The valve is the last restriction
Air that has travelled the length of the inlet still has to pass the valve curtain, the gap between the valve head and the seat, and then the port in the head. MIT lists choking at the valves among the limits on volumetric efficiency, alongside friction and the tuned length of the runner. A manifold calculation that ignores the valve is a calculation of a duct, not of the engine.
The curtain area grows as the valve lifts, until the port throat or the seat opening becomes the smaller passage. Past that lift, more camshaft lift does not pass more air. This is why a camshaft and a port belong to the same job. Lift, duration and the port's minimum area have to be looked at together.
Inlet valves and ports
A larger inlet valve can raise the area available at a given lift. It can also move the valve closer to the chamber wall, where the wall shrouds one side of the curtain, and it can force a thinner bridge between the inlet and exhaust seats. The gain is the unshrouded area, not the diameter printed on the valve.
Port shape decides the speed and the direction of the air. A port that is large all the way along slows the air at low engine speed and weakens the ram and mixing that a smaller, well-formed port can keep. A port that stays small becomes the choke MIT refers to once the valve is no longer the limit. The short side of a turned port, the inside of the bend onto the valve, is where a poor shape separates the flow. Enlarging that region without keeping the air attached is a common way to make a port flow less than it did.
The seat and the angles cut on it turn the air into the chamber. A seat that is too wide loses the speed at the curtain. A seat that is too narrow can flow well and then wear or overheat. Those widths are a specification for the head in question. They are not a single angle that suits every petrol and diesel chamber.
Exhaust valves
The exhaust valve opens while the cylinder pressure is still high, so the blowdown pulse described in exhaust system tuning starts at the seat. A restriction right at the exhaust seat costs more than the same restriction further down a large pipe, because the pulse energy and the residual gas both depend on it. Exhaust valves also run hotter than inlet valves. A larger exhaust valve is a temperature and a sealing problem as well as a flow problem. Material and cooling of the seat are part of whether it survives a higher exhaust temperature from boost, late ignition or a weak mixture.
The chamber
The chamber sets the compression ratio once the piston and the deck height are fixed, and it sets how far the flame has to travel. A modification that cuts material out of the chamber lowers the compression ratio unless the piston is changed to suit. It can also unshroud a valve. Those are separate effects and they should be measured separately: combustion-chamber volume on the one hand, and flow at the lifts the camshaft actually uses on the other.
On a direct-injection engine the port also aims the air at the injector spray. A port enlarged for maximum flow can spoil the motion the injection was designed around. The same caution applies to a diesel port shaped for swirl. Flow at a bench, with the valve held open and no fuel spray, is not the whole function of that port.
Sources
- Internal combustion engines, lecture 8: intake and exhaust processes. MIT OpenCourseWare, course 2.61, spring 2017. Valve choking as a volumetric-efficiency limit. Accessed 2026-11-11.