Pop ups and windows. Your thoughts.

[tree monkey]

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.

what was done in the 70's and what is done today are 2 different things. i've read blair's and jenning
s books. i also have porting software that says thay were not always right. but still worth reading

 

[Andyshine77]

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

Precisely how it works IMHO.

 

[tree monkey]

Precisely how it works IMHO.

if a domed piston is the best, then why don't the oem's do it to all there saws?

 

[Andyshine77]

if a domed piston is the best, then why don't the oem's do it to all there saws?

That's a good question? I guess I need to read through Jenning again.:smile2:

 

[Terry Syd]

I just went back and checked my memory of Blair's work. I got it wrong for the back angle, it tends to be more towards the exhaust port than the rear of the cylinder.

Here's a link to Blair's work - Design and simulation of two-stroke engines Gordon P. Blair.pdf - 4shared.com - document sharing - download

The discussion of transfer flows is on pages 264 & 265. The graphs are numbers 3.35 and 3.36

 

[Andyshine77]

Thanks for the link.

 

[Terry Syd]

This about the fourth or fifth time I've put the link on this forum, you must have missed the other times. I went ahead and downloaded the document on my computer so I have it available for easy access.

 

[tree monkey]

This about the fourth or fifth time I've put the link on this forum, you must have missed the other times. I went ahead and downloaded the document on my computer so I have it available for easy access.

i'm sure blair was the best in his time. but look at a 70's 250 dirtbike. no matter what you do to the porting, it would not compair to todays 250 stock. also his port grafs are for over square engines, chainsaws are under square.
i've done alot of wild things to chainsaws, and one thing i know is popups are not the ticket

 

[parrisw]

because it works

Hmm, well, that explains everything then. Thanks for the help!

 

[Terry Syd]

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.

 

[Justsaws]

A graciously curved fitted top over a well windowed waist sitting atop a full skirt, piston ####.

 

[Terry Syd]

Sounds very feminine when you put it that way.

 

[chainsaw kid]

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

028 super is a great saw, done up even better...torquey too! Problem is the comparable saws today blow it away and most people would laugh at an 028. Thats why it's always good to have the sleeper no one wants to steal. Pull it out and blow by the guy with the expensive saw.
That dome piston in that saw, I too like that design. The shape of the combustion chamber, location of the spark plug, dome piston, high compression....and they are still running 25-35 years later... Lets see how the new saws hold up 25-35 years from now.

Sorry for the slight derail.

Kevin

 

[AUSSIE1]

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.

 

[AUSSIE1]

if a domed piston is the best, then why don't the oem's do it to all there saws?

Just like other mods we do to gain up to 40 odd % over OEM's

 

[blsnelling]

My 440/460 majorly came alive with the popup. It was the last thing added, and nothing else was done at that time. My 372BB didn't really come alive until I windowed the piston and raised the cylinder wall on the sides. I don't think you can just make a general statement about either mod. It depends on the saw.

 

[mdavlee]

I don't like adding windows to a 372 cylinder. A popup did make the 2171 gain a lot more over standard porting .I want to cut the squish band and see how one does that way. The 2171 had the least work done and the strongest runner i've turned out.

 

[blsnelling]

I don't currently have windows in my 390XP piston. I plan on adding them once I'm done with everything else. That way I'll be able to tell if that mod alone made a difference. The 385XP that I did for WYK has a windowed piston. It made an awesome runner. From what I understand, this is not a new mod and has been used in the past by numerous different builders.

 

[wigglesworth]

My 440/460 majorly came alive with the popup. It was the last thing added, and nothing else was done at that time.

U should had gained a lot of intake of u turned the top down and milled the base.

 

[blsnelling]

U should had gained a lot of intake of u turned the top down and milled the base.

Probably about 3*. That's a by product anytime you do a popup.