Most 4v setups are DOHC. The 3v setup is still SOHC, which means less moving parts and better packaging - look at the size and weight of 4v heads vs 3v heads. Less parts also means cheaper; cheaper to make, cheaper to repair, and cheaper to modify.why not just make it a 4v?
i like your wordsMost 4v setups are DOHC. The 3v setup is still SOHC, which means less moving parts and better packaging - look at the size and weight of 4v heads vs 3v heads. Less parts also means cheaper; cheaper to make, cheaper to repair, and cheaper to modify.
Great explanation of why intake valves are bigger/ can be bigger than exhaust valves!!!Total number of valves really does not make a difference. What matters is the total combined cross sectional area for the intake valves and combined cross sectional area of the exhaust valves. For a given cylinder diameter, you will be limited by the number of holes or how big of holes you can put in the head. Too many holes and/or too large of holes will weaken the head which will result in a lower limit on peak cylinder pressures. Lower cylinder pressures means less horsepower. Therefore, the head is optimized taking into account peak cylinder pressures and flow in and out of the cylinder (volumetric efficiency).
The reason the combined cross section of intake valves are always larger than the exhaust valve cross section is based on the compressibility of the working fluid (i.e. air and fuel). Let us look into an example where we have the same size intake and exhaust valve. The intake charge is “cold” when compared to the exhaust gas, therefore it is denser than the hot exhaust gas. Cold gas will choke, or reach the a maximum flow limit through a given hole size before a hot gas flowing though the same size hole. In other words, you can push more of a less dense fluid though a hole than a fluid with higher density through the same hole.
Let us remember that the same amount of mass or “stuff” entering the cylinder will be exiting the cylinder. We want to maximize the amount of new charge entering the cylinder so we make the intake valve area large, or in our case have two intake valves. Now that we know that the exhaust gas is less dense, you can use a smaller size valve (or one valve in our case), to get that same amount of charge out of the cylinder.