As the piston falls the atmospheric pressure forces (remember that the gas column is about 50 km’s high) air into the cylinder. The amount the piston falls (the piston speed or piston demand is about the same) is dependent on where is it in the crank cycle. Suffice it to say that it is very much not linear or sinusoidal at this junction. The airspeed in the small gap around the valve is thus highly variable. The air is coming from a big pipe ( port) and has the ”squeeze” through the small gap and unqueeze into the other big pipe (cylinder). The whole reason why there is a loss of flow in the first place it that this process is lossy ( i.e. it costs energy/there is no free lunch) As there is continuity of mass and energy (ie the pipe is not leaky and total energy is constant/ first law of thermodynamics) it should all add up. All good and well but, while the total amount of energy is constant, the amount of energy after the gap has certainly changed. Where did it go ?
The main thing is that there is a conversion of states Pressure (port)>Velocity(seat)>Pressure(cylinder). Efficiency is not 100%.. where did to the energy ”dissapear” to?
Well it got converted to heat by air whirling around aimlessly. If you can reduce the whirling about a bit, this would be beneficial . One way is to reduce the amount to directional change in a manner that reduces energy loss (smooth cornering vs powersliding…. a soapbox racer) both on the way in and the way out. When you do the Unsqueezing in a controlled manner you can even recover some of the lost energy (pressure), a phenomenon known as pressure recovery.
As the speed on the seat is a lot higher than the before or after this applies more so to the areas directly before and after the seat. (as people have found out for ages)
Calver reported when doing the 1275 head on a small bore block, just sinking in the seat and widening the topcut accordingly did actually gain flow in low to mid lift range (0.200-0.350 inch lift)
If you look at the red areas and note the added extra bit of top angle I added ( figure above gray bit could help keep the flow tidier. ( and improve Cd and pressure recovery… )
(This is allways a balancing act as sinking in the seat will effectively lower the short side as well, then again the flow will be pretty much diagonal through the bowl after low lift so it would depend ( as allways) I guess).
Super Narrow Race Valve Seats ?
What about a super narrow seat then.. you only have to look down the port to see what kind of massive gain you must have.. It is bigger init !? As for the gains.. probably if you leave the valve off and measure bare port flow. But here is what happens if you put the valve in . Massive area improvement ? Not really unless you lift the valve very high or have a port that is very steeply downdraughted. Because of the shallow port port approach and the 90 degree bend the flow on higher lift is very much ”up and arcross” the bowl it makes sense ( but better ideas have failed 🙂 ) to have a shallow approach to the seat a filled in bowl and a wider topcut.. we’ll see
I have worked on heads with seats like this. It is horrible because there is hardly any metal left to work with. It was actually worse than this on one head than on this figure
( all figures adapted from a figure as published by GP Blair SAE R-186)