2065 exhaust porting help

[danders7]

I've been lurking here for the past month or two with an interest in doing some mildish port work on my Jonsered 2065 that I reasoned could use a little freshening up. I picked up a 50mm Meteor piston and jug and have worked on the lower transfers and the intake without too much issue, but I could use some help on the exhaust. Unlike the intake, which was pretty symetrical, the exhaust roof has a peculiar slant to it. Here's a slightly fuzzy picture of what I'm talking about:

The right hand side of the port opens at around 102.5 atdc, and the left side of the port opens a full 2 degrees later. Squish without the base gasket is around .030, so I'm trying to avoid raising the roof any more than I have to, if at all. If I were to try and raise the left side to match the right, any advice as to how much additional arc would need to be in the roof to avoid troubles snagging the ring?

Dave

 

[danders7]

Ok,let me ask this a little differently. I don't like the looks of the as-delivered port roof, but as goofy as it is I'm wondering if I might be better off leaving it as-is rather than risk too flat a port roof if I were to try and even things out. I was originally shooting for 160 deg on the exhaust side, so I do have a little room to raise the roof a bit.

Do any you more experienced guys see a problem with leaving the port roof alone? I know the shape is less than desireable, it almost looks like the port was cast upside down from what was intended. I just don't know how to correct it without ending up with a 180 degree exhaust.

 

[parrishturf]

Its been a while since you posted this. Have you made a decision?

Is this the stock jug or after market?

Are you sure about the squish, thats huge? (and may help later)

I have worked with a few jonsered saws and so far the squish has been way large on all of them.

How thick was the gasket?

Also the arc across the top of the port to me is less important than a smooth chamfer and a balanced shape.

It is posible to lap the base of the jug down .010 or .012" to gain a little room and compression.

 

[AUSSIE1]

You need to fix that roof. It needs to slightly open earlier in the centre. If your ex ends up around 160^ bring the In duration upto 158 if you are going to get the barrel turned, if not 160. To get alot nicer torque curve, you really should be doing some upper trans work.

 

[blsnelling]

+1. That is one of my main complaints with AM cylinders. They just can't seem to get the shape of the exhaust port right. IMHO, raise that roof as much as required to shape it correctly. If it ends up too high, then it was junk to begin with. It's just that important.

 

[danders7]

Thanks guys. I had re-measured the port opening and I think it's actually less than 102 atdc, and not having a way to lower the cylinder I was pretty certain I didn't have enough room to play with raising the roof so I left it alone. I've got about 6 or 7 tanks of fuel through it now and it doesn't run bad, but I've been wondering how much the combination of high squish, low transfers and wonky exhaust port are costing performance wise.

I ended up raising the piston skirt on the intake side by nearly .030 to get the intake duration in line with the exhaust - I wanted to hold open the option of having the cylinder turned down at a later date. Am I correct in thinking the cylinder could be dropped by .030 and with a new piston the intake timing would stay unchanged, and then the outside of the piston crown could be turned down to achieve desired squish? As I mentioned, I don't have any way of doing this myself so I'd have to find someone who can.

I saved off what looks like a nice example of a port roof from parrisw here http://www.arboristsite.com/chainsaw/182935-3.htm and when I get a bit of time I was going to try mocking up what the port opening should look like on paper and try to get an idea of how much material would have to be removed to correct it. If .030 won't do it, it'd kinda be nice to find out before the machine work is done.

 

[steve316]

Exhaust roof shape

Danders7;brad knows what he is talking about on the ex. port shape.If you are going to make a pop-up piston that will give you what you can cut off it, plus your gasket and what you need to remove to set your squish.as long as you keep ex. openning at 100 degrees or 101 you will end up with160 or 158 duration,the amount of blowdown may be more than you want.just a thought. good luck Steve.

 

[AUSSIE1]

Danders7;brad knows what he is talking about on the ex. port shape.If you are going to make a pop-up piston that will give you what you can cut off it, plus your gasket and what you need to remove to set your squish.as long as you keep ex. openning at 100 degrees or 101 you will end up with160 or 158 duration,the amount of blowdown may be more than you want.just a thought. good luck Steve.

Huh? What are you cutting? Any wonder people get confused with simple instructions!