reed porting...

[highstrung74]

i have a reed valve(arrow) and was wondering if i put the motor at tdc and used a marker to draw on the piston skirt intake side and remove any remaing material if this would improve the motor as opposed to drilling in the actual piston...thanks in advance.
Dallas Dean

 

[johnsteve]

Id like to see pictures of the arrow motor to see how they port and how the reed valve works

 

[sbest]

The piston still needs the skirt and cylinder wall to stabilize the piston. The inner surface of the skirt is not doing much of that task so that is why it is drilled. The outer edges and bottom of the skirt need to be strong so don't weaken their support.

Are you talking about drilling into the cylinder?

Yes, it can be done, but any metal you take out is reducing crankcase compression and you want that reed as close to the crank as you can get it for this reason.

The Arrow does not do this so I'd be leary of removing too much cylinder material. The Best answer (I always love it when I can say that) is reed direct into the crankcase with no piston interference.

Or even better rotary valves.

Steve

 

[johnsteve]

so the reed in the top picture is at BDC? That black pipe is the exaust I guess. The second picture the has the cylinder has been cut on the intake side. How much has been taken to the top of the cylinder and how deep is the cut? what kind of motor is that. Its not a china girl for sure?

 

[sbest]

Yes, the top picture is a cutaway of a Yamaha Blaster cylinder with the piston at Bottom Dead Center (BDC).
This is an example of a reed valve engine converted by the manufacturer from a piston port.
They did it by modifying the intake port and putting a hole in the piston skirt and include a boost port.
The reed is moderately close to the piston to reduce crankcase volume.
Intake and reed is on the left, exhaust on the right. The red is photoshopped in where I stuff these cylinders.
I know these cylinders quite well and am able to take them from 17hp to nearly 40.

This picture is just one that I grabbed off the net that illustrated cutting the cylinder rather than the piston skirt.
I do believe it is a China Girl cylinder, but there are so many small engines that look similar.
It is generic picture I grabbed off a google search and is what was often done in snowmobiles in the 1970s.
Problems are the reed is too far away from the piston and the huge volume added to the crankcase, but it works.
The cut can go as high as the transfer ports (some take it slightly higher to start charge roll) and deep enough to give the cross-sectional area need for flow. At least the area of the carb is typical.

Steve