Here is a pic of the best shape to prevent the disc from pulling into the port as it sweeps over it - and wearing the case.
The disc spins anticlock - so the outer leading edge of the closing side of the disc is well supported ( as its past the port hole ) and
the inner corner radius of the disc actually forms the closing timing point with the bottom left corner of the port.
Thus the port closes gradually from the outside inward, with the timing still set at the usual 140/90 numbers.
The only thing that changes is the last part of the port to be open at 90* is the bottom left corner.
We tried a huge number of shapes when dynoing at Zip the Hines 250 Superkart title winning engines and even had a full port shaped radius ( convex and concave )
on both leading and trailing edges plus combinations of straight edges and curved ie everything we could think of.
The port shape that was wider at the top, and smaller at the bottom with dead straight blade edges made the best overev power, but it wore out the dyno case in no time.
So we went back to the symmetrical shaped port as per the Aprilia pics.
Note that the later RSA port is rotated 90*, with the widest port chord on the centreline thru the crank, giving the same area as the older RSW port, but a smaller footprint of
the valves intrusion for any particular timing duration.
Having the blade not inside the port for the most time, ie with a "narrow " port angular width, gives better power for the same timings as a wide port.
Edit - added pic of later RSA port with widest chord on crank center line , also just added older RSA version.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
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