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Illustrated: 5 Valve Cylinder Head
Illustrated: 5 Valve Cylinder Head
Multi Valve Engines: Improving breathing is one of the keys for power enhancement. Unquestionably, in the 2-valve era valves used to be the bottleneck, hence the need for more valves.

3-Valve Engines

The earliest mass production multi-valve engines were 3-valves because of its simple construction - it needs only a single Camshaft to drive both intake valves and the exhaust valve of each cylinder. Today, there are still a few car cars using this cheap but inefficient design, such as Fiat Palio and all Mercedes V6 and V8 engines. Mercedes uses that because of emission rather than cost reason.

4-Valve Engines

A typical 2-valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4-valve head increases that to more than 50%, hence smoother and quicker breathing. 4-valve design also benefit a clean and effective combustion, because the Spark Plug can be placed in the middle.

4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduce more friction and hurt high speed power. Therefore the sportiest Honda and Mitsubishi still use DOHC.

5-Valve Engines

It is arguable that whether 5 valves per cylinder helps raising engine efficiency. Audi claimed it does, but fail to provide evidence to support. In fact, its 5V engines are no more powerful and torquey than its German rivals with 4 valves per cylinder.

Originally, 5-valve design doesn’t guarantee covering more head area than 4-valver. Nevertheless, if the head of Combustion Chamber is in irregular shape like the picture shown, the valves may cover larger area. Ferrari F355 make use of this to enhance high-speed breathing. Is there any disadvantage? Yes, faster breathing also harm low-speed torque if no counter measure is taken. Therefore it is more suitable to sports cars.

All existing 5-valve engines have 3 intake valves and 2 exhaust valves per cylinder, still arranged as cross-flow. The exhaust valves are larger, but in terms of total area intake valves are larger. In F355, by arranging the outer intake valves open 10° earlier than the center valve, it got the swirl needed for better air / fuel mixture, hence more efficient burning and cleaner emission.

The advantage of 5-valve engine is still under questioned. Formula One cars also no longer favor it. Even the Ferrari F1 cars which was once famous for 5V engine has switched back to 4-valve design a few years ago.
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