Backwards Piston?

[exSW]

On some jugs it is a nightmare, it may even require building up the tunnel on the outside with GB Weld so that the proper size and shape can be achieved. Other jugs may only require a bit of 'blending' - if you have the right tools to do it.

If done properly, it is a superior way to get more time/area for the transfers. A two-stroke can only begin to 'inhale' a fresh charge when the transfers close. If you raise the transfers 1mm, you shortened the 'stroke' by 1mm.

Measure the distance from the transfers to TDC - that is the working stroke of the engine. Say that distance is 23mm, now raise the transfers 1mm and that distance decreases to 22mm. - The 'displacement' of the engine has been reduced by 4.5%.

By that same token if you raise the piston by one mm do you increase the stroke?

 

[Terry Syd]

By that same token if you raise the piston by one mm do you increase the stroke?

Sorry, I'm not following what you mean by 'raise the piston'.

 

[Adirondackstihl]

By that same token if you raise the piston by one mm do you increase the stroke?

No

 

[exSW]

Sorry, I'm not following what you mean by 'raise the piston'.

Use a different piston with a lower wristpin. Or a longer rod. Or change the crank throw.

 

[Terry Syd]

Now I see where you are going. The answer is no. The effective stroke is measured from transfer closing to TDC - you haven't changed that relationship.

 

[exSW]

Now I see where you are going. The answer is no. The effective stroke is measured from transfer closing to TDC - you haven't changed that relationship.

If I have room to raise the piston all other things remaining static I've also raised TDC.

 

[Deets066]

If I have room to raise the piston all other things remaining static I've also raised TDC.

Then yes that distance would be larger

 

[Terry Syd]

Let's say you raised the 'deck height' of the piston 1mm, that is, the distance from the wrist pin to the top of the piston is 1mm higher. The transfer closing will also be 1mm higher - so the relationship/distance between transfer closing and the piston at TDC will remain the same.

 

[exSW]

Ok, so effective displacement of a two stroke is the distance from transfer close to TDC. Not the distance from BDC to TDC. So this means the power stroke effectively ends not at BDC but when the piston starts to clear the exhaust port?

 

[Red97]

Ok, so effective displacement of a two stroke is the distance from transfer close to TDC. Not the distance from BDC to TDC. So this means the power stroke effectively ends not at BDC but when the piston starts to clear the exhaust port?

The diplacement is bore x stroke.

No way of increasing displacement (cc of saw) without changing the bore or stroke.

The power stroke is from tdc to when the exhaust port starts to open.

Just put the darn piston in already

 

[exSW]

The diplacement is bore x stroke.

No way of increasing displacement (cc of saw) without changing the bore or stroke.

The power stroke is from tdc to when the exhaust port starts to open.

Just put the darn piston in already

Big meanie.

 

[huskihl]

Me thinks Joe ended Ross' fun

 

[exSW]

Me thinks Joe ended Ross' fun

Nah,it's all good.

 

[Red97]

Big meanie.

Me thinks Joe ended Ross' fun

I just want to see the thing making chips...

I enjoy the discussion tho.

 

[huskihl]

Nah,it's all good.

Is it for a shinny?

 

[exSW]

Yes. No pics,no write up until it lives. There's about three things going on that aren't the usual build.