the installation of a set of American Racing headers. Would the power gains be even greater on the supercharged combination?
In part one, we covered two scavenging effects offered by long-tube headers. Using both the kinetic energy of outgoing gases and reflected pressure wave, (properly designed) long-tube headers provide low pressure in the combustion chamber during overlap. This helps both evacuate the residual exhaust gases, while simultaneously drawing in the intake charge.
Of course, these negative pressure waves must be timed properly to arrive at the combustion chamber when the piston is just past TDC on the end of the exhaust stroke. Short primary tubes (like those used on most factory modular exhaust manifolds) don't provide sufficient length for proper timing of the pressure waves, thus they offer no beneficial scavenging effect. In essence, they are simple flow devices that provide an escape route for the exhaust gases. In this way, the power potential of the stock exhaust manifolds is determined by their ultimate flow rate. By comparison, long-tube headers offered additional power not through additional flow capacity (though this can be the case as well), but primarily by artificially enhancing the breathing of the motor. Think of long-tube headers as a super extractor, sort of a mini supercharger for the exhaust system.
The scavenging effect offered by headers is similar to the charge filling offered by long-runner intake manifolds. Know that our designations of long-runner intakes and long-tube headers should more accurately be designated optimized length, as the intake runner and exhaust primary lengths are tuned for the specific combination. We use the long-runner designation here only to differentiate them from short-runner intakes (like those used with the supercharger) and stock exhaust manifolds. The difference between the charge filling offered by optimized intake runners and the scavenging offered by long-tube headers is that the pressure waves are happening in reverse. On the intake side, the opening of the intake valve and downward movement of the piston creates a strong negative pressure wave that propagates outward to the plenum and is returned as a positive pressure wave to enhance cylinder filling. On the exhaust side, the open exhaust valve sends out a positive pressure wave that is returned as a negative pressure wave to help draw out residual exhaust gases and draw in the intake charge. Not surprisingly, the two pressure waves must work together to optimize power production, meaning the intake manifold and header must be sized to operate effectively in the same rpm range. This tuning effect of both the intake manifold and exhaust is offered despite the presence of boost. Boost from a supercharger or turbocharger does not minimize the effect of the ram effect offered by the intake, nor does it alter the scavenging effect offered by long-tube headers.