When comparing performance exhaust systems it's a good thing to understand the different exhaust systems on the market today and the performance demands of your engine. Understanding how a header works will help you decide on which exhaust system is best suited for application. Headers have to basic functions.
First is to simply relieve back pressure.( Super charged engines especially do not like back pressure. In many instances we see horsepower increases in the 80 to 100 range and a reduction in boost pressure.) For example, when viewing a Mercury manifold you first notice a big 4" exhaust hose. But the actual exhaust gas opening has about a 2 1/2" inside diameter. Most performance manifolds use a 4" exhaust hose and a 3" or 3 1/2" exhaust gas tube. All of our headers use a 4" diameter exhaust gas tube and a 5" diameter water jacket tube. At the end of the header the exhaust gas tube terminates and water is introduced into the exhaust stream. The 5" diameter water jacket tube is then reduced to 4" diameter for standard hose coupling. At no time does the exhaust gas go through a hole smaller than 4" diameter. When looking at these exhaust gas openings consider their area not their diameter. For example a 4" diameter tube has an area of approximately 12.56 square inches compared to 2 1/2" diameter tube with an area of approximately 4.90 square inches. That's 60% smaller! Know wonder why we see horsepower increases in the 80 to 100 range.
The second function is scavenging. In other words the sucking out of spent exhaust gases. With high performance long duration cam shafts the exhaust valve during the overlap cycle remains open as much as 50 degrees after top dead center while the piston is starting the intake stroke. Camshaft designers rely on the header to pull the remaining spent exhaust gases out. The header accomplishes this by using the inertia or momentum from the previous exhaust pulse to draw out the ones behind it. That's why long tube headers work better than short tube headers. They isolate the pulse to maintain a high velocity and create a low pressure behind it. To test this take a header and blow air from an air hose into any port, take a piece of paper and place it over any of the three remaining ports a note the suction that is created by the discharging air.
HEADER PERFORMANCE
Performance figures on headers very considerably. As a rule, the higher horsepower engines benefit more than lower horsepower engines. For example the 330 horsepower Mercury engine will gain approximately 75 to 100 RPM, where as the 365 horsepower Mercury engine will gain approximately 300 to 400 RPM (45 HP). Blown engines gain up to 800 RPM (80 to 100 HP).
When comparing our headers against other performance exhaust systems recent dynamometer test have shown the following using a 460 CI big block Chevrolet, 9 to 1 compression ratio, solid roller camshaft ( 255 @ .050 duration, 110 lobe separation angle, 680 lift), dual plane intake manifold and 850 Holley carburetor. Results taken at 5500 RPM.