Windowed pistons, when and why?

[FATGUY]

I do a small amount of work on repairing OPE. I've also done a bit of machining of cylinders, pistons, etc. for friends of mine that port saws. When you're buddies with guys like Andre and Brad, you tend not to be very motivated to learn to port yourself. They tell me what to machine, I do it, they do their thing, and voila, a ported saw. One of the things that I'm having trouble wrapping my mind around is why you would use a windowed piston in a saw who's lower transfers open in the base. Aside from lightening the reciprocating assembly, I see no advantage, yet very successful builders such as Terry and I believe Brad do it. Why?

 

[Jacob J.]

That's a pretty interesting question Nik. When I first started modifying saws for me, back in the late 90's, I tried putting a full-circle 266 piston in a 371. It worked pretty well. I got more chain speed and acceleration after raising the lower transfer shrouds up, like Terry did in his thread, to the transfer bridge. I know that as the piston bottoms out, and starts traveling up, the charge is burped up from the mixing area to the transfer tunnels and up into the squish area for compressing. I think a windowed piston slightly changes the timing and duration of the "burp." I guess my question would, why do some manufacturers choose to use a windowed piston in a cylinder with open transfers.

 

[FATGUY]

does that suggest it slows the charge going into the transfers down? Please forgive my ignorance.

 

[Chris-PA]

I've wondered about a windowed piston and open transfers too. It seems like the path length would be very short - the charge need only flow around the top "disk" of the piston above the windows to get into the combustion chamber. Not sure what effect this would have.

 

[tlandrum]

nik ,my thoughts on the windowed piston are this,and i may be way off base but its why i use them. by using the windowed piston and significantly raising the lower transfers your starting the fuel charge into the transfers sooner and i believe you get a more fully charged stroke. i feel that with the non windowed piston there has to be some stagnant charge left in the piston. i also believe that when you have a fresh charge completely clearing out with each stroke the piston will run cooler. windows in the piston also make it lighter and i believe will help it spool up faster. like i have said i may be way off in my thinking but i have built umpteen dozen 372's and have found that every one i have put the windowed piston in,even if its just windowing the oem piston, seems to spool up faster and have more overall performance than the non windowed version.

 

[barneyrb]

nik ,my thoughts on the windowed piston are this,and i may be way off base but its why i use them. by using the windowed piston and significantly raising the lower transfers your starting the fuel charge into the transfers sooner and i believe you get a more fully charged stroke. i feel that with the non windowed piston there has to be some stagnant charge left in the piston. i also believe that when you have a fresh charge completely clearing out with each stroke the piston will run cooler. windows in the piston also make it lighter and i believe will help it spool up faster. like i have said i may be way off in my thinking but i have built umpteen dozen 372's and have found that every one i have put the windowed piston in,even if its just windowing the oem piston, seems to spool up faster and have more overall performance than the non windowed version.

Terry, I agree completely with what you are saying. In a closed transfer cylinder I think you NEED the extra transfer ability of the windowed piston, not so much on open transfers. I also think when the windows are down to the point of letting flow through to lower transfers you actually get a "boost" or a higher velocity of charge at that point. GRANTED I am not a professional but I have some experience dating back to 1974 starting with bikes. Dang I'm getting old.....

 

[srcarr52]

Put a 372 piston at BDC and look at what little area there is for the a/f to go through to get to the transfers. The windowed piston and raising the lower transfers allows a little longer of a charge. Shortly after the piston starts to go back up the flow in the transfers is significantly decreased so moving as much of the charge before that is a must.

Also any weight you can remove add the rotating assembly will help with spool up. As you remove weight off the piston, you could remove the same from the crank.

 

[Dennis Cahoon]

Just for the record......a modern bike piston does not have windows. They have ports where the windows would be, but don't go all the way through......so......has anyone ever tried making ports instead of windows?:hmm3grin2orange:

 

[FATGUY]

I appreciate the responses gentlemen, keep them coming!

 

[FATGUY]

Just for the record......a modern bike piston does not have windows. They have ports where the windows would be, but don't go all the way through......so......has anyone ever tried making ports instead of windows?:hmm3grin2orange:

tell me what to cut, I'll be happy to.

 

[blsnelling]

I agree with all the theory that has been presented. That's been my theory as well. But...how do you account for the fact that the strongest 372 I've ever run has no windows in the piston? Just trying to muddy the water

 

[Jacob J.]

Put a 372 piston at BDC and look at what little area there is for the a/f to go through to get to the transfers. The windowed piston and raising the lower transfers allows a little longer of a charge. Shortly after the piston starts to go back up the flow in the transfers is significantly decreased so moving as much of the charge before that is a must.

Also any weight you can remove add the rotating assembly will help with spool up. As you remove weight off the piston, you could remove the same from the crank.

That's my thought too. If you look at the Pioneers, P-40 through P-62, they have quad ports and a full-circle, non-windowed piston much like the 371/372/385/390. Saw builders here in the late 70's/early 80's would cut ports in the Pioneer pistons to gain some spool-up and chain speed.

 

[Jacob J.]

I agree with all the theory that has been presented. That's been my theory as well. But...how do you account for the fact that the strongest 372 I've ever run has no windows in the piston? Just trying to muddy the water

Not only that, but I've run a strong 372 built by Phil's saw shop that had the stock slug, and stock lower transfer shrouds, but everything else was modified.

 

[srcarr52]

That's my thought too. If you look at the Pioneers, P-40 through P-62, they have quad ports and a full-circle, non-windowed piston much like the 371/372/385/390. Saw builders here in the late 70's/early 80's would cut ports in the Pioneer pistons to gain some spool-up and chain speed.

Then they got smart and added the boost port or third transfer.

 

[FATGUY]

I agree with all the theory that has been presented. That's been my theory as well. But...how do you account for the fact that the strongest 372 I've ever run has no windows in the piston? Just trying to muddy the water

looks like I'll need to order two pistons for my 2171 build. We'll have to try it both ways.

 

[blsnelling]

looks like I'll need to order two pistons for my 2171 build. We'll have to try it both ways.

I like it. It only takes a few minutes to swap pistons, and, they're not that expensive.

 

[LegDeLimber]

Got a couple of Tramadols clouding my brain right now (knee and that carpol junk),
So i'm too fuzzyed to pull the memry of what I was striving for.

got a feeling could inspire some thoghts on flow paterns in
the open-vs-closed pistion
wondering what flow looks like after it come out of openside
and round the top "dics" of the pistion.Sorta thinkin
might have a expanding turbelnt efect on charge flow
vs more direct/smopth flow from closed side pistn

But anyone that ported on old 70's~80's Yamaha reed valve stuff
care to chime in about how we viewd the 2-way flow throw those
holes in intak eside of piston
they made good tourqers at some expens of top end hors powr.

also wondering about the pistons witth the un-open side recesses
that i'v just seem to notice in some pics from srtrato engs

=
think i'm gonna go sleep off som e of the meds and come back lter

 

[komatsuvarna]

Not only that, but I've run a strong 372 built by Phil's saw shop that had the stock slug, and stock lower transfer shrouds, but everything else was modified.

I'm just a hack, but that's the way I do mine. This cylinder is finished.

 

[mdavlee]

The 2 372s I did last time there was the build off the one without windows was stronger in the cut but spooled a little better. Both had pop ups and the same port timing. It may have just been the 2171 was just a great runner or something. I've not put windows in the xpw piston or raised the transfer walls any. I didn't want to risk going back wards on torque.

 

[cowroy]

I'm just a hack, but that's the way I do mine. This cylinder is finished.

Is this the cylinder from the saw you had at J's? Did the piston you had in it have windows?