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pretty much all you're going to need to know:
Function
The basic function of the intake manifold is to get the air form the carb 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 obvious, 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
You 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 a fuel are mixture flow 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 carb has a sharp turn. The air flows straight down through the carb 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. 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 your doing, don't polish the intake ports.
Tuned Ports
Most people will think of fuel injection when you say tuned port, but it 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 example's 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 it helps, which is why a dual plane single 4 barrel manifold can give a wide power band, but less peak power.
Manifold Heat
Most production manifolds will have some sort of exhaust or coolant passage in it to heat the intake. This helps fuel atomization, but hurts power. Cooler air is denser and denser air makes more power. Any kind of performance engine should not use manifold heat.
Venturi
An hour glass shape in a carb that cause the air to increase velocity as it passes through the narrower section. As velocity increases, pressure decreases. This is how a carburetor flows fuel. The pressure in the venturi will be lower than the pressure in the fuel bowl, so the higher pressure will push fuel through the carb.
Booster Venturi
This is where the fuel enters the venturi and it is fact another smaller venturi it self. It's main purpose is to further increase the speed of the air and in turn lower it's pressure even more to gain more signal strength. The are many kinds of booster venturi, the ones that give the best signal strength and atomization are usually the most restrictive to airflow.
Signal Strength
This directly related to venturi size, shape, booster venturi, and air speed though the carb. The signal strength is how much the venturi can reduce pressure. A large venturi will have less signal strength than a smaller one, but will also flow more air. If the venturi is too big, it will have a hard time metering fuel at low rpm, if it is too small, it will be a restriction at high rpm.
Dry Flow Intake
With fuel and air traveling through the intake, sharp corners are a bad thing as velocity increases. Air is lighter that fuel and can take sharper turns. As an air fuel mixture goes around a sharp turn, the fuel separates and flows along the outside of the turn. Getting intake runners long enough to help low to mid range torque is hard to do with limited hood clearance. Multi-port fuel injection lets us inject fuel right at the intake port of the head, which leaves the rest of the manifold flowing only air. By doing this, we can have some sharper bends. Air still flows better in a straight line, but not having fuel separation is a big plus. The GM TPI manifold is a good example of a dry flow manifold. There is no fuel in the runners until right before the heads. The runners come out of the plenum and cross to the opposite side of the engine, making them long enough to help low-end and still give hood clearance.
Wet Flow Intake
They flow air and fuel of course. Carburetors and throttle body injection are wet flow systems. The intake runner shape is much more cridical because it must minimize fuel drop out.
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96 talon esi-t
san clemente, ca
as needles of ice
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