Pop ups and windows. Your thoughts.

[one.man.band]

cannot comment on intake flow over piston, but can say that flame front could suffer over very high domes.

gained 5 hp (with smoother idle and acceleration) on a originally 70 hp bike whose heads were bored for second plug. the domes on pistons were approximately 1" (w/valve pockets cut in).

the small diameter bore of a saw is less the than 1/2 than the motor i am talking mentioned, so in comparison most likely the flame travel issue would not be as evident, but still there.

 

[tree monkey]

research paper here:

Scavenging a piston-ported two-stroke cylinder : Rogowski, A R : Free Download & Streaming : Internet Archive

dated, november 1 1938

 

[tree monkey]

Treemonkey, I think you are confusing Bell with Blair. Bell's work was rather simplified, he didn't even use time/area, only timing figures. Blair on the other hand is at the engineering level.

If you take a look at Blair's work you will find a reference to 'Loopsaw'. That is a chainsaw engine that he uses for comparative work. He even does some modification of Loopsaw in one of his chapters (towards the end I think). Whereas Jennings time/area is relevant to oversquare piped engines, chainsaws are a different animal. I was able to glean a real nugget of information from Blair's work for the time/area of transfers on chainsaws.

Timberwolf had stated that he found a time/area for transfers less than the bottom range of Jennings formulas and charts to work for chainsaws. When I found Blair's reference to chainsaw transfer time/area he confirmed what Timberwolf said with an actual number to work with - I won't ruin the surprise, you can find it with the link.

was thinking of jennings

 

[tree monkey]

Precisely how it works IMHO.

you don't want the fresh charge mixing with the old charge. you want the fresh charge to push out as mutch of the old charge as posable with a min amount of fresh charge lost out the ex port.

 

[ncfarmboy]

There must be something with windows vs no windows the new Dolmar P&C's for the 5100/5105 and the 7900 no longer have windows. Don't know on the 7900 yet but the 5100 non windowed piston has wider skirts than the original windowed version.
Shep

 

[AUSSIE1]

My experience is limited, that's why I wanted to hear from people with more experience.

One thing I don't like is jumping on the bandwagon and doing something that has zero proof it actually works. Windows in pistons that have cylinders with bottom feeding transfers, simply does not work. The only thing I can see the windows doing in these cylinders is forcing some charge back down the transfers, plus unwanted turbulence, just like EC said.

Obviously compression is an issue with some saw, we know that. Do pup ups work? yes and no, IMHO it all depends on the design of the saw and the stock compression numbers, if you can get decent compression numbers by simply setting squish, you're good to go. What good does an extra 10psi really do in a work saw that already has 175psi? add to that you no longer have a plug and play top end.

So far this thread has been informative. Keep your minds open guys, don't stick to one idea just because. Every engine is different, and responds to different things.

Andy, the only way you'll have proof something works is to try it yourself. Bradley has said he's tried the windows and they work for him? Did he try the windows separate from "raising" the lowers?

The experience I've had with windows was with the 064 piston that already had the windows (372BB). I started off with std level lowers to slowly raising them a little more each time with excellent results, so I can't say whether the windows had a positive or negative affect in this application. Some say they had negative results raising the lowers. Had they windowed pistons at the same time?

175psi is a good figure for a work saw. I wouldn't sniff at that!

My 064 pistoned 371XP BB was running 205psi after many, many tanks through it which surprised me it ending up that high. That really had some balls that saw and I really gave that saw a decent pushing in big hardwood that was too big really for it's bottom end build strength. Next thing I know the saw is running a little sluggish, so I checked and it was down on compression. I flipped the lid and the piston had started to touch the squish band. A new piston, pin and small end brg and she is back together with a little less compression and an easier life.

 

[parrisw]

What about saws like the 200T, its an open port saw and has a windowed piston. If windows were going to screw up transfer flow in a saw, you'd think it would really do it in a open port! But the 200T is a little gutsy saw.

 

[blsnelling]

Most saws don't have 175 PSI stock, even with the squish tightened. A popup will often give you about 20 PSI. I feel it's well worth the effort to get in the 175-190 PSI ballpark.

 

[Mastermind]

I like popups when adding compression is needed. I'm thinking of Wiggs 460 with 30psi less than mine and the same port timing....mine was quile a bit stronger.

Like has been said though. There is a point where too much is reached and the engine goes backward.

Windowed pistons in bottom feeding transfer jugs I ain't a fan of. I can't see doing anything to lose case pressure...

 

[Dennis Cahoon]

Most saws don't have 175 PSI stock, even with the squish tightened. A popup will often give you about 20 PSI. I feel it's well worth the effort to get in the 175-190 PSI ballpark.

If you only knew how to tighten-up the combustion chamber Brad!......Hahahahahaha!

 

[parrisw]

I like popups when adding compression is needed. I'm thinking of Wiggs 460 with 30psi less than mine and the same port timing....mine was quile a bit stronger.

Like has been said though. There is a point where too much is reached and the engine goes backward.

Windowed pistons in bottom feeding transfer jugs I ain't a fan of. I can't see doing anything to lose case pressure...

How does it loose case pressure? That little bit of material gone from the piston is going to change case pressure?

 

[tree monkey]

How does it loose case pressure? That little bit of material gone from the piston is going to change case pressure?

it 's not going to change a thing in a work saw one way or the other. been there done that. the best thing i ever done was to stop playing with the tach and stopwatch. thay cant show torqe, you have to feel it.

 

[Dennis Cahoon]

Never owned a tach!....what are they?.....something you eat!......Hahahahahahaha!

 

[parrisw]

it 's not going to change a thing in a work saw one way or the other. been there done that. the best thing i ever done was to stop playing with the tach and stopwatch. thay cant show torqe, you have to feel it.

Agreed about the tach, I have one, it lives most of the time in the bottom of the tool box.

Everyone's got an opinion, go with what ya feel.

All this talk about popups and windows. Makes me want to take out my windowed popup piston in my 372 and stick a stock on back in and see if there is any diff.

 

[chainsaw kid]

Agreed about the tach, I have one, it lives most of the time in the bottom of the tool box.

Everyone's got an opinion, go with what ya feel.

All this talk about popups and windows. Makes me want to take out my windowed popup piston in my 372 and stick a stock on back in and see if there is any diff.

I'd be curious to see what you find out. When you going to do it/or do you have the time?

 

[parrisw]

I'd be curious to see what you find out. When you going to do it/or do you have the time?

I don't know when. I'm building a 390xp right now. I have a couple other saws which I was going to do next, but they could wait. It would take a bunch of work to put a stock piston back in, since its machined for a popup, for a stock piston to fit I'd need to setup for cutting the squish band on 372's, and it would change ex and transfer timing. I'm just setting up now to be able to cut squish bands, never attempted it before. Got a plate made on Monday for cutting 390's.

 

[tree monkey]

I'd be curious to see what you find out. When you going to do it/or do you have the time?

been there done that. i dont do popups unless i have no other choice

 

[HEAVY FUEL]

been there done that. i dont do popups unless i have no other choice

That popup in a stock 6400 was rather interesting.....

 

[mweba]

Posting this for HEAVYFUEL.

Cause he asked nicely

 

[HEAVY FUEL]

Thanks Mitch!!