Posted: July 30, 2011 in Induction Systems


     The basic function of the intake manifold is to get the air from the carburetor or throttle body directed into the intake ports. It may seem like a simple thing, but what really goes on inside is quite complex. The design of the intake manifold will have a significant effect on how the engine runs.

Air Flow

     Getting air into an engine is the key to making power and there are many ways to increase the air flow into the engine, some are obvious and some are not. Other than forced induction and nitrous, there are 3 ways to increase air flow. The first is better port and valve shapes to improve flow. The second and less realized is harnessing the inertia of the airflow to better fill the cylinders. This is why cams keep the valves open past TDC and BDC. If all the induction parts are matched to the same rpm range air can continue to fill the cylinder even as the piston begins to move upward. This is due to the speed of the intake charge giving it inertia to resist reverse flow, to a point. The third and not known to many people is induction wave tuning, this is related to inertia tuning, but is more complex and harder to tune to a specific rpm range.

Porting Goals

     Your goal with any port modifications should be to get as much flow and velocity as you can with as little restriction as you can. When working on a flow bench, pay close attention to how much metal you remove and how much the port flows. If you have a 100 cc port that flows 100 CFM, then you modify the port by grinding 5 cc’s of metal away and the port now flows 110 CFM, you gained flow and velocity (a good thing for a street engine). If your modified port flows 103 CFM, you gained a little flow, but lost velocity. You will need to cc the ports often and measure flow often to get good results. If you don’t have access to a flow bench, it’s best to remove as little metal as possible. Most pocket porting jobs give very good  results when less than 5 cc’s of metal is removed. More than that, you need a flow bench to see if what you’re doing is helping or hurting.

Port Shape

     Any sharp edges or corners make a restriction to airflow. Air is light, but it does have mass and will flow better if it does not have to negotiate sharp corners and around obstacles. With a wet flow manifold (fuel flows through the manifold as well), sharp turns in the manifold will cause fuel separation at higher rpm. Fuel is heavier than air, so when the air/fuel mixture flows around a corner, the heavier fuel will not be able to turn as good as the lighter air. If you look at a basic 4 barrel intake manifold, the area directly under carburetor has a sharp turn. The air flows straight down through the carburetor as then has to take an almost 90° turn to get to the cylinders. At high rpm the fuel has a hard time staying mixed with the air and puddles on the port floor. Another thing that causes fuel separation is low velocity. This is especially a problem with large ports at low rpm, the lower the velocity is, the more time the fuel has to drop out of suspension. Fuel is heavier than air, so the longer it has to separate, the more it will. Getting high velocity is very easy, but getting it without making a restriction is a little more difficult. You need large ports to flow well at high rpm, but large ports will decrease velocity at low rpm.

Port Polishing

     Polishing the intake ports can show slight improvements in air flow, but can hurt power. A rough texture will make some turbulence at the port walls. Fuel has a tendency to run along the port walls, especially on the outside of turns and the floor. A rough texture will help keep the fuel suspended in the air. Unless you really know what you’re doing, don’t polish the intake ports.

Tuned Ports

     Most people will think of fuel injection when you say tuned port, but it has nothing to do with fuel, it’s all about air. The idea of tuning the ports to an rpm range has been around even before fuel injection. When a port is “tuned”, it takes advantage of the induction waves to increase volumetric efficiency of the engine in the desired rpm range. Induction wave tuning only helps over a narrow rpm rage, but it is very effective. Tunnel rams or individual runner manifolds are good examples of tuned ports. All manifolds will have each runner tuned to a specific rpm range, but most will have runners with different lengths. When you change the length of the runner, you change the rpm range that the intake is designed for, which is why a dual plane single 4 barrel manifold can give a wide power band, but less peak power.

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