As we've mentioned in our introduction to building exhaust systems, the exhaust manifold design, exhaust header design has a major affect on engine performance, and plays an important role in car tuning. In terms of exhaust header design, both the primary pipe diameter and primary pipe length will determine the engine's overall power band as well as its peak power point. But, before you begin with your exhaust header design, you need to take into account the number of cylinders, the engine capacity, and the maximum usable RPM as these will all influence your design.
The primary pipe diameter and the primary pipe length of exhaust manifold are the two important factors in the exhaust header design. To a large extent, engine capacity, the number of cylinders and the maximum usable RPM will influence primary pipe diameter and primary pipe length. Thus, a 1600cc 4-cylinder or a 2400cc 6-cylinder naturally aspirated modified street car with a maximum usable RPM of 5,500 RPM, should have an exhaust manifold with a primary pipe diameter of about 1? inch and a primary pipe length of 34-36 inches, while a 2000cc 4-cylinder naturally aspirated modified race car should have an exhaust manifold with a primary pipe diameter of about 1? inch and a primary pipe length of about 32 inches that feeds into a 2? inch collector. Ideally, the primary pipe lengths should have the same lengths but can be within 2 inches of each other. On a four-cylinder engine, the and all four primary pipes should join together in a single collector before feeding into the front pipe of the exhaust system, while on a six-cylinder engine, the primary exhaust pipes available from cylinders 1, 2, and 3 should join into one collector and the primary pipes from cylinders 4, 5, and 6 should join into another collector. A Y-pipe could then be used to join the two collectors before feeding into the front pipe.
The best exhaust header design for all round race performance would have 1? inch primary pipes that are 32 inches long. These dimensions provide the best power curve over the widest RPM range and would be ideal for rally cars. An exhaust manifold with longer primary pipes would provide better top-end power but with less pulling power, while an exhaust manifold with shorter primary pipes would provide better low-end torque. On a turbocharged engine, an exhaust manifold with short primary pipes will help with acceleration until boost pressure is reached and the turbocharger spools up. Thus, you should have the exhaust manifold tuned to your specific needs during the design stage.
In our page on cylinder head porting, Henry (aka Double H) explains that the primary pipes in the exhaust manifold should at match the exhaust port diameter on the cylinder head; but to reduce reversion, a primary pipe that is slightly larger than the exhaust port is better. Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder. Any exhaust gases that are drawn into the combustion chamber will displace the air/fuel mixture being drawn in through the intakes valves and will increase the temperature in the combustion chamber, thus reducing the volumetric efficiency of the engine, as well as engine power. Preventing reversion will reduce the contamination of the air/fuel mixture by the spent exhaust gasses and will improve the efficiency of the engine. An anti-reversion header or AR header that is specifically designed to inhibit reversion would be your best choice. Anti-reversion headers have a built-in lip that restricts exhaust gas flow back into the combustion chamber.
In our guide to exhaust header design, we did not mention log-type headers as these headers are always less effective than exhaust headers with equal length primary pipes that joint together in a collector. However, on a turbocharged engine, you may not have enough space for an equal length header and a turbocharger. This space limitation would necessitate the use of a log-type header. In addition, the primary pipes of the exhaust manifold must come together at the collector before it feeds into the turbocharger and the size of the collector will be determined by the size of the turbocharger's turbine inlet.
In some cases you may even need to retain the stock cast-iron exhaust manifold. If this is the case, you should examine the stock exhaust manifold closely for imperfections that could restrict exhaust gas flow, much the same as you would do when porting the cylinder head. The aim would be to make the internal surface of the exhaust manifold as smooth as possible while keeping the shape and size of the primary pipes as uniform as possible, without weakening the manifold. By smoothing down the internal surface, you would not only improve exhaust gas flow, which would be crucial to reducing turbo lag, but you'd also reduce carbon build-up. Remember, however, that widening the primary pipes would reduce exhaust gas velocity and would result in a thinner manifold wall, both of which would have a negative effect on turbo lag! A thinner manifold wall would have greater exhaust heat loss, which would mean a reduction in the heat energy that is used to drive the turbine.
When designing your own turbo exhaust header, you need to ensure that your header is strong enough to support the weight of the turbocharger, and that is can withstand the heat buildup caused by the turbocharger. This means that you have a choice of two materials when designing the exhaust header: steam pipe and bends, or stainless steel tubing. Stainless steel tubing may be easier to bend and shape, and would require less welding and grinding but you should ensure that the bends are formed in a mandrel bender that does not deform the inner radius of the bends.