Why is the exhaust valve smaller than the inlet valve?

Answer 1

Can the exhaust valve be too big? Indeed it can unless certain steps are taken to design other aspects of the engine to suit the valve size. The main adverse effect of large exhaust valves is in the overlap period at the end of the exhaust stroke when the exhaust valve is closing and the inlet one opening. While both valves are open at the same time some rather nasty things can happen. Fresh inlet charge can skip straight across to the open exhaust valve and disappear down the exhaust port leading to high hydrocarbon emissions and poor cylinder filling. Also the exhaust gasses can reverse flow at low engine speed and come back up the exhaust pipe and into the cylinder leading to contamination of the fresh inlet charge. To combat these effects you can reduce cam duration but this then adversely affects high rpm power. Alternatively you can increase the cam lobe centreline angle (LCA) so the exhaust cam closes earlier and the inlet one opens later. This reduces the 'valve overlap triangle' but again might not be what you want at higher rpm. What we can deduce from this is that large exhaust valves might have little adverse effect on low tuned or standard road engines with short duration cams but progressively worse effects on higher tuned engines. In fact it leads us to the inevitable conclusion that the higher we tune an engine the smaller the exhaust valve might need to be. This flies in the face of everything most people believe about tuning where bigger is always better.