Originally Posted By: mjoekingz28
Hello. Just wondering why intake valves have more diameter than exahust ones.....
Thanks
With only so much surface area on the top of the combustion chamber for valves, it works out better to give the intake more circumference than the exhaust, because the intake is limited to a much lower pressure differential.
Its about maintaining equal mass flow. The exact same mass of air/fuel comes in the intake on each stroke as goes out the exhaust, but the stuff that goes out the exhaust is at much higher pressure and temperature (PV=NRT, but "T" has gone way way up due to combustion and the cylinder volume at BDC is exactly the same, therefore pressure is far higher).
So the exhaust valve can be smaller and the very high pressure differential still pushes all the gas out thru a smaller valve opening. On the intake side (without forced induction, that changes the game...) the absolute MAXIMUM pressure differential to push the air/fuel into the cylinder is sea-level atomspheric pressure (about 14psi). In reality its less, because the cylinder is NOT at perfect vacuum after the exhaust stroke, and unless you're at sea level, the outside pressure is less than sea-level atmospheric, and with the throttle partly closed the intake manifold pressure is even lower still.