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Inlet and Exhaust - Air Inlet

 - Better air inlet design always leads to more power.

Improving the breathing of the engine should be the first mod that is made to any engine and especially to any forced induction engine. Everything from the air filter to the exhaust port affects the inlet air supply. The whole inlet system, the cylinder head and the camshafts all affect air flow. Increased boost isn't much good if the flow is inadequate to allow the boosted air to get to the cylinders in an efficient manner. In fact, improving the efficiency of the air flow might even cause a slight loss in boost pressure while upping the power of your engine, because back pressure on the inlet air is being reduced.

Air Filter and Air Box.
The filter and box must be as free flowing as possible.
 - Most stock filters are very good.
 - A lot of stock air boxes are very restrictive in order to reduce under bonnet noise.

Changing a good stock filter for an after market air filter normally won’t make any difference at all to air flow (there are a few exceptions). If the stock air filter is clean and free from grime etc then chances are its doing its job quite well. The air-box on the other hand is a different matter. The stock air box and other sound muffling devices usually strangle the air flow in an effort to keep things as quiet as possible. Some aftermarket air boxes and other items can really make a big difference in air flow into the engine.
On a 2.0L turbo engine just changing the air-box and remapping the engine can give really good power increases. However, sometimes a high flow aftermarket air filter is required and they will make a power difference when other modifications are used. The standard air filter may me adequate for the stock power, but when the exhaust and other upgrades are made then an aftermarket air filter must be considered. A good example of this is the Subaru WRX. The standard filter needs to be changed when any mods are undertaken.

Air Flow Meters.
 - Hot wire meters are the best for flow.
 - Air vane meters can be modified to give better flow characteristics.
 - Any mods to the air sensors must usually be accompanied by a fuel remap.

After the air-box, the air-flow meter is the next restriction that will be hit. The modern hot wire type of air flow meter isn’t much of a problem, but the vane type and the Karmen vortex type (Mitsubishi) can cause some serious flow problems. The air flow meter can be scrapped in favour of a MAP based input if you are using an aftermarket EMU. But if you want to retain the air flow meter then other options must be investigated. Vane meters can be replaced by hot wire meters.
The spring on the vane type meter can be slightly weakened, which will improve air flow, but a remap must be done to avoid an over rich mixture being burned.

There is no easy was to improve flow with Mitsubishis Karmen air-flow meter. There is an expensive option available called the Tbone modification, and it will produce excellent flow and retain perfect road manners, and it is essential on highly modified Evos and GSR Lancers.

Throttle Plate Sizing.
 - Most engines will benefit from a larger throttle plate.

The next spot for flow problems is at the throttle plate(s). If the throttle plate is too small for your state of tune, then a bigger one must be fitted. Any replacement throttle plates must have a compatible idle air control and throttle pot. Otherwise, the stock throttle body must be thick enough to allow machining to accept a bigger throttle plate. In this case, you should machine the body to accept a bigger plate from a different throttle body – this saves the hassle of manufacturing a new throttle plate. A bigger throttle plate will improve air flow but idling and town driving might suffer, so if this is an important consideration, then maybe sticking with the existing throttle plate is a good idea – but electronic throttle control can overcome this problem but can be a bit expensive. Regardless of the size of the throttle plate, you should ensure that the throttle body is streamlined for good airflow. Grind out lips and ridges in the flow passage. Also streamline the throttle spindle and fixing screws as much as possible. In top turbo engines an individual throttle plate controls air flow into each cylinder. A good rule is to have the butterfly about 1 ½ times the size of the inlet valve diameter for top end power, for low down power the size should be 1.3 times the size of the inlet valve diameter.

Inlet Manifold Plenum.
 - A good plenum will equalise the air delivered to each combustion chamber.

The inlet manifold is very important consideration for good airflow. With regard to the inlet plenum, it should deliver equal volumes of air to each cylinder and the runners and openings must be formed to give maximum air flow. The most important job of the plenum is to equalise the air to each cylinder. With big increases in boost, large imbalances can become apparent. There are different ways to tackle this depending on the layout of the engine and the type of forced induction used.

Inlet Runner Design.
 - Runner length and diameter dramatically affect the power delivery.
 - Runner modification is a major tuning tool.

The runners are a real tuning tool. Their diameter and length have an influence on the shape of the power curve. The diameter fixes the rpm at which the engine makes max horsepower. A big fat runner will let the engine breathe freely at high revs but will be down on power at low rpms. Not good for turbo engines. Long runners will push up the bottom end power, but once past the peak power will quickly diminish. Short runners will kill the bottom end, but the engine will continue to deliver good power well beyond its peak.

How to Modify the Stock Manifold.
 - The manifold can be replaced or extrude honed for better flow.

Mods to the manifold can be difficult and expensive but there are exceptions. Sometimes there can be a more performance oriented manifold from a different model car that can be simply bolted on. Otherwise, the maniflod can be extrude honed if the casting is thick enough. Alternatively, the manifold can be cut and appropriately sized runners can be fitted. Obviously, this can be an expensive option.

Cylinder Head Modification.
Simple machining can dramatically improve the cylinder head.
 - A good head design will transform an engine.
 - Most of the changes are universal across all heads… valves and ports machining.

A cylinder head can make a huge difference to any engine. Some of the most legendary performance machines can thank their cylinder head for their performance. A good cylinder head will change an ordinary, mundane engine into a beast. The GTR is a good example of this. For some performance oriented blocks, there are modified and aftermarket blocks available off the shelf. Most of these will give improved flow and combustion and are cheaper than a ported stock head. There are basic changes that can improve flow in any head. The valve seat and the area under the valve seat have sharp ridges where the valve seat meets the port and it can be easily ground out.

Cam Selection.
 - NA cars can benefit from racier cams.
 - A gain in the top end normally means a reduction in low down power.
 - Turbo and centrifugal blowers perform better on mild, production type cams.

In turbos and centrifugal blowers, you don’t have to worry too much about camshaft and valve train changes. Forced induction engines will work well with any cam that isn’t too wild.. so the stock cams are usually ok. The better the design of the cylinder head and the more efficient the inlet and exhaust paths, then the less cam duration and lift we need for good power. For Roots or screw blowers it is possible to use very wild cams. Pressure in the inlet port is always going to be higher than in the exhaust port, so during valve overlap fresh air/fuel will flow right out past the open exhaust valve. This is good for scavenging exhaust gas and it cools the piston, chamber and exhaust valve. But fuel economy goes south and the cat can overheat. For nitrous engines we don’t want nitrous flowing out the exhaust port during overlap. Because exhaust gas is way up we have to add duration to the exhaust lobe. When we cam the car to run best on nitrous, the NA performance becomes lethargic.

The following general points need to be remembered when choosing any cams. Massive amounts of valve overlap is not the way to go on modern, well build engines. Because of the use of free flowing inlet and exhaust systems and free flowing cylinder heads we no longer need big overlap to scavenge the cylinders of exhaust gas. We rely more on exhaust headers to do this. We can reduce the overlap and trap more air/fuel in the cylinders and thus produce more power.