Wakizashi

[Stihlbro]

I see what you are seeing with the piston. I say modify the piston and give it a try. A lighter piston never hurts. You can gain some piston cooling by not keeping the mixture under the piston. A sign of mixture staying around the bottom of piston is a blacken build up.

 

[Mastermind]

Personally, I'm a fan of violent opening pressure on the transfers......so I like the bottom end to be as tight as possible.I like a saw with stuffers. So........taking more off the piston seems counter to what I look for.

 

[Four Paws]

Personally, I'm a fan of violent opening pressure on the transfers......so I like the bottom end to be as tight as possible.I like a saw with stuffers. So........taking more off the piston seems counter to what I look for.

But what if you could induce an improved flow pattern? Funnel more charge into the transfers?

Increasing case pressure can be accomplished several ways -

1) reduce volume = stuffers/JB Weld/etc.
2) increase blowdown
3) decrease intake duration

Which is the best?

 

[Mastermind]

Funnel more charge into the transfers?

You can't move more than the piston will displace.......can you?

 

[Red97]

You can't move more than the piston will displace.......can you?

No, it's all how ya trap it in the chamber...

 

[Deets066]

I see what Josh is sayin, if the piston is slowing down the flow or giving the charge poor direction.

It's a guessing game at best, that's what I like about it, cuz nobody has a Definate answer

 

[Four Paws]

You can't move more than the piston will displace.......can you?

Of course not. But, you can reduce pumping losses.

 

[Deets066]

Of course not. But, you can reduce pumping losses.

That sounds kinda dirty Josh

 

[Four Paws]

Personally, I'm a fan of violent opening pressure on the transfers......so I like the bottom end to be as tight as possible.I like a saw with stuffers. So........taking more off the piston seems counter to what I look for.

The 7900 Dolmar has a really tight case, and 30-ish degrees of blowdown. My thought is it lacks case volume to hold much extra charge than what it needs. If it had a larger case, could it have less blowdown? The 6400 utilizing the same case has 6 transfers. I think that shows the correlation between case volume and displacement.

I see what Josh is sayin, if the piston is slowing down the flow or giving the charge poor direction.

It's a guessing game at best, that's what I like about it, cuz nobody has a Definate answer

Speaking of piston interaction. The 7900 Dolmar went from a windowed piston to slab sided piston. Did the flow characteristics improve (feeding from base - no short circuit through piston windows), or was the reduction in weight the reason the saw gained?

 

[drf256]

Good stuff guys.

I had the same thoughts on the echo 490. @Red97 opened the bottom of his piston. I did not.

Not sure the percentage of time that the piston stays at BDC makes the mod worth it for flow. For lightening maybe, it's still fully supported. And it does dwell at BDC for some time.

Josh, did you measure how many degrees the change to the piston will be open vs. stock?

 

[Deets066]

[QUOTE="

Speaking of piston interaction. The 7900 Dolmar went from a windowed piston to slab sided piston. Did the flow characteristics improve (feeding from base - no short circuit through piston windows), or was the reduction in weight the reason the saw gained?[/QUOTE]


It gained with a slab sided piston? Slab side should be heavier and create more velocity.

 

[paragonbuilder]

The 7900 Dolmar has a really tight case, and 30-ish degrees of blowdown. My thought is it lacks case volume to hold much extra charge than what it needs. If it had a larger case, could it have less blowdown? The 6400 utilizing the same case has 6 transfers. I think that shows the correlation between case volume and displacement.



Speaking of piston interaction. The 7900 Dolmar went from a windowed piston to slab sided piston. Did the flow characteristics improve (feeding from base - no short circuit through piston windows), or was the reduction in weight the reason the saw gained?

What does the 6400 have for blowdown?

 

[mdavlee]

What does the 6400 have for blowdown?

20-24 I think. 7300 is less than the 7900 too.

 

[Mastermind]

The 7900 Dolmar has a really tight case, and 30-ish degrees of blowdown. My thought is it lacks case volume to hold much extra charge than what it needs. If it had a larger case, could it have less blowdown? The 6400 utilizing the same case has 6 transfers. I think that shows the correlation between case volume and displacement.



Speaking of piston interaction. The 7900 Dolmar went from a windowed piston to slab sided piston. Did the flow characteristics improve (feeding from base - no short circuit through piston windows), or was the reduction in weight the reason the saw gained?

The six open transfers of the 6400 add a huge amount of volume to the case. As you thought, the 6400 has (and runs stronger) with much less blowdown.

The new piston is wonderful in my mind. It produces even more case compression with less weight. I say that because the area inside the piston is trapped, and adds to the force feeding the lowers.

 

[Four Paws]

Anyone else have thoughts, examples or real world experiences to share?

 

[exSW]

So, Joey @Stihlbro mentioned he has been studying his 620P. I think studying is the most important thing a guy can do to understand the specifics and intricate details of any subject.

That said, I have been studying wakizashi as well. Specifically how the piston and case feeds the transfers.

I have been pondering this piston modification.

View attachment 45265

I think it will allow more charge to flow unrestricted to the transfers. Plus offer the benefit of a lighter piston and increase volume for fresh charge.

View attachment 45266

680 piston


Your modification


Bottoms up

 

[Four Paws]

Wow that piston is majorly melted.

Guess I am on to something...or not?

 

[Mastermind]

Wow that piston is majorly melted.

Guess I am on to something...or not?

I think you are. The saw you are working on is a wide open design, so flow over velocity......yeah?

Timing numbers off the cuff without knowing the carb size. 98/116/78?

 

[exSW]

Wow that piston is majorly melted.

Guess I am on to something...or not?

It was you that told me a 680 is basically a 575 with a bigger bore. This is what Shindaiwa did they must have had a reason. Love to see the inside of a 695.

 

[exSW]

Wow that piston is majorly melted.

You know that may tell you something right there. How hot,how fast like we said how new that p&c looks otherwise.