Custom 4-2-1 Header

I was under the impression that in some applications, the presence of exhaust backpressure increases the low-end torque produced by the engine. I would imagine that the midrange flow would be what you're aiming for since I'm assuming that majority of the torque at low speeds is sourced from the electric motors (since max torque for an electric motor is at 0 rpm). Someone with more engine construction and tuning experience can probably do better than my speculation.

 

A properly tuned 4-2-1 system would probably reduce fuel consumption and midrange power but If the tube length was correct it would place the Cat converter farther from the engine requiring a longer warm up time for the cat. converter that might end up lowering your mileage. The end result might be better highway and worse city mileage.

 

Header design is complex and scientific. Where the collectors are placed (e.g., length of tubes from flange to primary collectors, and from primary collectors to secondary collector) is critical. Fractions of an inch make a difference. Additionally, pipe diameter helps determine the torque curve (which is why some believe tube headers reduce torque over cast iron - generally, they have bigger tubes which DOES reduce torque in the mid-range). Finally, 4-2-1 vs 4-1, and for a 4-2-1 sequential vs non-sequential (that is, whether you join 1-3 and 4-2 for sequential, or 1-2 and 3-4 for non-sequential) all must be considered.

For the basics, check out this from Wikipedia Manifold (automotive) - Wikipedia, the free encyclopedia:

"When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. With the exhaust valve open, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprising three main parts: The high-pressure head is created by the large pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse. The remaining exhaust gas forms the low-pressure tail component. This tail component may initially match ambient atmospheric pressure, but the [ame="http://en.wikipedia.org/wiki/Momentum"]momentum[/ame] of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse. Performance headers work to increase the exhaust velocity as much as possible. One technique is tuned-length primary tubes. This technique attempts to time the occurrence of each exhaust pulse, to occur one after the other in succession while still in the exhaust system. The lower pressure tail of an exhaust pulse then serves to create a greater pressure difference between the high pressure head of the next exhaust pulse, thus increasing the velocity of that exhaust pulse. In V6 and V8 engines where there is more than one exhaust bank, Y-pipes and X-pipes work on the same principle of using the low pressure component of an exhaust pulse to increase the velocity of the next exhaust pulse.
Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will create backpressure against which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds."

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Plus, if you want to get into details of 4-2-1 vs 4-1, and sequential vs non-sequential (that is, whether you join 1-3 and 4-2 for sequential, or 1-2 and 3-4 for non-sequential) check out this highly detailed discussion, with torque curves and pictures of headers at Header-Exhaust Design Effects on Engine Power - Team Integra

Like everything else in hotrodding, all of this has been worked out in detail by others for over 100 years, and there are books to help design using rules-of-thumb or even formulae. Check out "Scientific Design of Exhaust & Intake Systems (Engineering and Performance) (Paperback)" and "How to Build and Modify Intake and Exhaust Systems (Motorbooks Workshop) by B Watson (Paperback - Jan 12, 1995)."