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390xp squish.


Gerbutt
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If I'm understanding you correctly that sounds pretty good to me. The idea of squish band is to squish the gas mixture towards the centre of the cylinder, to promote good turbulence and rapid combustion. 
What you don't want is "end gas" stuck down the end of a narrow passageway where the combustion struggles to get to, that would be a waste of energy and won't do much for performance. You will get this if the squish band is too deep (radially).
Edit: whether 3mm is the right number is one for the experienced tuners, I think I would punt for a slightly bigger number,   say 5 or 6mm, depends somewhat on bore size and the combustion chamber shape.


Thanks for your input bud, that was nice and easy to understand and makes total sense [emoji106] It may be a tad larger than 3mm, it was just a guess looking at it.

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I only deal with inches when it comes to squish. I'm just dumb that way. The usual accepted spacing is .020", or near enough. As mentioned earlier, the fuel is 'squished' out of this band and into the combustion chamber. Often, whatever fuel is left in this band does not combust fully. So the closer the tolerances up to about .020", the better. Any closer and you are risking damage, especially if you are using something like a £20 tool from a former British Colony in Asia VS something like an expensive Mitutoyo or SPi Micrometer to gauge the depths.

 

If you are considering doing port work, you may want to first spend time learning on something like a used, worn out two stroke motor VS a functioning 390XP top end. I began my porting days working on <$100 Echos that needed repair(so I could see the results VS using a destroyed engine). I would also spend a good amount of time on one of the outdoor power equipment forums porting threads beforehand to better grasp the nuances of porting. I learn something new there every time I visit. I don't build ultimate performance engines, and nor do you have to, but its good to know where to spend your limited time and efforts for maximum returns, let alone simply preventing making mistakes(like ring ends, free porting, bad timing, damaged plating, clearance issues, etc etc etc). In general, with porting, less is more.

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I think .020 is the same as 0.5, I will aim a little higher than that because my micrometer is a cheap one. I’ve just got the gasket paper through and that’s meant to be certain thicknesses so will check them on that.

I’ve only done little changes on this saw, just a bit wider on exhaust and less so on intake, not raised or lowered or touched the transfers apart from smoothing out lips. I would like to match the transfers to the case but finding it hard to get an answer whether I’ve marked the right part in the picture above. I’m sure it is right because there’s nothing else that doesn’t match, but want to be sure first. I do t expect much from the changes, certainly nothing like the change that Spud made to my 346xp, that things a little beast!!

Thanks for your wise words of warning bud [emoji106]

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0.02" is 0.5mm, no drama. You need to move the gasket from the cylinder to the crankcase and see how different the cylinder lower transfers are to the crankcase opening and then gauge if you can open the lowers to improve flow and resistance. Some high revving saws don't like too much opening as it kills transfer velocity but from your picture, you are looking at small amounts

In an ideal world, you could split the crankcase and attach one half to the cylinder and see and feel the differences that can be blended but don't get that chance too often.

Like Wes says, ring ends, free porting, skirt width, bevels are all dangerous areas to the uninitiated and need to be understood before you grind metal on an expensive top end!

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Thanks for the advice there Spud. I don’t think I fully understand what you are saying, but would you say that the little triangle on the cylinder transfer that I marked in yellow is the part that needs blending? From what I can see everything else matches perfect, according to the gasket.

I’ve just made a new gasket so it’d be great to grind that part before putting it back together.

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Hiya Spud, what sort of top rpm would you expect from a 390xp? Just checked mine and it’s at 11900, does that seem a little low? The plug colour looks right and it’s nice and responsive. It has the blue coil, would that be making the readings hard to find? Thanks.

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1 hour ago, Gerbutt said:

Hiya Spud, what sort of top rpm would you expect from a 390xp? Just checked mine and it’s at 11900, does that seem a little low? The plug colour looks right and it’s nice and responsive. It has the blue coil, would that be making the readings hard to find? Thanks.

Spud ported my 390 some years back . I think it has the blue coil but obviously the limiter never cuts in when in wood . It definitely  revs higher than standard  and is more responsive all round .

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Spud ported my 390 some years back . I think it has the blue coil but obviously the limiter never cuts in when in wood . It definitely  revs higher than standard  and is more responsive all round .


I can remember watching the vids of it cutting if it’s the same one, sounded like a right beast!

After a bit more reading I think they are usually at 13,000rpm. Strange thing with mine is I’ve just ported it and I’ve not touched the H jet but it’s at 11,900. All I can think is when I did the duel port muffler a couple of years ago and I turned the H jet up a tad, I must have been scared and gone way to far - but it’s always seemed to cut fairly well. I think next week I’ll go down the yard with the tach and have a play and then stop the saw when flat out in a cut a check the plug.

At least after all this tampering it’s still going [emoji3]
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