Re: Transfer port timing.. How much can I raise the ports? Keep em flat,or angle them
Well Joe, now you're asking "trade secrets" so good luck getting anyone who has a "dog in the hunt" to tell you anything at all.
Here's my understanding so try to hang with me. "Raising" transfer port timing should never be by a certain height, but by a certain port duration. This is even more critical than on an exhaust port where you can take a certain "figure" for raising it and really kinda guess at how highly tuned that engine is.
There comes a time (and it's a VERY fine line) where transfer port duration is too much and the engine can be allowed to "back flow". This is a particularly bad situation because you get really nasty things like flow separation and liquid dropping out of the charge. The other down side to transfer ports being raised too much is that you begin to limit your blowdown time (if you have a reasonable exhaust port duration). This has the effect of actually LOWERING where your maximum HP can be put out because as the RPM's increase blowdown time becomes ever more crucial to get the last charge out.
As far as roof angles go, I've actually heard conflicting theories to this.
I have heard that flat port roofs tend to have higher peak hp because the new charge is smashed out into the cylinder while it's moving at high speeds pushing the old charge out. That seems to defy the law of common sense to me. The way I see it, a more angled port roof would raise the rpm that the engine puts out its power because it would direct that charge into the head chamber more effective even as the RPM's increase...
I can tell you in practice, it's very difficult to get exactly a given "angle" on a port roof (like trying to shoot for 4 degrees versus say 5 degrees) and you're at best mostly approximating a desired angle.
