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Valve seats are critically important really, basically they seal the chamber and allow the gases in and out to state the obvious.
Whats not so obvious is the impact the valve seat angles can have on the efficiency of inducted and exhausted flows.
So what about the angles and what works best?
Unfortunately theres no single one size fits all we can use on all heads as each head (different makes/marques/models) all have differences that dictate what will work best for it, things such as port downdraught angle, short side radius shape, valve shape, chamber shrouding (if any).

What i can show you here is the results of some optimisation on an intake valve seat, the port isnt modified at all, the differences generated come entirely from the seats unless otherwise stated.

Ok, so lets have a look at a typical valve seat that you might find on just about any head.
This is a VW/Audi head i did recently, all its had done at this point is to lap the valve to its seat so it seals ok, the guides were absolutely worn out though, no wonder the engine didnt make any power.
45 degree seat angle, 75 lower cut into the valve throat.





Heres a link to a video i made. Have a listen to the whistling sound that occurs when the valve hits about 2.5mm off its seat.
It got so loud at one point the camera started to overload.

Valve lift test.

http://www.youtube.com/watch?v=SyUio7RaL0Y


That whistling sound is caused by the port oscillating , created in this case by a "backcut" thats been applied to the valve head, we've all heard of that operation at one time or another.
Its created a seat combination that performs very badly, worse than stock in this case, it destroys low lift flow, although it does recover later on and is better than the standard valve shape at higher lifts on this head.



Hers a comparison of two intake valves.
Stock item on the right, modified on the left is the one making all the noise.




Heres a look at the modified seats created some 24 lift tests( thats about 528 individual measurements) later to get somewhere close to optimal.
I still think theres some flow left in this one because the lower seat angle flows worse at 75 degrees but better at 60 degrees.
I think if i went about half way between those two at about 67 degrees id be pretty much spot on.
I wasnt able to try that out though as the cutter needed for it is a special order item, i could have gone with a 70 cutter i suppose but the gains for the cost of the cutter made it uneconomical in this case.





So whats all that effort get us from an otherwise standard port?

Heres a graph of the best curve generated by the above.




As you can see, its flow gained for really not much work ( once the developments done!) and at this point the ports not even touched.

The red line incidentally is the flow of the port with no valves in and represents the maximum possible flow it can make as is.
If the lift curve can intersect with it (not always possible) then at some point the valve will no longer be the restriction and youll be getting the maximum you can out of it, which is the whole point.


This final graph shows the results of both seat and port mods on another same head from the same maker, the development work from the other head having been implemented on it.
Yellow-modified intake.
Red stock intake.
Blue-modified exhaust.
Black0stock exhaust.





Gains on newer heads probably wouldnt be quite so marked as the above because manufacturers have gotten smarter and make them better, still, theres usually something left to get.

I hope this is a useful article.