Jonsered 920 w/GD - Interesting numbers

[Deputyrpa]

I'm interested in some comments. Really interested!

So I was able to obtain an OEM "open" transfer cylinder for the Jonsered 920 to replace the closed port cylinder. I put in a new VEC piston in it and slapped on the open port cylinder with Motoseal.
The numbers I got on the wheel are:
Exhaust: 118
Transfers: 128
Intake: 72

10 dungarees of blowdown is some interesting stuff. I will have it back together tomorrow and see how she runs. I'm guessing she'll have some grunt, but not rpm. What do you all think about those numbers?

 

[Ketchup]

I would think the same thing. Those are your numbers without a gasket?

That’s not many dungarees. Did you take any photos of the case, transfers, piston? What’s your squish?

920/930 have a good reputation. Interested to see what it tachs out at.

 

[cus_deluxe]

are you positive on the exhaust height? 124 degrees of duration is not much.

 

[Deputyrpa]

I would think the same thing. Those are your numbers without a gasket?

That’s not many dungarees. Did you take any photos of the case, transfers, piston? What’s your squish?

920/930 have a good reputation. Interested to see what it tachs out at.

Yes, that's just with Motoseal. I took the cylinder off again to compare it to the original (they're both OEM) cylinder. I'm having some coffee, and I will compare them with a ring and caliper in a couple minutes. Pics too.

 

[Deputyrpa]

are you positive on the exhaust height? 124 degrees of duration is not much.

Yeah, I'm by no means an expert, and those number even raised my eyebrows. I recalibrated the wheel with a piston stop each time and re-measured it several times to be sure.

 

[Deputyrpa]

Here's a pic of both cylinders, and each one separately showing the relationship between the transfer port and exhaust port.

 

[Deputyrpa]

Here a pic of the cylinders with a ring pushed in with a piston, rotated 180 to keep perspective. the bottom edge of the rings are positioned at the opening edge of the transfers. The measurements between both cylinders are idenitcal. So the area from the top of the exhaust ports to the bottom side of the rings is all that is open when fuel begins to come in. It's less than halfway.

 

[Ketchup]

Just from photos the transfer upper to exhaust height looks pretty normal. I would run it before grinding anything.

 

[Deputyrpa]

Just from photos the transfer upper to exhaust height looks pretty normal. I would run it before grinding anything.

I didn't plan on grinding it. Those OEM cylinders are hard to come by, and my mistakes are easy to come by. lol! I'm interested to run it too. With the OEM cylinder with a gasket, it was a torque monster. The reason I obtained that open cylinder was because Jonsered found that port design for the 930 gave more power, and retrofitted the later 920s with that design. With a gasket delete, it should be even better. The squish with the gasket delete is .031

 

[ErnieG]

I didn't plan on grinding it. Those OEM cylinders are hard to come by, and my mistakes are easy to come by. lol! I'm interested to run it too. With the OEM cylinder with a gasket, it was a torque monster. The reason I obtained that open cylinder was because Jonsered found that port design for the 930 gave more power, and retrofitted the later 920s with that design. With a gasket delete, it should be even better. The squish with the gasket delete is .031

One of my 930's has a gasket delete and it's a great running saw for sure ...they take a gasket delete very well !

Ernie

 

[ErnieG]

I didn't plan on grinding it. Those OEM cylinders are hard to come by, and my mistakes are easy to come by. lol! I'm interested to run it too. With the OEM cylinder with a gasket, it was a torque monster. The reason I obtained that open cylinder was because Jonsered found that port design for the 930 gave more power, and retrofitted the later 920s with that design. With a gasket delete, it should be even better. The squish with the gasket delete is .031

If you have a local machine shop or your handy with a grinder you can turn that 920 cylinder into a 930 ...I've done that with two of mine .
Ernie

 

[Deputyrpa]

If you have a local machine shop or your handy with a grinder you can turn that 920 cylinder into a 930 ...I've done that with two of mine .
Ernie

Besides the different transfers, what is the difference?

 

[ErnieG]

Besides the different transfers, what is the difference?

That's it's ...just open transfers

Ernie

 

[Deputyrpa]

That's it's ...just open transfers

Ernie

See my pics above. I have both types of cylinders.I'm installing the open one with a gasket delete.

 

[ErnieG]

See my pics above. I have both types of cylinders.I'm installing the open one with a gasket delete.

That would be my choice also !

Ernie

 

[Redfin]

Curious, are you referring to “open” and “closed” transfer by the lower tunnel opening to the case?

 

[Deputyrpa]

Curious, are you referring to “open” and “closed” transfer by the lower tunnel opening to the case?

Yes, for lack of a better term. But they're not "open" as in trenched.

 

[drf256]

Were the stock pistons the same, or was one windowless? Are the cutouts in the case the same on both models?

What Jawney is trying to say is that “open” transfers refers to them having no bridge between the lower and upper. They are just open, with a full view of the outer tunnel and the piston itself acts as the inner tunnel.

What you have there is a partial/hybrid case fed and side fed lower transfer on a closed transfer jug.

 

[Deputyrpa]

Were the stock pistons the same, or was one windowless? Are the cutouts in the case the same on both models?

What Jawney is trying to say is that “open” transfers refers to them having no bridge between the lower and upper. They are just open, with a full view of the outer tunnel and the piston itself acts as the inner tunnel.

What you have there is a partial/hybrid case fed and side fed lower transfer on a closed transfer jug.

The pistons were the same and the case notches are as well. Yes, I know they are not open, so I put quotations around the word "open" for simple distinction's sake.

Spike60's Saw College on afleetcommand's YouTube channel provided an interesting history of Jonsered's 900 series progression to this notched cylinder design.

Irrespective of the difference in the jugs, the timing numbers are still quite different and interesting. 10 degrees of blowdown is just odd, but I'm no enginerd. If I could get the starter pawl springs to do their thing, I will run it today. Thanks.