The intake valve are responsible for ensuring the correct mix of air and fuel gets into the cylinder. They have to work at very high rates and hence need to be built in a very sturdy fashion.
The intake valve look like a trumpet and once their job is done they need to close properly. This will help in preventing any loss of power from the engine and will also prevent flame coming out of the gap and damaging the intake part.
Larger valves will help in getting a higher mixture of air and fuel thus increasing the power. An alternative to this is to increase the number of valves while keeping their size the same. Earlier there were 8 valves where 1 exhaust and 1 intake valve was there in each cylinder. After this 16 valves were used with 2 each of exhaust and intake in a 4 cylinder engine. Now engines come with even 20 valves.
There have been good innovations in this area like the VTEC engine of Honda where the rate of closing and opening is dependent on the RPM of the engine to give the maximum power at a particular speed.
For the valves to work properly the valve springs should be functioning properly. If there is any problem in the spring the valves will not close fully or because of the bounce back will reopen a bit after closing. Again this can lead to catastrophic issues inside the engine. If the pressure of the spring is not good the valve will not close and with extra pressure the cam will wear off faster. The retainers of the valve are also important and it is generally advised never to use new springs on older retainers.
The valve springs come with a coating to resist getting corroded, if there is any scratch it will become the cause of spring failure. As a general advice you should keep the speed of the engine lower till the engine oil is able to reach the operational temperature.
Earlier dual springs were used per valve which allow better performance tuning however after 1996 some of the car makers moved back to single spring. See to it that the car you are doing tuning in has a dual spring.
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