Pop ups and windows. Your thoughts.

[AUSSIE1]

BB's are a bigger pump.

Some bottom ends have run a range of different capacity top ends.

 

[parrisw]

Lowering the cylinder does not increase crankcase compression. You are interested in the trapped volume, so on a piston ported saw, so you'll measure the volume of the underside of the cylinder when the intake has just closed, a reed valve engine would be the whole cylinder.

Ya, I guess that's correct.

I still don't think widowns affect case compression at all.

 

[srcarr52]

Ya, I guess that's correct.

I still don't think widowns affect case compression at all.

Well, not a lot.

 

[parrisw]

Well, not a lot.

LOL, theoretically ya it does, but how much? If you could measure it, it would be so minimal.

 

[mt.stalker]

To further this topic.

Anytime you increase the volume of the crankcase with the piston at the just closing the intake port, you are decreasing you crankcase compression ratio which is key to getting the air fuel mixture pushed up the transfers. Also increasing your intake duration decreases the crankcase compression because there is less compression stroke inside the case. It's the trapped area vs. the area at BDC that creates the pressure needed to push the air/fuel up the transfers.

I know some older saws can really benefit from more crankcase compression (like a 272 or 288). I've had great results with filling in the crankcase in any area that I can with epoxy on these two saws. A. Bell also has a complete chapter on filling crankcases in his book, there is also another chapter on internally super charged engines which increase the crankcase compression even more but they were made illegal in racing in the 70's and the extra moving parts makes them not a great fit in smaller lightweight engines. So IMO crankcase compression is something to be concerned with.

And one more example is the Husky 357XP vs. the 359, same case and crank, but different cylinders and the 357 has inserts on the crank to make it full circle. You can put a 357 cylinder on a 359 but it will suck until you put the crank inserts in.

I'd also like to see other options then cutting a pop-up, I've build mandrels to true up squish bands since the factory machine work there is often terrible. I don't see how I would chuck a whole cylinder in the lathe to get a boring bar in there to cut it, without over cutting and getting into the cylinder wall.

I could see using the pulse mig or a tig welder with a long nozzle to fill in the combustion camber but then finishing it comes back to the above point.

This is so cool , reading these chats between you and Randy and Jacob and others too numerous to mention , your experiances . It is so interesting / educating to us rookies we can't thenk all of you enough . God i love this stuff .
Thanks again,
John

 

[Andyshine77]

Ya, I guess that's correct.

I still don't think widowns affect case compression at all.

I don't have the numbers to back it up, but two well respected saw builder told me just the opposite. On top of that the fastest 372's I've ran didn't have windowed pistons.. Again this is simply what I've been told, I think they're correct, but I'm not 100% sure of it. So far it looks like they may be right.

 

[mdavlee]

I did the 2171 and 372 with popups. One got windows and the other didn't. The timing numbers were almost identical. I think the exhaust was 1° different and the blowdown and intake were the same. The one without windows was way stronger than the other one. Both were blowing around 200 lbs of compression.

 

[Andyshine77]

I did the 2171 and 372 with popups. One got windows and the other didn't. The timing numbers were almost identical. I think the exhaust was 1° different and the blowdown and intake were the same. The one without windows was way stronger than the other one. Both were blowing around 200 lbs of compression.

Good deal. Pop ups really seem to work with the 372's, again I think it has to do with the transfer design.

 

[mdavlee]

I would like to try one without a popup and cutting the chamber and see how it turns out with similar numbers. I know I won't put windows in a 372 piston and raise the lowers again.

 

[parrisw]

I don't have the numbers to back it up, but two well respected saw builder told me just the opposite. On top of that the fastest 372's I've ran didn't have windowed pistons.. Again this is simply what I've been told, I think they're correct, but I'm not 100% sure of it. So far it looks like they may be right.

Fair enough. But how does it affect base compression much at all? Yes it does cause your loosing that small bit of area from cutting the windows, but other then that there is no way it can affect base compression, if it does, then tell me how??? Not just because I said so. That's crap!!!!!!!!

 

[parrisw]

I would like to try one without a popup and cutting the chamber and see how it turns out with similar numbers. I know I won't put windows in a 372 piston and raise the lowers again.

I'm going to do this when I have time. I wish I had an endless supply of OEM jugs and time to play with. I'll need to order up another meteor piston, and setup for cutting 372 squish bands. I have another 372 jug to play with as well. I'd be worried about doing it on BB kits due to the lack of plating all the way up, really don't know how far you can go.

 

[Jacob J.]

I'm going to do this when I have time. I wish I had an endless supply of OEM jugs and time to play with. I'll need to order up another meteor piston, and setup for cutting 372 squish bands. I have another 372 jug to play with as well. I'd be worried about doing it on BB kits due to the lack of plating all the way up, really don't know how far you can go.

I think I have an OEM jug or two that you can have for testing builds. I'll throw in some pistons and rings as well.

 

[mdavlee]

I don't even have any form of 372 to try it out on. The only 70cc I have is the 576.

 

[parrisw]

I think I have an OEM jug or two that you can have for testing builds. I'll throw in some pistons and rings as well.

:smile2:,

I have one Jug, and one well used oem piston that I'd rather not use.

 

[Mastermind]

I have to try to picture this stuff in my head. A full skirt piston is carrying a load of air inside it...like a bucket. Cutting a window in that would let it leak out the side instead of it being directed into the lower transfer inlet. It seems like the sideways flow would hamper the upward flow into the transfer tunnel.

I'm sure this makes no sense at all to anyone...but I understand it.

 

[WoodChuck'r]

Lotta good reading in this thread.

Kudos to Andre for starting the conversation and as well to others for chootin' the **** on a good subject.


This stuff is way outta my league but I'm definitely a big fan of talk like this. I've been watchin' this thread but I don't have much to add to any of what's bein' said.

Good stuff though guys. Good stuff. Thanks for keepin' an ignorant fool like myself somewhat in the know.


Keep it up d00dz. Keep it up. :msp_thumbup:

 

[parrisw]

I have to try to picture this stuff in my head. A full skirt piston is carrying a load of air inside it...like a bucket. Cutting a window in that would let it leak out the side instead of it being directed into the lower transfer inlet. It seems like the sideways flow would hamper the upward flow into the transfer tunnel.

I'm sure this makes no sense at all to anyone...but I understand it.

I see your thinking. But here is my thinking. Don't think of it as a bucket, I think of it as pushing air from the rings down, since the piston isn't a air tight seal on the jug. The parts of the piston from the rings down, just take up volume IMHO, but I could be wrong! LOL

 

[Mastermind]

I see your thinking. But here is my thinking. Don't think of it as a bucket, I think of it as pushing air from the rings down, since the piston isn't a air tight seal on the jug. The parts of the piston from the rings down, just take up volume IMHO, but I could be wrong! LOL

The most important lesson I've ever learned is to realize I'm not as smart as the engineers that designed these engines. I stick to enhancing what is there rather than redesigning it.

There has to be a reason they didn't want a windowed piston in that jug in the first place.

 

[parrisw]

The most important lesson I've ever learned is to realize I'm not as smart as the engineers that designed these engines. I stick to enhancing what is there rather than redesigning it.

There has to be a reason they didn't want a windowed piston in that jug in the first place.

MEH, what the hell do they know anyway.

 

[Mastermind]

MEH, what the hell do they know anyway.

:biggrin::biggrin::biggrin: