As we've mentioned in our introduction to building exhaust systems, the exhaust manifold design, or exhaust header design has a major effect 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.
Naturally Aspirated Street Cars
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 1½ 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 1¾ inch and a primary pipe length of about 32 inches that feeds into a 2½ 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 pipes 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.
All Round Race Performance
The best exhaust header design for all round race performance would have 1⅝ 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, we explained 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.
Ultimately, determining the correct primary pipe diameter and primary length that provides the best engine characteristics and performance will require that you have your car dyno-tuned.
In our next section we'll look specifically at the design of an exhaust header for a turbocharged car.