Cylinder Porting : Increasing Duration And Flow

[90GTVert]

I took a look at the new cylinder on the engine. With 1 base gasket I got 128 degress of transfer duration and 178 degrees exhaust. The piston didn't quite clear the ports at BDC. I went to 2 base gaskets and the piston clears at BDC. Durations moved up to 133 transfer and 179 exhaust. The Venus is using 130/184 and has a nice wide powerband, but doesn't wanna make power revving to 9,000 or above. I have to assume a good bit of that is the pipe, but I still want more from this build.

I know T2 used to make power at 9000+. It would cruise at 9700-9800 with ease. The ZX says it's for 9500, but I've definitely seen it kicking over 10,000RPM on occasion. I'm gonna shoot for durations good for around 9500-10000RPM and I think that should suit the pipe well. I wish I had measured the duration of the cylinder I raised the exhaust port on with great results, oh well. I'm thinking the transfer durations should be suitable. I think I'd want the exhaust around 188-190 degrees. I hate to kill low end with higher durations, but I was quite pleased with T2 in the past so I would only imagine it will get better.


I marked the cylinder where the exhaust port roof would need to be for 190 degrees. 190 is actually the center of the huge mark. I know that's not precise, but I can always check the porting with the degree wheel when it's close. Using a pen or thin marking device would be better for precision. Dying the walls and scribing a line is another good choice.





I started working with the exhaust port, but I didn't go all the way to where I needed it to be. I'm gonna wait till I get my other bits, then I have room to smooth it out without going over the duration I want. I widened the port just slightly. Never measured. Maybe 1-2mm total.





I checked out the skirt by the boost port as well and decided to grind it out a bit.









Pulled the cases apart again and checked this cylinder against what I had matched to the old cylinder. The boost and transfers will need a little matching.



I brought the boost port toward the cylinder. You should see the difference in the left and right side in this pic.

[90GTVert]

This information provided by 190mech.

Did you measure your chord width?Try for about 63% bore diameter.That should wake it up!!

[90GTVert]

I decided to go a little (maybe a lot) more radical on port timing. I kinda wanna see what a 103cc does over 10,000RPM... aside from grenading. My initial figures/goals were 190 exhaust duration and 133 transfer duration. Wellll... I kicked it up a notch. The finished durations are 196 exhaust and 136 transfer. The exhaust duration should be good for 10,500-11,000RPM. The transfer duration is intended for 12,000-13,000RPM accodriding to Bell's stats, but I have liked the additional transfer duration in the Venus. The longer durations are said to reduce the narrow, "pipey", powerband. I know the Venus will chug along happily at 4,000RPM or pull to about 9,000RPM with it's transfer durations intended for 10,000-11,000RPM according to Bell's book. Hopefully I haven't just made a big mistake, but time will tell.

Here's the exhaust port in it's finished form. Chord width was increased to just a hair over 34mm (63%).





I filed the piston's crown to get the 3 degrees of transfer duration that I needed, and to get the final 2 degrees of exhaust duration without enlarging the port farther. Then I gave teh piston a little smoother finish.