INTAKE SYSTEM

THE AIR INTAKE SYSTEM

by Double H

A cold-air induction system.

The whole point of performance tuning and engine modifications, whether it involves supercharging, turbocharging, NOS, or gas flowing the cylinder head is to improve the efficiency of the air flow in and out of the engine. This process is often described as improving engine breathing. The main aim in improving engine breathing, is to identify restrictions that impede the air flow in and out the engine, eliminate these restrictions and improve air flow, or use a pump to force more air into the engine. Our sections on Turbochargers, Superchargers and NOS dealt with forced induction and getting more air into the engine, while our section on Exhaust Systems dealt with getting air efficiently out of the engine through the correctly tuning and designing the exhaust system.

In this section we'll discuss the air intake side of the engine and ways in which we can improve the efficiency of the air flow into the engine. We'll look specifically at improving the efficiency of the air filter and air filter box, sizing and positioning of the mass air-flow sensor and the throttle body, improving the inlet ducting, cold air induction, how to tune the intake manifold runners and how to design the intake manifold. The information discussed here is meant for naturally aspirated engines but it may also be applied to engines that use forced induction systems. Let's begin with the air filter ...

THE AIR FILTER AND THE AIR BOX

One of the first things most people think of when considering air-flow restrictions is the air filter, but in most cases, it's not the air filter that's the problem, but the air box or the air filter housing. The best way to check whether the air filter or the air box is restricting air flow, is to use a water manometer to check air flow at various RPMs before the air box, before the air filter, and between the air filter and the mass air-flow (MAF) meter.

THE AIR BOX

A cold air induction system.

Should either the air filter or air box be restricting air flow, replacing the air box and air filter with a high-flow cone air filter, such as a K&N air filter, is the best option. But you must box the cone air filter to ensure that hot under hood air does not enter the air intake. On naturally aspirated engines, under hood temperatures can be as much a 30º above the ambient air temperature, and on forced induction engines, it can be as high as 50º above the ambient air temperature! As you know by now, higher air temperatures result in lower air density and, consequently, lower power output. Higher air temperatures can also lead to detonation, especially on forced induction engines.

USE AN AIR FILTER

Another popular school of thought suggests that an air filter reduces horsepower, and that your engine will make more horsepower if you run without an air filter. This I call pretzel logic – it makes perfect sense but is fatally flawed! The correctly sized air filter will have a slight effect on air flow, but only slight, while running without an air filter will have a major effect on engine life and engine power! The purpose of an air filter is to prevent dust and dirt from getting into the combustion chamber and grinding away at the piston rings and cylinder bore, not to mention your turbocharger and supercharger. Pretty soon the slight power gain obtained from ditching the air filter will be lost through worn piston rings and seals. Do the smart thing, always use an air filter. It's not there for show and it's not optional!

THE MASS AIR-FLOW SENSOR (MAF)

A hot-wire mass air-flow sensor. Notice the wire mesh?

On electronic fuel injection (EFI) cars, the air-flow sensor or the air-flow meter could be an air flow restriction, depending on which type of air-flow sensor is used. There are three common types of air-flow sensors used on car engines:

• The hot-wire mass air-flow sensor, which does not cause much air flow restriction.
• The vane-type air-flow sensor, which has a spring tensioned swinging flap.
• The Karmen vortex type air-flow sensor, which is used mainly by Mitsubishi.

HOT-WIRE MASS AIR-FLOW SENSOR

The hot-wire air-flow sensor, which is often called the mass air-flow sensor, doesn't present much restriction to the air flow as it is usually about the same size as the bore of the air intake duct. If you need to increase the bore of a mass air-flow sensor, you can swap out the mass air-flow sensor for a larger one from the same car manufacturer, but you would need to reprogram the engine control unit (ECU) so that it can accurately measure the increased air flow rate. If your stock ECU is not reprogrammable, you may need to install a programmable aftermarket ECU.

These air-flow meters usually have a fine wire mesh at either end of the bore. Removing this wire mesh will lead to a 1½ to 2% increase in power at maximum RPM.

VANE-TYPE AIR-FLOW SENSOR

The vane-type air-flow sensor, with its swinging flap is more reliable than the hot-wire mass air-flow sensor but it does present some air flow restriction. You can swap out these air-flow sensors for larger units, but you would need to reprogram the ECU to ensure the correct air flow readings.

Alternatively, you could reduce the spring tension on the swinging flap. However, this will lead to the flap swinging open further, and reaching full-open before peak air-flow is reached. The ECU calculates the air-flow rate on how far and how fast the flap swings open; therefore, the ECU will need to be reprogrammed. However, the flap should never reach full open before peak air-flow is reached as the ECU would not be able to read increases in air flow once the flap is full open. No amount of reprogramming will get the ECU to increase fuel delivery if it is not able to sense an increase in the air flow rate.

KARMEN VORTEX AIR-FLOW SENSOR

The Karmen vortex-type air-flow sensor that is used my Mitsubishi is an oscillatory air-flow sensor that measures the vibrations of the downstream vortexes caused by the barrier placed in the air stream. These air-flow sensors represent the greatest restriction to air flow, and they are the most difficult to adapt for better air flow. Some tuners use a bypass around the air-flow sensor, but this is terrible for modified street cars as the air flow will bypass the air-flow sensor at idle and during cruise conditions.

The best solutions on these engines is rather expensive. It entails a second air intake controlled by a secondary throttle plate, a MAP controlled ECU and secondary fuel injectors. This secondary air intake branches into the air intake duct after the air-flow sensor. Under normal conditions, air flows through the stock air intake and through the air-flow sensor; but under heavy acceleration and at high RPM, the secondary throttle plate is opened by a pressure-sensitive solenoid switch to allow air through that secondary air intake system. A MAP sensor controlled ECU is then used to deliver the extra fuel through a secondary set of fuel injectors.

THE THROTTLE BODY

A well machined throttle body

The throttle body is another potential restriction in the air stream. Again, you should use a water manometer to measure the air flow at various RPMs before and after the throttle body. Generally, the throttle bore should be the same diameter as the air intake duct. If it is not, you may need a larger throttle body. Sometimes the stock throttle body has enough wall thickness to allow you to machine it to a larger bore, eliminating the need for a compatible idle air control. Most throttle bodies will allow at least a 2 mm increase in the throttle bore while most will allow an increase of up to 5 mm, and even 7 mm.

Increasing the throttle bore would not require a reprogramming of the engine control unit (ECU) but you would need to replace the throttle plate with a larger one when you increase the bore on the throttle body. Here you have two options: either have a throttle plate machined with the correct radius and angles around the edges, or bore the throttle body to a size that will allow you to fit a larger throttle plate from another engine. In most case, the latter will be the easiest and the most cost effective. Anotheroption that will be even more cost effective would be to fit a larger throttle body from another engine. However, this option is complicated by the need for a compatible idle air control.

Another thing to consider on the throttle body is the smoothness of the throttle bore. Ridges, lips and rough surfaces in the throttle bore will disrupt air flow; therefore you should smooth them out be grinding away lips and ridges. This you can accomplish using a die grinder. However, if the surface is rough, you may need to machine the bore and fit a slightly larger throttle plate. It would thus be better to take the condition of the throttle body into consideration when opting for a larger throttle body as it may make more sense to bore your existing throttle body and fit the throttle plate from a larger throttle body.

A twin-plate throttle body

You should also slip stream the throttle spindle and the fixing screws but take care not to weaken the throttle spindle too much.

MULTIPLE THROTTLE BODIES

When you fit a larger throttle body, you compromise a bit of drivability. This is because a small movement of the accelerator will lead to a larger increase in air flow through the larger throttle body. This could make the car jerky in traffic, and could lead to wheel spin on wet roads. A solution is to fit a multi-plate throttle body with two throttle plates. In this set up, only one throttle plate operates at low RPM while the second throttle plate only comes into play under heavy acceleration. This is accomplished by having the second throttle plate only opening when the first throttle plate is about 40% open.

Another solution is to have one throttle body for each cylinder. This would provide the best throttle control as the throttle bodies are close to the cylinder head. It will also provide the best top end power.

THE INTAKE MANIFOLD

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 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.

INTAKE RUNNERS

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.

COLD AIR INTAKE SYSTEMS

A cold air induction system.

An alternative to modifying the stock air intake system that we've discussed thus far, is to either design and install your own cold air intake system, or install an aftermarket system. Your cold air intake system can be either a short ram induction system or a cold-air induction system. Of the two, a cold-air induction system would provide more horsepower, but it has its disadvantages. Let's start with the short ram induction system.

THE SHORT RAM INDUCTION SYSTEM

A short ram induction system may be good for 4 to 8 horsepower and is relatively simple to design and install. It generally consists of a short piece of metal tubing and a high-flow air filter that is usually a conical filter. The metal tubing should have the smoothest possible bends and would usually be slightly larger than the stock intake system. Don't go too big with the tubing as you don't want to lose air flow velocity. In fact it wouldbe best to determine the correct diameter of the tubing by using a water manometer to measure the air-flow rate as we've described in our section on the air filter and air box.

If your car has a crankcase breather hose that runs from the valve cover to the air intake, you may need to make a fitting for it on your short ram induction system and fit a hose of the appropriate length. In most cases this would be the only modification you'd need to make. The air filter would be relatively dose to the engine so you shouldn't require any modifications to the engine or body.

THE COLD-AIR INDUCTION SYSTEM

Depending on the design, a cold-air induction system would double the power made by a short ram induction system because it would pick up colder ambient air either from below the car's grille or from the car's front wheel arch, whereas the short ram induction system picks up hotter air in the engine compartment; and engine compartment temperatures could be 30 to 50 degrees higher than the ambient air temperature!

Obviously the cold air intake pipe must be longer to reach the colder air, and it is going to need more bends. However, if there are too many bends, the horsepower gained by getting colder intake air could be offset by a loss of air-flow in the intake system. So try to keep the cold air intake pipe as straight as possible and as short as possible as the longer the pipe, the more friction the incoming air will experience as it passes through the pipe.

Also, you need to be careful where you place the air filter of your cold-air induction system as it could be contaminated by dirt, or blocked by snow or mud. While these aren't really serious problems, if the air filter is positioned too low, it could suck water into the engine, which would be disastrous. Water is far less compressible that gas, and once it gets into the combustion cylinder it will cause hydraulic-lock as the pistons will encounter a virtually solid mass of water. The result could easily be bent conrods, a snapped crankshaft, and a totaled engine!

(Credit to Custom Car US)

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