Been doing some work at the drawing board and calculator recently, and thought I'd give some insight into what I'm thinking here.
One of the big reasons for defining the displacment to the 535cc with the 87mm bore is because it permits larger valve size. I touched on this subject earlier in this thread, but I have a little time now, so I can go into the concept a little further.
Basically, when designing an engine, the first thing decided is the displacement, along with what bore and stroke it will have. That's the start. The stroke defines how high the thing can rev, by the piston speeds associated with that stroke length. The bore defines the outer limits for valve sizes, and how much flow you can get in/out the engine for making power.
So, there's a progression of sizes dictated by these things.
The displacement and stroke together define the bore size.
The bore size dictates maximum valve size. In a typical closed combustion chamber, with the term "closed" meaning that the chamber area is smaller than the overall bore area, and there is some squish area associated with it, there are some limits to available valve size. Typically, the intake valve size will be limited to about 53% of the bore diameter, or else the edges of the chamber are likely to impede the flow around certain parts of the valve, called "shrouding". This is counterproductive because even though the valve is bigger and should produce more flow area, some of that flow area gets shrouded by the combustion chamber walls or bore walls, and so not all of the flow area can be fully used. So, you could probably get as much flow from a smaller valve that is less shrouded, and more optimally flowing around all the entire valve. This is key, because an oversize valve requires an oversize throat and oversize port, and if it doesn't flow to its optimal extent, then the response of the flow is sluggish, and robs throttle response and can also reduce low rpm torque.
Next, the valve size dictates the throat diameter of the port right behind the valve.
And that throat diameter dictates some critical dimensions in the port runner, and also may influence the size of the throttle body or carb.
So, we can see now that right from the "get go", we will be able to set our entire inlet pathway to flow more, and flow better, if we use the biggest bore size that we can fit in the engine. This is critical, because we do need to get a certain amount of flow to produce certain torque/hp amounts, and also rpm speeds.
And, as we stand the valves up more vertically for improved combustion chamber shape, we get less room for valve size because the angle is coming down closer to the actual bore diameter, and doesn't have as much angle upward toward the center which can provide more room for valve size. This is kind of a "compromise" situatiion where we want to get enough valve for flow, but still get the chamber efficiency where we would like it.
I have done some basic drawings and figures, and it looks like we could get a 1.8" inlet valve into this chamber with an 11 degree valve stem angle from vertical. It so happens that a 1.8" valve is 53% of the 87mm bore of the 535. And we might get a 1.45" exhaust valve in there too, but we might need to use a slightly wider angle on the exhaust valve, like 13 degrees or so. Or if we can get enough exhaust flow from a smaller valve, then we might not need to widen the angle there because a smaller valve might fit. This is still up in the air about which final valve sizes and angles we'll finally settle upon, but I am doing some preliminary drawing and calculating what will fit.
Since we have a set limit with the 34mm throttle body and injector housing on the GT, I am trying to get the rest of the design to work maximally with that size. The idea being to get as much as we can get out of the existing parts that people already have on the bike, to make the package more affordable, because they won't have to go out and buy a whole new bigger throttle body and injection system. And for regular UCE owners, they can get the GT piston and throttle body and injector housing as reasonably priced bolt-on parts from RE, if they want to maximize the rpm.
Now, for the exhaust valve, we typically will see closed chambers with exhaust valve diameter about 40% of bore diameter. That's a 1.37" exhaust valve, or basically 1-3/8". This is a bit smaller than the stock exhaust valve, and it requires a very good design to make the most out of that, but it should do really well for exhaust extraction and low rpm torque production. I am going to discuss this deeply with Mondello's to see how small we can make this exhaust valve without restricting the top end hp. I think that this valve will end up being between 1.375" and 1.45". I am leaning toward 1.45", but we'll see if they make a convincing case to go smaller. If we can go smaller, we can enlarge the intake valve a little more and move it over toward the center to avoid shrouding, or we can use a more vertical valve angle, or maybe a little of both.
We are going to try to center all this around the flow capacity of a 34mm throttle body, which is around 205 cfm. We want to reach that same 205 cfm flow capacity with all the other aspects of the inlet system, so it can make the most of what we can get in thru the stock system. For people who want to use carburetors of larger diameter, we can custom port for that alternative. The basic package is going to be designed to extract the most power possible using the stock injection parts, as a cost containment strategy.
Topic: Ace GT Head Project (Read 247627 times)