• Husqvarna Motorcycles Made In Sweden - About 1988 and older

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Port knotching the piston on top?

Bigbill

Bigbill

Husqvarna
Pro Class
Has anyone ported or flared the piston on top like dremeled a pie shaped wedge towards the exhaust port? The exhaust port will open a tad sooner. Porting the exhaust port by raising it 1/8" arch to 1/16" at the corners and polished maybe too much? My '81 250cr the exhaust port was arched 1/8" center, 1/16" to the sides. I opened up the port for more exhaust flow and polished it. The intake side I polished it. Knifed the rib in the transfer port and equaled the sides.(tapers into the port) I advanced the stator 1 click. Put the UFO in the carb and jetted it. But my thoughts are pie notching the top of the piston?

http://www.dragonfly75.com/motorbike/pistonramp.html
 
The ring is put a certain distance down from the crown of the piston. Would not cutting into the piston tend to get the ring hotter. Especially where you mention. I notice when they went to water cooling the ring is a bit higher, at least in the displacements I have to examine.
 
Bigbill said:
Has anyone ported or flared the piston on top like dremeled a pie shaped wedge towards the exhaust port? The exhaust port will open a tad sooner. Porting the exhaust port by raising it 1/8" arch to 1/16" at the corners and polished maybe too much? My '81 250cr the exhaust port was arched 1/8" center, 1/16" to the sides. I opened up the port for more exhaust flow and polished it. The intake side I polished it. Knifed the rib in the transfer port and equaled the sides.(tapers into the port) I advanced the stator 1 click. Put the UFO in the carb and jetted it. But my thoughts are pie notching the top of the piston?

http://www.dragonfly75.com/motorbike/pistonramp.html
Click to expand...


Bill,
I did that same type of piston modification on my first ever 'dirt" bike back in 1971. It was on a rotory valve Kawasaki 85 motor. I didn't know much, if anything about modifying motors back then as I was 13 years old. But a friend of Dad's raced flat rack and gave me a few pointers on easy mods. One was flaring or filing a wedge on the exhaust side of the piston top. I don't remember how deep the flare was but the top edge of the flare was just slightly above the top ring edge. That was the only performance enhancing mod I did and I remember how the bike suddenly had a lot more power and I could now do wheelies with the bike (perfect for a teen age boy back then).
 
I was thinking about knotching it closer to the compression ring. But maybe since the piston is domed why not knotching some of the dome away leaving the distance to the ring the same. The exhaust knotching adds more low end while the intake knotching adds more top end. Since my porting adds more mid to top this could be interesting. Just food for thought.

When I experimented with porting I did one change on each bike. But on my 81 Husqvarna 250cr and the 85 Honda 125 had all the improvements done to them. The 250cr running a 11t front sprocket was a rocket. But the Honda had all the power on top. What little a 125 has.

It's all about the flow. In with the gas mix and out with the exhaust. Speed up and smooth out the flow.
 
Piston exhaust notching will NOT add low end. All you are doing is changing the port timing so it wouldnt matter if you notched the piston top or raised the port the duration is increased, so either way you will not be adding low end. The only possible reason to notch the top of the piston as opposed to raising the exhaust port is if you have already raised the exhaust port as far as you can, or if you did not want to modify the cylinder.
 
Yes, just as you lose some compression if you raise the exhaust port since it will be open longer before the piston closes it.
 
justintendo said:
wouldnt you be losing compression somewhat by removing material from the piston?
Click to expand...

No question about it. Notching the piston must increase the volume of the fuel/air mixture at TDC in direct proportion to the amount of material removed. Its pure physics, Boyles law applies: (P1xV1/T1 = PTxV2/T2). That said, the loss of compression may be offset to a degree by effectively raising the exhaust port.

Here's a question for the real 2-stroke gurus out there: Doesn't the exhaust port open as the piston ring clears the port? Or is that a negligible effect given the piston to wall clearance (might have just answered my own question)
JT
 
Kartwheel68 said:
Piston exhaust notching will NOT add low end. All you are doing is changing the port timing so it wouldnt matter if you notched the piston top or raised the port the duration is increased, so either way you will not be adding low end. The only possible reason to notch the top of the piston as opposed to raising the exhaust port is if you have already raised the exhaust port as far as you can, or if you did not want to modify the cylinder.
Click to expand...

It will make the percentage of the time the chain is in tension less and make more noise as more pressure when the exhaust opens. You are most likely right on the low end though there may be a way to create an exhaust system where that is not the case.
 
We had a big discussion about notching the dome of a piston to alter the port timing in a 1982 250CR. This was done because there was not enough height in the cylinder sleeve to safely raise the exhaust port. I recommend making a new sleeve with higher height so the desired porting would be possible. I am not a fan of having to hack up a piston every time you need to replace one. Make it out of aluminum and have it nikisil lined so you can always replace with an off the shelf piston until you roach the cylinder. Then replace your custom liner and the process starts again.

I realize this was done in 1982 and it was a cool thing to do to have a 250CR to get 390/430 type mid range with insignificant loss of top end. Making the header longer on the expansion chamber is key to this.
 
It seems to me that there wound be more of a loss of compression by notching the piston than raising the port alone . Whether you make a 1 mm cut on the piston or the port you would be changing port timing the same amount but loss of material on the piston should lower compression even more . Which way will perform better ? Don't know . Any increase or decrease in vibration , again , don't know .
 
jimspac said:
I am not a fan of having to hack up a piston every time you need to replace one..
Click to expand...

Wouldn't someone who modifies the exhaust timing from stock be trimming in the cut outs of the piston at the transfers and maybe something on the intake timing as well?
 
I am sure that would happen but the mod I was referring to involved using a YZ250 piston that is 4-6mm longer in the distance from the wrist pin to the start of the dome. If I use a YZ piston in one of my 250WRs it will be with a 130mm center to center connecting rod so to not affect the stack height. I am not adverse to cutting windows into the piston walls.​
 
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