Comparing some 50-70cc port molds

I now have 3 transfer port molds made from 3 different cylinders. A Honda NSR50, Kawasaki KX60 and a Metrakit Pro Race cylinder for a Derbi.

It was very interesting to see the major differences in the transfer designs. I was looking at these with the optimal transfer design being based on the RSA 125 cylinder and the comments made by Frits and Jan in their posts on the Pit-Lane and KiwiBiker boards along with Frits article on transfers I posted.

The first difference to note is the size of the A&B transfer port inlet against the size of the port window where the charge exits into the cylinder. The Honda and Kawasaki use the old widely held idea of forcing the charge through a narrowing duct to increase velocity. The port size is cut in half from entry to exit. The Metrakit however shrinks by about 15-20%, focusing on the principle of "mass transportation" rather than velocity.

Next up is the inner radius of the transfer ducts. The Kawasaki had the ideal design with the largest radius you could fit. This optimizes flow and I have also read that it helps oil transfer to the piston as it can attach itself to the wall and gradually flow up to the port exit and the piston skirt increasing lubrication and reliability. The MK and Honda both have less than ideal shapes, especially the MK. I believe these 2 cylinders are limited by having full length cylinder studs the run through the cylinder that obstruct the port shape from being optimal.

The Metrakit cylinder and its "A" transfer port was the only one to have any upward angle. This is done to effect the central column of fresh charge by impacting it higher to keep it from flowing out the exhaust port. This is all explained in Frits' article on the leaning tower of Pisa and transfer theory that is posted here. I did finally get a large bubble at the bottom of the Kawasaki port. It might not look like it but it has no upward angle.

Lastly is the way in which the inlet charge is orientated when it leaves the duct. The MK most closely mimics the RSA cylinder here as well.

Very interesting to see the differences in designs from one manufacturer to the next. As 2 stroke R&D peaked and then went the way of the dinosaur some companies adopted certain RSA traits while others never did.

Here are some photos and specs of the RSA cylinder to compare with.