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Understanding Performance Headers
The exhaust system of an internal compression engine is an integral component for maximizing efficiency and HP. A well balanced exhaust system is tuned to suck out the cylinder exhaust and allow for an overfilled condition. Headers are generally used to produce this effect. It is not uncommon that the volumetric efficiency will be increased to 101-105%. Camshaft overlap, allowing the exhaust and intake values to both be open near top dead center (TDC) will additionally assist in moving the next combustion charged in the cylinders.
Header Dimensions
It is important have the correct header diameters and lengths for proper tuning. Balancing a high rev’ing engine with a performance cam increases the need for a properly designed exhaust system; this is typically accomplished with an engine dyno. However for most applications making a reasonable choice based on an accurate understanding will suffice.
Peak torque is typically produced with a mean exhaust gas velocity of 250ft/sec. A larger pipe diameter relative to the size of the cylinder will reduce the gas velocity unless rpm’s are increased. Therefore, the rpm required for peak torque can be moved up or down by increasing or decreasing the diameter of the header pipe.
Header pipe length will also modify the peak torque curve. Shorter pipes increase high speed power whereas longer pipes increase low speed power. Regardless of the pipe length, the peak HP is still fixed at a given RPM by the pip diameter.
As an example, a 2.5L 4 cylinder engine with competition cams should have a primary tube diameter of 1.875in – 2.125in with a secondary pipe diameter of 2.0in – 2.5in for a 4-2-1 header system. The primary pipe length would be approximately 20in to 30in long and the secondary pipe length would be 8in to 18in long. For a 4-1 header, the pipe diameter for the same engine should be 1.875in - 2.25in by 29in to 38in long. The diameters would be reduced by approximately .125in to .25in and the length should be 8in to 14in shorter.
Unequal Lengths
In order to avoid a sudden rush of power while achieving a more balanced torque curve, unequal primary pipes are often used. This is especially useful for street tuning when an even torque curve is needed throughout a wide RPM range. A good rule of thumb would be to add 4in and subtract 2in from the ideal tuned length.
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