Headers make more power three ways; by using gas column inertia pulse tuning to help evacuate the cylinder during the exhaust stroke, acoustic pressure wave resonance tuning to create a low pressure reflected wave rarefaction pulse during the overlap period and finally through a reduction in back pressure. If the first two reasons sound like a passage from “Hooked on Physics”, check out this simplified analogy. Some say a well built engine’s sound is music to the ears. Well this really is true; basically a header is a glorified musical instrument. A header is tuned in much the same way that an organ pipe is tuned but in this case the header is tuned to produce the right conditions in the cylinder to make power instead of a specific note on a musical scale.
|This is the header that takes the place of the exhaust manifold with the close coupled cat we illustrated on the prior page. You can see it is much better from a flow and tuning perspective. Sure enough it makes around 10 more wheel hp over the manifold alone.|
The optimal length for the header's primary tube is one that has a fundamental note corresponding to the degree point in crank rotation of the 4 stroke cycle when the exhaust valve opens. At this point the piston is traveling downwards and is near the bottom of the stroke. When the exhaust valve opens, a high pressure pulse of hot expanding exhaust gas travels down the exhaust port at approximately 300-350 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a period of suction or a low pressure rarefaction behind the main wave of the pulse. This first negative pressure wave helps evacuate the cylinder of burnt exhaust as the piston nears TDC and slows down.
|The primary reason why headers make more power isn't just the increase in flow, it is because they are tuned like a musical instrument. These 350Z headers almost look like a musical instrument.|
Depending on the engine, the pulse can have a positive pressure of around 70-90 psi at the valve when the exhaust valve opens and anywhere from 5 to 15 psi at the end of the primary tube with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure. This low pressure rarefaction follows several milliseconds behind the initial high pressure pulse. You want to have the low pressure rarefaction be in the area of the exhaust port as the piston approaches Top Dead Center(TDC) on the exhaust stroke to help evacuate the cylinder of burnt exhaust gasses as the pumping action out of the cylinder from the piston traveling upward decreases. The primary pipe's length and diameter controls this as the speed of the pulse is fairly constant and the length and diameter govern at which rpm this phenomenon works the best, hence the analogy of tuning like a musical instrument. A header, just like an organ, is tuned by the dimensions of its pipes. This is how the first power producing phenomenon works.
|On late model cars with close coupled cats, short runner headers are popular because they typically can use the stock catalytic converters in the stock location. These are about the only headers that have a chance of getting CARB certified for California street use. Short runner headers usually make power over a narrower range, typically more high rpm power because the tuning is compromised for fitment. On some cars they can still make a huge difference in power over stock.|
The second reason headers make power is by pulling as much fresh fuel/air mixture into the cylinder as possible during the overlap period. The overlap period is in between the end of the exhaust stroke and the beginning of the intake stroke in a 4 stroke engine, where both the intake and exhaust valves are open at the same time for a few degrees of crankshaft rotation as the piston travels around TDC. Engine designers use overlap to help the engine breathe better in the mid and high rpm range. Overlap helps blow the last bits of stale residual exhaust gas out of the cylinder while allowing an inrush of fresh fuel and air mixture. Less burnt gas and more fresh fuel and air mixture mean more power. It is best if the low level vacuum or rarefaction can be maintained past the initial low pressure front in the primary pipe to help scavenge the cylinder of burnt exhaust gas and pull in a fresh charge of fuel air mixture during overlap. A well designed header uses acoustic energy to maintain low pressure near the exhaust valve during the overlap period.
|Headers help turbo cars just as much if not more so than NA cars. The thing is a that proportionally, the header does not help anywhere near as much as the turbo does so people tend to forget about them for turbo applications. A header on a turbo car can also do an extra positive, decrease spool time! Here is the Full Race header on our Project Evo IX.|