>(snbip) Of course, reducing the flow capability of the bypass in
>fact *does* reduce the amount of charge, assuming that was
>the limiting factor in the first place. Higher velocity
>means more drag. In the case of the Fox, it appears that
>there is no effect on the power.
>
> Brett
As you say, Brett, "assuming that was the limiting factor in the first place". In our stunt engines, I think we would be hard pressed to find an engine that is port size challenged. Or, to put it another way, where the ports are not large enough (cross section), and are thus a limiting factor in the amount of charge arriving in the cylinder. And for many of them, it is like a water hose that may be more than large enough, but if you do not have adequate pressure you cannot get the water from "here to there". Squeeze the hose or partially close off the nozzle and you greatly increase the pressure and the distance that it will flow.
But your comment is very correct in that doing this will not increase the amount of water coming out of the hose, and, if you squeeze it too far, you will begin to limit the amount of flow even though it is moving faster and farther.
In our engines we need to force the fuel from the case up to the top of the piston and in a very small fraction of a second. 10,000 rpm, for instance translates into 166 strokes a second. But, in only a small fraction of the stroke period is the intake port open. So, we are not only fighting the time period available, but the distance needed to travel. That distance is small, yes, but the distance remains crutial. And then we are attempting to move comparitively larger amounts of fuel because we are running with a rich mixture. So, as you observed, it is always a compromise between adequate size, and adequate flow pressure.
Bigger ports allow us to dribble more fuel in under lower pressure. But the low pressure may limit the ability to reach the chamber in time (hence the Schnurle "run away".) Raising the ports, which increases the time for the fuel to enter the combustion chamber is one way to help. But this alters the timing and affects the power and engine run in other ways. Reducing the size (cross section) of the ports to increase pressure is another way to help. But one needs to insure that an adequate cross section is maintained for adequate fuel flow or power will, again, be affected. Everything is a compromise.
Interestingly, the boost port was added to the Schnurle design to increase the power of the engine at high speeds ("boost" is a good term.) It works fine under lean conditions and high speed. At lower speeds and richer mixtures it is generally not needed and can work against us. But, going back to Brett's statement, even the Schnurle Fox 40, with its very narrow transfer ports, can benefit for our purposes with the blocking off of the boost port showing that on this engine, at least, even these narrow transfer ports are adequate for our purposes.
How far can we go? I don't know. I have had no success in trying to block off a portion of the much larger transfer ports (compared to the Fox 40) on the FP 40, for instance. If a new sleeve were cut and narrower side ports were cut into the sleeve, this may work. Or (and this is a real possibility) I just didn't do an adequate job in blocking off the right portion of the transfer ports in the case on the 40 FP.
But, on the Fox 35 this is different. Despite what many think, they did make subtle changes in this engine over the years. And there are some years where the single transfer port of this cross-flow engine appears to be too big for the speed and mode in which we normally run this engine. On these it will benefit with a partial restriction of that port, which (for reasons I don't understand) doesn't appear to be too crucial in how you do it. Still, not every Fox 35 will benefit, and it may, indeed, negatively affect those that do not benefit.
But, back to the original question: velocity? Borrowing a line from Ted, "I don't measure no stinking velocity." Although this may provide some "interesting" observations, especially if we could measure it from one engine to another and under varied conditions, the real question is what does any particular modification do to the engine run? I don't really feel there would be a consistent correlation here in measured velocity from engine to engine.
