Pop ups and windows. Your thoughts.

[Dennis Cahoon]

That's right!.....a domed pop-up.......makes a BadA$$ Racesaw......Work saw, a waste of time!

 

[Jacob J.]

Here is a 2 stroke piston, what say you on flow across the piston with this.

The piston in my old Clinton saws looks similar to that. I've heard those being called a "wave separator" style piston.
The Clinton piston is ramped more towards the exhaust ports and has a shelf on the intake side.

Pretty much all of Stihl's pro-grade models in the 70's used domed pistons at one point or another with the exception
I believe being the 090. But they did a lot of playing around with combustion chamber shapes and sizes on the 090.

 

[parrisw]

That's right!.....a domed pop-up.......makes a BadA$$ Racesaw......Work saw, a waste of time!

Ok fair enough. I really respect your thoughts and opinions, but why is it a waste of time?

The piston in my old Clinton saws looks similar to that. I've heard those being called a "wave separator" style piston.
The Clinton piston is ramped more towards the exhaust ports and has a shelf on the intake side.

Pretty much all of Stihl's pro-grade models in the 70's used domed pistons at one point or another with the exception
I believe being the 090. But they did a lot of playing around with combustion chamber shapes and sizes on the 090.

I was always under the assumption that the ramps direct intake flow up into the combustion chamber so it doesn't just flow straight out the exhaust.

 

[srcarr52]

IMHO a pop-up is a band-aid on a hard to solve problem. The factory combustion chamber on stock saws are too large for emission and reliability reasons. A lower compression engine is less finicky about carb tuning and proper fuel. Without welding in the combustion chamber, cutting the squish band, or making a two piece head (all three are difficult processes) the only other solution is to create a pop-up.

A decent pop-up will bring the compression into respectable range for higher octane fuel (premium only) and the saw can still be just as reliable with a little more attention to the proper carb tuning. Yes a pop-up does cause issues with flow, heat distribution across the dome and flame propagation but the benefits of compressing the air fuel mixture closer to it's ideal cycle will out way the losses from them. Of course things can be done to minimize the losses by making a smooth transition from the pop-up to squish area of the piston, matching this area as close to the dome of the cylinder as possible to stop predetonation and hopefully not mess too much with the flame propagation. I like to have the clearance of the pop-up at the base close to my squish number and gain about 0.040"-0.060" of clearance on it's way up, also putting a healthy radius at the top to keep the piston from getting a hot spot.

I'm not sold on windows in pistons. Obviously for a cylinder with lower transfer ports in the cylinder wall they are necessary but for a base inlet transfer port I don't know if they actually have any flow benefits, maybe even a detriment due to the two flow paths converging causing turbulence. The lightening of the piston, and the added removal of heat are always a good thing thought. The 372 I built I put windows in and it is a great runner but I didn't get time to test it in stages so I have nothing to compare against. This is something I would like to study when I get my dyno running (hydraulic pump with a throttle valve kart engine dyno).

 

[Terry Syd]

Steve, I agree that the tach isn't the best device for recording objective information, but it is all we have. A saw, stuck on a log and allowed to self feed into that log will give an rpm read out. If we tweak the HS needle, we may get another couple hundred rpm in that cut. Now 200 rpm at 10,000 rpm is only 2%. Most people would never pick up a 2 % change, but the tach can.

We can use the tach to find things like peak horsepower (best cutting speed), the rpm of peak torque, the width of the powerband and at what rpm the saw wants to bog. If we make a mod to the engine, we can see if any of those reference points change and by how much (and record them for future work)

It ain't the best, but it beats listening to the sounds of a muffler mod (ie: "sounds bad azz")

 

[stihl038x2]

Here is a Dyno, he talks about it in one of his other vids, but I couldn't find it.

Steve

<iframe width="560" height="315" src="http://www.youtube.com/embed/H0gLFuT9aW4" frameborder="0" allowfullscreen></iframe>

 

[RTK]

Yep, I haven't heard of any dynos available for saws. The tach is something everybody can have access to.

I've seen more than one dyno that has been set up for a saw or other 2 stroke applications. Most that have them tend to be pretty tight lipped when it comes to doling out data and from what they have shown not all that hard to make your own.

 

[sachsmo]

On the old "clamshell" design, a welded pop-up is a good option.

For a race saw/toy.

Would not use one on a "work" saw.

 

[parrisw]

Ok, you guys talked me into trying to cut squish bands. Here is a start.

Got a center pilot hole drilled so far and squared up the piece of alu stock. Now just have to lay out the cylinder mounting holes and get some long bolts.

 

[wyk]

<iframe width="420" height="315" src="http://www.youtube.com/embed/zwdMSCJULgU" frameborder="0" allowfullscreen></iframe>

That comes out to a little short of 20hp, folks.

 

[Dennis Cahoon]

<iframe width="420" height="315" src="http://www.youtube.com/embed/zwdMSCJULgU" frameborder="0" allowfullscreen></iframe>

That comes out to a little short of 20hp, folks.

......that's the trouble with dnyo's!......people Bull$hit you about how much hp they're making to sell their products.......064 with 20hp.....Hahahahahahaha!

 

[Dennis Cahoon]

......but why is it a waste of time?


I already told you in post #6.........IMO......"You can get all the compression you need in a work saw without using a pop-up piston."

 

[Andyshine77]

So Andy, what's your thoughts/experiences?

A popup can be worked beyond your typical lathe spin. Sometimes the gains can out weight the negs. For me a reworked popup piston with top ring pin shifted for a wider port and one ring on a windowed piston (not that I care for windows) produces more than I can get from a kit piston. Very few have the means to access a popup let alone further machining methods. For sure you see some lowers over worked to the point you could see they would struggle with velocity.

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.

 

[parrisw]

......but why is it a waste of time?


I already told you in post #6.........IMO......"You can get all the compression you need in a work saw without using a pop-up piston."

Right! LOL, sorry. I'm listening. Setting up today to be able to cut squish bands. Figured I'd give it a try.

Got some more done for the 390, don't even know yet I'f I'll have to cut the squish band or not.

Pay no attention to those stray holes! LOL, I assumed the bolt pattern was square, well it wasn't, LOL.

 

[komatsuvarna]

My expierence is very limited, but I guess Ill spit it out. In my expierence, piston windows in a 372 does nothing good. I know everybody ports different, and that may be why my expierence was different, but I won't be doing another windowed 372.......

Popups, Ive only done a few, haven't had one go backwards yet. I will say that I think it does disrupt flow, but maybe the added compression off-sets the flow disruption. I guess it depends A LOT on the saw too. Some of the Huskys (372 namely) are known for being a little on the lower side. I also had a damn good runner, that only blowed 130psi.....

EDIT, I don't have a picture of any my popups I don't think, But I lay the edges way back, looks more like a slight bubble.

 

[one.man.band]

sorry...don' have a direct link to this, it's a file i have saved on my hard drive very long time ago. but it should be available by a search.

national committee for aeronautics
technical note #674

title research paper: scavenging a piston ported two stroke cylinder

"......an improvement in scavenging efficiency of approximately 3% was realized by substituting a round top piston for a flat top piston."

(good info on intake and exhaust port angles in that paper).

using some on-line engine simulation calculators, i found that by plugging #'s in for a small displacement engine bore, stroke, rpm that an increase of 0.1 ft/lbs. of torque could be gained by raising compression.

hope it helps.

 

[Terry Syd]

Are you sure it was #674? All I got with that number was a paper on air cooling (pretty good research in any case)

 

[one.man.band]

research paper here:

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

 

[wyk]

......that's the trouble with dnyo's!......people Bull$hit you about how much hp they're making to sell their products.......064 with 20hp.....Hahahahahahaha!

Yup. I asked him how much it put on the dyno before he ported it, and he claimed a 044 or 440 did 5.1 hp. Still, it's just his word. Even piped and on meth, that seems pretty high.

 

[RTK]

sorry...don' have a direct link to this, it's a file i have saved on my hard drive very long time ago. but it should be available by a search.

national committee for aeronautics
technical note #674

title research paper: scavenging a piston ported two stroke cylinder

""......an improvement in scavenging efficiency of approximately 3% was realized by substituting a round top piston for a flat top piston.(good info on intake and exhaust port angles in that paper).

using some on-line engine simulation calculators, i found that by plugging #'s in for a small displacement engine bore, stroke, rpm that an increase of 0.1 ft/lbs. of torque could be gained by raising compression.

hope it helps.

research paper here:

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

Looks like it depended a lot on port lay out and timing in that paper, or did I mis something