I have a little time this morning, so I thought I'd write what's on my mind.
I'm doing a lot of thinking about getting the most from this upcoming ACE GT Billet Head, and there's a lot that goes into that.
The first matter is the valve angle(stem angle). This is about 26 degrees for each valve on the standard UCE head. It was 40 degrees on each valve in the old Iron Barrel head. As a relative matter, the side-draft inlet system which we have on these engines can get a straighter port into the engine with wider valve angles and deeper chambers. This favors flow, typically. However, on the UCE, we have a little bit of downdraft angle in the EFI throttle body and injector body, and so that helped to work with the 26 degree valve angle.
Modern heads have even lower valve angles which stand up more vertically, such as 23, 20, 18, 15, 12, 11, 10, etc.. As the valve angles get more vertical, typically the ability of the head to make more power is improved. Most of this comes from the more efficient combustion chamber that is shallower and more compact, for faster and more complete burns, yielding more power. However, as the valves get stood up more vertically, this has some effects on the port, because it either needs to get more downdraft angle, or there needs to be more turn in the port. We really can't get more downdraft angle because we have to hook up to the stock EFI inlet system where it is, or we start to get into clearance problems with the frame. So that's out. Then, we will have to put more turn into the port, making it less of a straight shot in. This requires some really good port work to make the most out of that, but it can be done.
The other thing that happens as the valve angle gets more vertical is that there is less room for the valve size, because they are no longer "tilted" upwards somewhat into the deeper part of the combustion chamber like the
standard head has. We are restricted almost to the bore diameter distance in terms of our valve size combination. We have a 3.425"(87mm) bore in the GT. And we need to have at least a little bit of room between the valves in the center, and have enough room on the edges to avoid flow shrouding by the chamber walls and bore walls. So, it's a tight fit for valve size with these modern valve angles. If we are going to get the flow we want out of these smaller valves and ports with more turn in them, we are going to have to be good at maximizing what we are working with.
So, what the current school of thought says on these matters is to make the exhaust valve as small as you can, and move it over toward the exhaust side as far as reasonable, while still having it work good. Then, that allows you to move the intake valve over more toward the middle, and use a larger size intake valve to make use of all the rest of the room that you got from moving the smaller exhaust valve over. This puts a lot of emphasis on having a very good working exhaust valve and port, which will be high speed because of the smaller size, and that should help with extraction effects during overlap. The exhaust valve is always smaller because the exhaust operates with high pressure to help it get out, which the intake does not have. We just want it to be as small as it can be to be effective enough with our overall flow, so that it isn't a restriction but gives us more room for a bigger intake valve. So, that is the direction we will be going.