Part Three: The Transfer Ports

[Terry Syd]

Some guidelines from Blair on transfers. There is a wealth of information in his work - stuff that has been empirically confirmed. You don't have to understand the math, it is the theories in the text, supported by the graphs and pictures that tell the real story. The math is for the next engineer to use on a new design.

3.5.5.1 The main transfer port
The orientation of the main port, which is the scavenge port immediately beside the exhaust port, is already stated as the designer's first priority and this has some potential for empirical guidance. If one examines the port plan layouts of all of the cylinders known to provide successful scavenging, several factors stand out quite noticeably:
(a) The upsweep angle of the main port, UPM, is rarely larger than 10°.
(b) The value of AM2 is usually between 50° and 55°.
(c) The target point for MT2 is usually between 10% and 15% of the cylinder bore dimension, BO.
(d) The target point for MT1 is approximately on the edge of the cylinder bore.
(e) The port is tapered to provide an accelerating flow though the port, i.e., AMI is greater than AM2, and AMI is rarely larger than 70°.
(f) The larger the angle, AMI, the more the target point, MT1, is inside the cylinder bore. The narrower the angle, AMI, the farther outside the cylinder bore is the target

 

[P.M.P.]

I just grind the fùcking things. If they don't run good, I grind some more.

You guys think too much.

YOU ROCK

 

[mdavlee]

There are so many more variations on transfers than any other aspect of porting, to me anyway. It seems as they are where a better part of the power is made.

I believe the main reason my hybrid is a good runner is because of the way the trans were shaped and timed stock. They were perfect, when you find that certain cylinder that you just know it is gunna snort.... don't mess it up cuz they are hard to come by.

My 46 right now I just leveled them up so no taper towards the intake. I did have to open the lowers some with the piston I'm using. I do want to put some deep fingers or boost ports in it when I get a chance.

 

[drf256]

This is the area we try to fill wif finger ports.

 

[Deets066]

This is the area we try to fill wif finger ports.
View attachment 68755

Some people like to fill other areas with their fingers....

 

[drf256]

Some people like to fill other areas with their fingers....

Which areas, besides anatomic ones?

 

[Deets066]

Which areas, besides anatomic ones?

You seem to have covered all the areas in your last 2 posts

 

[Al Smith]

You want to talk magic,the illusive push pull system with a tuned pipe and a tuned intake .I think it was Villers MC early on that had one that worked to a certain extent but the true both sides in resonance was very limited to RPM range .--I've only read about it and know very little about it .

 

[Terry Syd]

Al, I think it was the 'Cotton' that had the original 'blooey pipe'. It was just a diffuser pipe without the reverse cone. It was tuned to pull the transfer charge up from the crankcase, but lacked the ability to pack the cylinder just before closing. I've read about how bloody loud the things were. Crikey, a non-muffler expansion chamber was loud enough, I can't image the racket those things made.

 

[CJ Brown]

I read about 70 pages of Blair's work last night. The math, as he stated, was for the development of a computer simulation program that would allow engineers/designers to see the effects of design changes without the very costly and time consuming method of trial and error. But as @Terry Syd stated above, there is so much more in there.

And the very first thing I discovered as I started reading was that my understanding of scavenging was completely wrong. I always understood it as being a function of the exhaust, but it is actually a functi0n of the fresh air/fuel charge.

Unlearning, unlearning...

 

[bill saf]

Jmo The one thing that I see missing that should be taken into some account for scavenging is the crank counter wights as in this that was posted. The wights act some what as ( fan, blower ) and shuts off flow to the transfers as it swings by. we all talk about what is going on the the top end and nothing about whats going on downstairs.

View attachment 68732

 

[CJ Brown]

I am hoping a discussion around the crankcase will be a part of this series, as it plays an influential role in how the air fuel charge makes it's way to the combustion chamber.

 

[Mastermind]

I am hoping a discussion around the crankcase will be a part of this series, as it plays an influential role in how the air fuel charge makes it's way to the combustion chamber.

I think we'll talk about exhaust (mufflers and tuned pipes) next (on Monday) and then back up to discuss blowdown and case compression in more detail.

 

[Stihlbro]

Oops!

 

[Lightning Performance]

Ya need more area to flow more stuffs..
But if ya got limited stuffs, what the hell good is the ability to move more of it??

Or:
You can focus on just moving your limited stuffs more efficiently.

But that has limits and isn't much fun.

I'm not limited to just fun stuffs.

He's talking about the scavenging effect. As the exhaust is expelled it's inertia or momentum leaves a low pressure area behind it.......that pulls on the transfer ports.

I notice saws with a lot of blowdown have very restricked mufflers. This would seem counter productive to making power but conducive to EPA emissions laws. These saws do poorly with little case pressure time.

what is the manufacturing benefit of the lip on the bottom of the lowers of just about every 2 port jug?

The only possible reason I could ever think of was for turbulance. Resuspend fuel but it all goes out the door when rpm's are up or you turn the saw on it's side. That leads me back to poor casting practices. Skipped casting flash before plating I guess.

 

[Stackowood]

This question is somewhat related to transfer ports. On a strato piston (MS193T shown below), what is the purpose of the cutout area just above the cutout for fresh air?

As the piston travels up and down this cutout area exposes the two transfer ports on either side of the piston, but gasses can't flow anywhere except side-to-side between the transfers. Is it for cooling the piston crown?

 

[Terry Syd]

I expect it is to get the right thermal expansion of the piston. With that much fresh air being blown across the piston from the strato function, there are going to be rather significant thermal gradients from one part of the piston to another.

Some Huskys even have a calibrated hole (different sizes for different models) that vents fresh mixture through the piston near the front of the cutaway to help maintain a more uniform expansion.

 

[Stackowood]

I expect it is to get the right thermal expansion of the piston

Sounds reasonable. Lots of fresh air coming in, but also less air/fuel mix to cool the intake skirt compared to non-strato. Thank you.

 

[Mastermind]

And weight reduction is likely considered as well.

 

[drf256]

This question is somewhat related to transfer ports. On a strato piston (MS193T shown below), what is the purpose of the cutout area just above the cutout for fresh air?

As the piston travels up and down this cutout area exposes the two transfer ports on either side of the piston, but gasses can't flow anywhere except side-to-side between the transfers. Is it for cooling the piston crown?

View attachment 71893

I'm glad u asked this. I was wondering the same.

I finally got a strato jug and slug , a 362, on my bench. It is a spent one that @dall sent me (many thanks).

The port you refer to is even smaller than what you see on your piston. I'm still trying to understand how the entire thing works as there is no true compression I can see occurring, even in the bottom cutout area. It simply rides across the strato port opening and then opens a space between the two side transfers and the strato port.

It must be using case vacuum to pull some of the air in its space back into the transfers.