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ananthkamath · 4th November 2005, 09:13

Chethan's right except for the backpressure part. The Backpressure theory is like an old wives' tale among hot rodders.

Exhaust velocity is a much more accurate measure of how well the system evacuates the cylinder. Engine output at a given RPM is solely a function of BMEP (Brake Mean Eff. Pressure) and therefore is dependent on how much charge can be burnt. The exhaust aids in "supercharging" the cylinder by creating a negative pressure thereby sucking fresh charge and evacuating the spent charge in larger amounts and shorter times.

However it is not possible to harness the full effect at all engine speeds because the exhaust length is specific to a certain engine speed. Header tube diameter must therefore be optimised so as to maintain good velocity at low speeds, and flow enough gas at target speed.

The normal 4-2-1 headers are designed to harness wave phenomena in the exhaust tracts. Meaning that one cylinder is "supercharged" by either bouncing the wave off of a closed exhaust valve of the other cylinder, or by using negative pressure from the exhaust flow of the other cylinder.

Therefore it is imperative that cylinders 1&4 and 2&3 (or 1&3 and 2&4 in some cases) be joined into a single pipe. So max, that answers your question.

4th November 2005, 09:13	#4 (permalink)
ananthkamath BHPian Join Date: Jul 2005 Location: Detroit, Michigan Posts: 999	Chethan's right except for the backpressure part. The Backpressure theory is like an old wives' tale among hot rodders. Exhaust velocity is a much more accurate measure of how well the system evacuates the cylinder. Engine output at a given RPM is solely a function of BMEP (Brake Mean Eff. Pressure) and therefore is dependent on how much charge can be burnt. The exhaust aids in "supercharging" the cylinder by creating a negative pressure thereby sucking fresh charge and evacuating the spent charge in larger amounts and shorter times. However it is not possible to harness the full effect at all engine speeds because the exhaust length is specific to a certain engine speed. Header tube diameter must therefore be optimised so as to maintain good velocity at low speeds, and flow enough gas at target speed. The normal 4-2-1 headers are designed to harness wave phenomena in the exhaust tracts. Meaning that one cylinder is "supercharged" by either bouncing the wave off of a closed exhaust valve of the other cylinder, or by using negative pressure from the exhaust flow of the other cylinder. Therefore it is imperative that cylinders 1&4 and 2&3 (or 1&3 and 2&4 in some cases) be joined into a single pipe. So max, that answers your question. __________________ Miata Turbo Is GO!!