[nbpro]

Intake manifolds direct the air and fuel mixture to each cylinder in the engine. They have a direct impact on the performance of the engine. The amount of horsepower an engine produces and at what RPM is dependent on how fast air and fuel can be moved through the engine, how much and how fast the air can be brought in and how fast it can be burned and exhausted from the engine.

There are many intake manifold configurations. Each of these have different characteristics as to the RPM at which they're most efficient, the torque range at which they're most proficient, the amount of air they can move and how well they dissipate heat. In order to efficiently increase power in an engine, this whole scenario must be met. Moving air and fuel through the engine means exactly that. What comes in must go out. Many performance shops promote their products in terms of increased horsepower their product will generate. They don't say, however, that this is only in conjunction with these other components. A higher-performance intake with the same carburetor or fuel injection won't do much in the way of performance. The fuel hasn't been increased to make use of the high-performance intake, the camshaft is only going to open the valves for the same amount to allow the same amount of fuel in, and the exhaust is only going to let the same amount out.

There two styles of manifolds, both with many variations as to runner length, amount of air it will flow, how many carbs can be used, how large a throttle body can be used, the ability to be drilled for port fuel injectors, how high it is and how well it dissipates heat--just to mention a few. The two styles are a dual plenum and an open plenum. The dual plenum has a wall inside that separates the primary and secondary sections of a carb on a carbureted engine. The dual plenum gives higher vacuum off idle and better fuel flow for low- to medium-horsepower engines. This will give better low-end torque for better acceleration from a stop. However, this type of intake manifold is not good for high RPM. The open plenum is better for high RPM, from 3,000 to whatever limits are on the engine and the particular manifold. On a fuel-injected engine, the manifold--regardless of design--should match the mass air flow sensor and throttle body.
Conclusion
High-performance intake manifolds are designed to create more vacuum to suck more fuel from a carburetor if the carb has enough capacity. High-performance intake manifolds don't do much for fuel injection at normal RPMs. By itself, an intake manifold can produce marginal performance upgrades at high RPM, and in some cases increase the torque at low RPM with no further modification