2-stroke piston window, rear boost port.

[John Orchard]

I did a bit of homework & thought regarding piston window's, thought I'd give it a try.  Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.

Not worried about any more power but thought anything that may give cooler running is worth a try plus even a slight reduction in piston weight wouldn't be a bad thing?

To tell you the truth, I wouldn't notice any increase in power, I'm not getting what I've got to the ground now!  It does seem a touch cooler in the combustion chamber; plug is a little darker.  And my imagination is feeling there is less vibration (I'm a wanker lol).

Any thoughts?

[Tahitian_Red]

Interesting stuff John.  How did you cut the window?  Would two windows work better?

Hopefully reliability isn't reduced. 

[John Orchard]

I drilled three 9mm holes next to each other, then dressed them with a couple of different shaped files.

Mx KDX400 piston is stock with one round 10mm hole in the same spot, the width of the rear boost passage on the KDX is about that width.  The width of the CR500RE boost port is quite wider but I didn't open the hole to the full width of the CR port.

Most dyno reports I have read on the mod quote best results from not having too much height in the hole.

My KDX piston does not have reinforcing around the window; looks like it's just drilled?  I monitor the piston condition regularly on both bikes (the benefits of an air-cooled bike ;-) ) but I don't think there'll be any issues.  Plus I don't use the bikes a lot anyway.  Not like when I was a kid; I'd go through tank after tank of fuel in a day lol.

[John Orchard]

Probably 2 x 10mm holes might be just as good, and maybe less stress effected?

[Rossvickicampbell]

as a matter of interest the Mugen piston in the 1980 Honda CR250 kit had additional ports added like this - 2 though I think.

[evo550]

I'm assuming that's the intake side?
Where would the hot gasses escape to ? The only route would be the intake port, but by the time the piston got that low in the stroke to expose the back of the piston the hot gasses would have all travelled up through the transfer ports into the combustion chamber.
Would it also effect crankcase compression?
Lozza will know....

[John Orchard]

Many modern reed-valve 2-strokes have a rear boost-port running up from the intake port to the same height as the transfers.  Once the piston uncovers the side transfer and rear boost port, the arriving low-pressure (vacuum) pulse from the exhaust draws air/fuel up into the combustion chamber.  And with it, hopefully, some of the mixture from under the piston  :-)

If the mixture under the piston is getting displaced with each stroke, there would be a cooling effect under the piston with each new air/fuel charge.

[John Orchard]

You can see in this pic that the intake port runs up to the transfer port height.

[John Orchard]

This is a KX250A5 cylinder & piston.

[evo550]

  Reports state an increase in 1 to 2 horsepower on a 500, plus a cooler running piston crown from escaping hot gases from under the crown.

If you are talking about hot gasses escaping out the window in the piston the only time its open would be when its lined up with the intake port, I'm thinking?
If a fresh charge is drawn in when that hole is open to the intake port it may cool the piston crown, but heading out?...everything seems to be going in the wrong direction.

Many modern reed-valve 2-strokes have a rear boost-port running up from the intake port to the same height as the transfers.  Once the piston uncovers the side transfer and rear boost port, the arriving low-pressure (vacuum) pulse from the exhaust draws air/fuel up into the combustion chamber. 

I thought the fresh fuel charge in the crank was moved up through the transfer ports to the combustion chamber by the descending piston increasing (compressing) the air pressure in the crank, not being sucked up by escaping exhaust gasses..
8 pages here we come....

[pokey]

SFA pressure in the bottom end so anything that gets the charge to the chamber is a blessing. thats why they have those fancy turbo cranks  and plugged counterweights that I think the jury is still out on if they do actually do anything exept lift the owners ego.The thing that does the most is a well designed expansion chamber.

The charge does get sucked up as well as pushed. when the sonic waves heads to the end of the pipe it creates a low pressure zone in the combustion chamber,. higher pressure in the bottom end moves the charge to the top. when the sonic wave hits the cones in the chamber it bounces back and pushes a bit of that fresh charge back into the combustion chamber to "supercharge "

 and the cycle continues

Whatever you can do to get the charge to the top end better  has got to be good, but its got to actually work

[John Orchard]

The piston window is continually open to the intake/rear transfer.

[pancho]

My limited knowledge of high performance two strokes indicates to me that with modern exhaust expansion chambers, crankcase compression considered so important in days gone by is now a non event.

[evo550]

The piston window is continually open to the intake/rear transfer.

John, that's my point, or I've misunderstood your first post.
You've put the window in the piston to allow hot gasses to escape OUT through it.
So if it is open to the intake port aren't the escaping hot gasses from under the piston crown traveling in the opposite direction to the fresh charge coming in through the intake port?
If you want hot gases to escape from under the crown wouldn't you put it on the exhaust port side?

[matcho mick]

seriously,you guys crack me up,