Husqvarna 445 piston question

[MAF143]

I'm working on a Husqvarna 445 for a friend and have a question about a region on the piston that opens the upper transfer ports to the crankcase between the strato air transfer and the opening of the transfer port to the combustion chamber.

As the piston is going up in compression the blue area short circuits the upper transfer port to the crankcase right before the strato ports are opened to the transfer ports by the red area. And as the piston is coming down in the power stroke, the blue area opens the upper transfer port to the crankcase via the hole in the piston after the stratos charged the upper transfer tunnels with air. I see what happens and when it happens, but I'm not sure what purpose it serves and how this timing and the size of the hole in the piston acts to improve the functionality of the saw.

Just trying to understand what this passage is for, how it works, and why is it there? My brain may be able to understand if someone explains it to me but I'm not even close to smart enough to figure it out on my own...

I'll be widening the exhaust and mildly porting this saw just for fun (it's a clammy), but I would like to understand what this blue area does and how it may effect adding duration to the intake. I'm thinking of increasing intake duration from 142* to 155* or so. Also blowdown on this saw was very short, only 12*.

Are there any other saws with this piston port that I can read up on? I couldn't find anything similar in my searches, but that may be cuz I'm not really sure what I'm searching for... rookie here, but very curious...

 

[MAF143]

An old MS362 piston that I have from an early run of them has the same void in the piston as the blue void in the 445 piston but it doesn't have the vent hole to the crankcase.

 

[huskihl]

The blue area has nothing to do with the transfer ports and is only there to lighten the piston.
The red area is the strato cut out. On the way up, right about the same time the intake opens, the top of that strato cutout will line up with the transfer ports in the cylinder and the vacuum from the case will pull fresh air through the ducts, into the piston, and up into the upper transfers. On the way down, that fresh air enters the cylinder first and purges out the spent exhaust

 

[farminkarman]

The blue area has nothing to do with the transfer ports and is only there to lighten the piston.
The red area is the strato cut out. On the way up, right about the same time the intake opens, the top of that strato cutout will line up with the transfer ports in the cylinder and the vacuum from the case will pull fresh air through the ducts, into the piston, and up into the upper transfers. On the way down, that fresh air enters the cylinder first and purges out the spent exhaust

Well said Kevin. Here is an x-torq animation that shows the process you described:

 

[Partner]

In the second well-known company it's called 2-mix

 

[MAF143]

Thanks for the explanation and the video guys. That blue void was throwing me. Glad to know that there's no function to it other than weight reduction.

 

[Tomos770]

I'll be widening the exhaust and mildly porting this saw just for fun

Dont forget to polish those XXX inperfections on the piston.... everything else is also in the video below...

 

[huskihl]

Dont forget to polish those XXX inperfections on the piston.... everything else is also in the video below...

Husky doesn’t put those x’s there to slow it down…

 

[Tomos770]

....but to reinforce those cut-outs?

 

[legdelimber]

Seems like the hash marks have been pondered at other times also.
I don't recall if there was a definitive answer though.
Was it an in-house marking system, or for turbulence or something else?
Kinda hard to think of them as something structural, given the hand cut looking irregularity.
Or could they have been for a quick visual inspection of castings being filled in?

I can just imagine someone in R&D saying, Gee boss we've got a whistling noise coming from the intake tract and it's killing our decibel rating for the new saws.
Bossman says: Hmm... let's try doing some hash marks on the pistons. It'll be cheaper to change production if they don't work.

 

[huskihl]

They keep the air flowing smoothly

 

[Tomos770]

It sound counterintuitive.....

What is that hole in upper opening doing there?

 

[huskihl]

It sound counterintuitive.....

What is that hole in upper opening doing there?

Weight loss

 

[Tomos770]

Why didnt they put 3 holes in it instead....?

It would be even lighter....