While it's not easy to modify an existing intake manifold because it requires specialized aluminum welding, this section will discuss what to look out for when choosing an aftermarket intake manifold, or having your existing intake manifold modified. The intake manifold actually consists of two distinct parts that can be tuned separately. The two parts are:
- the plenum, which is an air chamber that distributes air to the various cylinders
- the intake manifold runners, which connect each cylinder to the plenum
The main purpose of the plenum is to equalize the air flow to the various cylinders, but its volume and shape, as well as the shape of the bell mouths, which are the opening to the runners, are also important. Generally, a plenum volume of approximately 80% of engine capacity for naturally aspirated engines to 150% of engine capacity for turbocharged engines works best.
In terms of function, the best plenum design would have the air duct feed the center of the plenum. Unfortunately, due to space limitations and production costs, manufacturers tend to build plenums that are fed from one end, with the plenum blocked off at the other end. This results in air rushing to the far end of the plenum and creates a slight imbalance of air flow to the individual cylinders are the air will tend to flow past the first cylinder and collect at the far end of the cylinder, which is usually at the last cylinder. Consequently, the first cylinder will run slightly lean while the last cylinder will run slightly rich.
The easiest solution to this problem is to fit a second throttle body to the far end of the plenum and fit a double air filter, intake system. But this only works on naturally aspirated engines that have sufficient space for a second intake system. On supercharged and turbocharged engines this solution is not feasible and you would need to modify the plenum or fit an aftermarket intake manifold with a more efficient air flow and air distribution design. There are three things you must consider when modifying the plenum or selecting an aftermarket intake manifold.
- First, the plenum should increase in size rapidly well before the first cylinder.
- Second, the plenum can taper towards the end from after the first cylinder but it should not taper to less than 1½ times the diameter of the intake runners.
- Third, the plenum should extend well beyond the last cylinder.
As is the case with the primary exhaust pipes, the diameter and length of the intake manifold runners influence the power curve of the engine. The intake runner diameter influences the point at which peak power is reached while the intake runner length will influence the amount of power available at high and low RPM.
A larger diameter intake runner, relative to the diameter of the intake valve, will result in improved engine breathing at high RPM and will take peak engine power to a higher RPM but will have little low RPM power. This may be good for a modified race car or a drag car, but will not be good for a turbocharged car with a large turbocharger. For a good responsive modified street car or a rally car you would want an intake manifold runner diameter that is approximately 80% the size of the intake valve diameter on a two-valve cylinder, or the same size as the intake valve diameter on a four-valve cylinder, as this will produce better port velocity and more intake inertia. For a high performance modified race car or a drag race car you would want an intake manifold runner diameter that is approximately 90% the size of the intake valve diameter on a two-valve cylinder, or approximately 110% the size of the intake valve diameter on a four-valve cylinder.
In terms of intake manifold runner length, a longer intake runner produces better torque at low RPM, while a shorter intake runner produces better power at high PRM. Generally, an intake manifold runner that is in the region of 200-300 mm long will sustain power at high RPM but little power at low RPM while an intake manifold runner that is in the region of 300-400 mm long will start building power from low RPM but will run out of power soon after peak power is reached. But note that the intake port on the cylinder head forms part of the intake runner. Thus the intake runner length is measured from the intake valve seat to the intake runner bell mouth, and not from the end of the intake manifold.
As with tuning your exhaust system, the intake manifold is best tuned on a dynometer, but it is much more difficult to make changes to the intake manifold so starting from a good base is best.