Porting pistons (windows, not changing timing via crown-porting)

[Cerberus]

I'm curious why more people don't address the piston when porting a top-end? For instance, on big bore 660's, the piston windows are inherently shrunken and w/o opening them they barely is enough to feed the transfer tunnel (depending how big it is I guess, I make my tunnels very very big with the thinking I want minimal resistance through BDC, also that I want lower velocity to prevent excessive charge spillout)

I've seen videos of guys polishing casting-imperfections out of the piston's windowing but never actually opening-up the windows, is it because of how much displacement it adds to the bottom-end? On that note, I'd been under the impression that torque likes 'regular' even slightly larger bottom end displacement (proportionately) and that it's high RPM that seeks for tiny crankcases, where stuffing is desirable, and had thought that was not the case on chainsaws....til I read a comment this morning that spoke of "earlier 056 cases with smaller crankcase volume" that I guess worked better when you put 660 jugs on them (implication being a reduction in crankcase volume boosted things, not hurt them...and so far as I can tell, anything ground-off the piston windows, or the lower transfers for that matter, is part of lower-end-displacement, no?)

Thanks for any insight, just find it really weird to have never ever seen anyone port a piston remotely as I do and it just seems so obvious to me...wanna figure out where I'm off-base!! I see an OEM piston's windows and they look more un-finished than the jug!

 

[Cerberus]

Oh and while 10-30% of the piston is silica/one, alumina/um is ~2.4g/CC, so a real modest cleanup dropping your piston 100gram to 95 is also adding 5cc's to bottom-end... IF this is universally undesirable, I'd at least be curious on thoughts on my specific situation: I see the additional bottom-end-displacement as good for my context IE using a big-bore cylinder, because now I'm not altering the top//bottom displacements as badly.

Thanks for insight on this, am seeing the BDC phase of the stroke as one wherein I want the piston seeing as little resistance as possible, and want my charge having lower relative velocity because our mufflers suck, so I make very large transfers and piston-windows with a thinking the piston can easily swoosh through the BDC phase, pump that charge up into a pressure-equalized chamber (25.5deg blowdown in this case) and "therefore" have more energy remaining for strong squeeze&bang of the next charge....quotes on 'therefore' because my logic may be way off there & hoping to be corrected if necessary (also I've read Blair & Jennings' entry books so if I'm missing something that someone knows is explained therein, I'd greatly appreciate a nod to what chapter(s) I may need a re-read (3rd/4th re-read, to be fair ))

 

[Bjorn]

Hello, Here is a modified piston. First picture stock second modified

 

[srcarr52]

Oh and while 10-30% of the piston is silica/one, alumina/um is ~2.4g/CC, so a real modest cleanup dropping your piston 100gram to 95 is also adding 5cc's to bottom-end... IF this is universally undesirable, I'd at least be curious on thoughts on my specific situation: I see the additional bottom-end-displacement as good for my context IE using a big-bore cylinder, because now I'm not altering the top//bottom displacements as badly.

Thanks for insight on this, am seeing the BDC phase of the stroke as one wherein I want the piston seeing as little resistance as possible, and want my charge having lower relative velocity because our mufflers suck, so I make very large transfers and piston-windows with a thinking the piston can easily swoosh through the BDC phase, pump that charge up into a pressure-equalized chamber (25.5deg blowdown in this case) and "therefore" have more energy remaining for strong squeeze&bang of the next charge....quotes on 'therefore' because my logic may be way off there & hoping to be corrected if necessary (also I've read Blair & Jennings' entry books so if I'm missing something that someone knows is explained therein, I'd greatly appreciate a nod to what chapter(s) I may need a re-read (3rd/4th re-read, to be fair ))

At a density of 2.4g/cm3 a 5g reduction in piston weight is ~2.1cc to the crankcase, which is almost negligible to the crankcase compression. On the bore sizes in question that would be a difference of ~0.010"-0.020" difference in intake port height.

But the 5g (5%) reduction in reciprocating mass is significant in reducing the energy loss due due to the acceleration profile and friction.

I think your assumption that people don't address the piston is incorrect, they do, they just don't show it.

 

[Mastermind]

At a density of 2.4g/cm3 a 5g reduction in piston weight is ~2.1cc to the crankcase, which is almost negligible to the crankcase compression. On the bore sizes in question that would be a difference of ~0.010"-0.020" difference in intake port height.

But the 5g (5%) reduction in reciprocating mass is significant in reducing the energy loss due due to the acceleration profile and friction.

I think your assumption that people don't address the piston is incorrect, they do, they just don't show it.

View attachment 345663 View attachment 345665 View attachment 345664

Dude ain't been here since July the 8th.

 

[srcarr52]

Dude ain't been here since July the 8th.

I guess I'm flexing for nothing then.

 

[ManiacalMark]

Dude ain't been here since July the 8th.

Or was it the 8th day of July?

I guess I'm flexing for nothing then.

No I think this could be an interesting thread on a topic that I don’t recall having gone down the rabbit hole before, not even in racesaw days.

 

[Maintenance Chief]

I guess I'm flexing for nothing then.

For what its worth I think you might be correct. Some saws have alot more snap after reducing piston weight.

 

[Sagebrush33]

With or without the dude, I'm here for the learnings.

 

[smokey7]

I think his adderall got in the way of simpler smaller posts that wouldnt make our eyes glaze over and roll back.

 

[Jason628]

I like to keep my questions simple like I am. You're most likely to get an answer that way as most people do not have the time to read a novel. Personally I would not feel comfortable taking a bunch of material away from the pin bosses.... But, it is a clone so it's not a huge loss if it scatters itself.