Pop ups and windows. Your thoughts.

[Andyshine77]

Not so long ago they were all the rage. why? There was never an ounce of proof they did much of anything positive in most cases.

I could never understand piston windows in closed port saws doing anything other than dropping case pressure. Some said it helps deal with heat, I have a hard time swallowing that pill.

Unless you have enough material and a severely compression challenged top end, I've seen little gains with pop ups in most work ported saws. To be completely honest I've seen things go the other way more than once, even when port timing numbers were kept the same. The 372 seems to be one of the saw where pop ups are advantageous, and I think I know why.

So what say the masses, and people with larger mental capacity than myself?? The more scientific the better.

Keep in mind this is about work saws, not high end competition saws.

Andre.

 

[Jacob J.]

I like having windows in my pickup, so I can see where I'm going.

 

[parrisw]

I've wondered too. I'm pretty certain they both work in 372's, I've tried with and without and they seem to run better with them.

 

[Jacob J.]

Andre-

If you have any doubts about windows, try putting a Husky 380 piston in a Stihl 064 and see how much you lose
in terms of top RPM. I think you'll lose around 3-4 grand. Just a hunch.

 

[wigglesworth]

I don't really like pop-ups, but in some cases, it's all u can do to get the comp up to respectable levels. I have not noticed "huge gains" in the saws I've done with em. Also creates lots of turbulence across the crown surface.

As for windowing pistons, I really don't see a need in it, other than lightening a slug a bit. In saws like the 372, where the serve no purpose in opening another point to help feed the transfers, I don't understand why folks do it. Now in older conventional jugs, with the lower trans in the side of the jug, it makes sense to have em there, as they will flow mix thru the windows into the transfers as the crankcase pressure increases.

But then again, what do I know......:dunno:

 

[Dennis Cahoon]

You can get all the compression you need in a work saw without using a pop-up piston. IMO a waste of time! The original pop-up design was used with a domed piston to pump up high compression for an alcohol running stock appearing racesaw without making a removeable head.

 

[parrisw]

You can get all the compression you need in a work saw without using a pop-up piston. IMO a waste of time! The original pop-up design was used with a domed piston to pump up high compression for an alcohol running stock appearing racesaw without making a removeable head.

Yes, if your setup to cut the squish band. I have not ventured into that yet, I guess I should give it a go some day.

 

[parrisw]

What about cutting a domed piston as apposed to a popup where just the center is rised. That would eliminate the turbulence caused by the popup.

 

[tree monkey]

Not so long ago they were all the rage. why? There was never an ounce of proof they did much of anything positive in most cases.

I could never understand piston windows in closed port saws doing anything other than dropping case pressure. Some said it helps deal with heat, I have a hard time swallowing that pill.

Unless you have enough material and a severely compression challenged top end, I've seen little gains with pop ups in most work ported saws. To be completely honest I've seen things go the other way more than once, even when port timing numbers were kept the same. The 372 seems to be one of the saw where pop ups are advantageous, and I think I know why.

So what say the masses, and people with larger mental capacity than myself?? The more scientific the better.

Keep in mind this is about work saws, not high end competition saws.
agreed,i cut the squish band.
Andre.

What about cutting a domed piston as apposed to a popup where just the center is rised. That would eliminate the turbulence caused by the popup.

the ms 260 is domed, and i cut it flat

 

[parrisw]

the ms 260 is domed, and i cut it flat

Why?

 

[Andyshine77]

One reason I made this thread is the fact I have done a few experiments, and have seen the results, in most cases my thinking was right, but not always. Something I also see done on work saws, is a lot of lower transfer work. Without carb and other intake work, it seems you loose some top end. You need pressure/volume to push the charge through the transfer, make the lowers too big the charge just sits in the case or transfers. Sometimes smaller is better. Just thinking out loud here.

 

[tree monkey]

Why?

because it works

 

[Andyshine77]

I don't really like pop-ups, but in some cases, it's all u can do to get the comp up to respectable levels. I have not noticed "huge gains" in the saws I've done with em. Also creates lots of turbulence across the crown surface.

As for windowing pistons, I really don't see a need in it, other than lightening a slug a bit. In saws like the 372, where the serve no purpose in opening another point to help feed the transfers, I don't understand why folks do it. Now in older conventional jugs, with the lower trans in the side of the jug, it makes sense to have em there, as they will flow mix thru the windows into the transfers as the crankcase pressure increases.

But then again, what do I know......:dunno:

This is 100% correct IMHO.

 

[tree monkey]

One reason I made this thread is the fact I have done a few experiments, and have seen the results, in most cases my thinking was right, but not always. Something I also see done on work saws, is a lot of lower transfer work. Without carb and other intake work, it seems you loose some top end. You need pressure/volume to push the charge through the transfer, make the lowers too big the charge just sits in the case or transfers. Sometimes smaller is better. Just thinking out loud here.

most people over port a cylinder to be a good worksaw

 

[Jacob J.]

because it works

Do you cut the squish band in the cylinder flat as well?

 

[tree monkey]

Do you cut the squish band in the cylinder flat as well?

yes

 

[Terry Syd]

A domed piston makes sense, there won't be anything in the way to upset the transfer flow. The domed piston may run a bit hotter as the crown has more area, but that is nothing compared to the crown area of the pop-up.

Has anybody run into a pop-up piston in any other two-stroke sport or application? I can't recall seeing it in any kart or aero engines. It seems to be a mod only used by chainsaw enthusiasts.

 

[tree monkey]

A domed piston makes sense, there won't be anything in the way to upset the transfer flow. The domed piston may run a bit hotter as the crown has more area, but that is nothing compared to the crown area of the pop-up.

Has anybody run into a pop-up piston in any other two-stroke sport or application? I can't recall seeing it in any kart or aero engines. It seems to be a mod only used by chainsaw enthusiasts.

a domed piston does upset transfer flow. the transfer port roof angle has to be right also

 

[Terry Syd]

Perhaps were not talking about the same kind of dome. I'm referring to the dome type pistons you find in motorcycles, that is, a slow curve on the top of the piston.

If you take a look at Blair's work, you will see a graph of the actual flow directions of the ports. The transfer flows tend to naturally flow upward from the port angles by about another 15 degrees. The up angle is even more when the port first opens. The same thing exists for the back angle, the actual flow is more towards the back of the port than the port angle.

 

[wigglesworth]

Has anybody run into a pop-up piston in any other two-stroke sport or application? I can't recall seeing it in any kart or aero engines. It seems to be a mod only used by chainsaw enthusiasts.

The 028 super has one stock, and IMHO, is the ideal shape to keep flow disruption to a minimum.