Intake

Intake manifold design, short runner, long runner, individual runners and throttle plates

Of all the areas involved in engine design, intake manifold design is often one of the most difficult. A good intake manifold must provide adequate flow characteristics during both high speed and low speed operation, must help to mix the fuel adequately with the incoming air, and must meet physical packaging limitations. It is these simultaneous requirements that can make a good all around intake manifold difficult to design. Furthermore, an intake manifold needs to be optimized based on a variety of engine variables, the most significant of which is the camshaft. Similar to the exhaust system discussion, the intake manifold can be designed for a variety of purposes. An intake manifold designed for primarily high rpm operation will have long intake runners, or the length from the throttle plate to the intake valve. These long runners are best suited to high airflow rates, and allow the best flow numbers at high operating speed. However, these same characteristics inhibit intake velocity and cylinder filling (Volumetric Efficiency) at lower engine speeds. Of course, the opposite holds true as well, an intake manifold with short runners will have excellent flow and mixing capabilities at low rpm, with a minimal vacuum signal from the piston, but will be restrictive to flowing at high rpm and high air speeds. This increased resistance to flow will inhibit high rpm power production.

There are some fundamental differences between the intake system and the exhaust system in terms of optimizing efficiency and power production. In a multi-cylinder engine, the intake ports are typically fed from a common intake manifold. It is the job of this manifold to direct air to the appropriate cylinder’s intake port. However, in (for instance) a V8 engine, that requires that one air inlet source (throttle body or carburetor) feed all 8 cylinders. Since each piston is drawing air in at a different time that is only separated by a fraction of a second, to some extent each piston sucks some air from every other piston. With all of the cylinders fighting for their own air, there is sure to be some loss in effectiveness for the manifold to deliver air to each cylinder. One way to combat this is by having individual throttle plates to feed some of the cylinders, or the ultimate solution is to have one throttle plate per cylinder.

This method is most commonly used on 2 stroke outboard motors, but there are some production cars that utilize it as well. Although the advantages are greater with a carbureted engine that benefits from a stronger and uninterrupted vacuum signal from each cylinder to ensure proper fuel metering, it is occasionally used on modern fuel injected engines. One such instance is the 2009 BMW M3 4.0L V8, which utilizes individual throttle. plates for each of its 8 cylinders. The benefits include improved low rpm throttle response and improved upper rpm power. The primary drawback is the added cost and complexity due to the required number of throttle bodies, and the required linkage to open them all at precisely the same rate.

Modern intake manifold designs are heavily tested and refined with sophisticated computer modeling programs to provide the best balance between efficiency, peak power, and power production across the entire engine operating range.