Stihl 090 Max Squish?

[Dibbs]

I've pulled the cylinder off my 090 and have decided to send it to an engineering firm to mill/lathe the bottom end to bring down my squish.
It is currently at 2mm (0.080") with 0.5mm (0.020") Gasket and I was wondering what is a reasonable amount to be removed for a saw with a 66mm piston?
I've also removed the decompression valve and brazed it shut as I want to keep all the compression inside my cylinder.

 

[tdi-rick]

From Bell's book he recommends 0.7-0.9mm for 100-125cc engines and 1.0-1.4mm for 175-250cc engines , so I'd err on the side of caution and try for 0.9/0.030" at an absolute minimum, and maybe 0.035" to be safe, purely because of the bore size.

 

[Dibbs]

Thanks tdi-rick, I just downloaded the book and it makes for some interesting reading, if anyone else is interested it can be found here.

 

[William Balaska]

Why are you interested in decking the cylinder? you could just eliminate the gasket, once over 180 psi you will have to run race gas to avoid preignition. The 090 is pretty much king stock, hard to believe you need more!

 

[parrisw]

Why are you interested in decking the cylinder? you could just eliminate the gasket, once over 180 psi you will have to run race gas to avoid preignition. The 090 is pretty much king stock, hard to believe you need more!

Once you're addicted to POWER, too much is never enough!!!!!!!

 

[Outlaw5.0]

Why are you interested in decking the cylinder? you could just eliminate the gasket, once over 180 psi you will have to run race gas to avoid preignition. The 090 is pretty much king stock, hard to believe you need more!

180 psi is fine on premium fuel. I switch to 100 octane unleaded race fuel at 200 psi. Sunoco 94 Ultra is really good for those 170-195 psi saws, but just normal 92-93 octane is fine.

 

[Lakeside53]

These are USA octane - meaninlless in Ireand (if it's the same as the UK) -they use a different system. Our 93 is about the same as UK 98.

I also question pumping up such an old design... I sure wouldn't push the rpm too much.

Brazed the deco to do what? when it's shut.. well.. it's shut...

 

[Dibbs]

These are USA octane - meaninlless in Ireand (if it's the same as the UK) -they use a different system. Our 93 is about the same as UK 98.

I also question pumping up such an old design... I sure wouldn't push the rpm too much.

Brazed the deco to do what? when it's shut.. well.. it's shut...

Thanks for the replies.
Sorry for the time lag, I've been busy at work for the last few days.

98 RON is Premium here and Northern Ireland is part of the United Kingdom.
I have no intention in winding the RPM up beyond 11,000 for tuning and 8,000 with the wind vane free. The thought of that steel flywheel becoming dislodged at 13,000 RPM is a very scary one!
I brazed the decomp as an experiment to see if any compression was being lost through it soon after I bought my compression tester.
My compression was at 140 psi before brazing and was 140 psi after brazing, so, for the benefit of those who wondered about compression loss through decomp valves, don't! (not at 140 psi anyway, I'll retry the experiment after milling has taken place, with new rings fitted and a fresh decomp valve)

 

[William Balaska]

Sounds like a new piston is in order as well at 140 psi

 

[harrygrey382]

I missed this one first time... Wow Dibbs your're really looking for some grunt aren't you! Never been a fan of decomps personally - plugged the 076 in fact (although found it was the little sealing ring, not the valve itself).
So you got the cylinder booked in?

 

[Dibbs]

Sounds like a new piston is in order as well at 140 psi

The piston is fine, it's has one slight score down it and by my best guess is that I'd blown something back into the cylinder when cleaning the saw with compressed air.

So you got the cylinder booked in?

It's already with an engineering firm, 1.5mm coming off it which will leave me with about 1mm to "play" with when sorting out gaskets to suit.
1.5mm amounts to 5.1cc removed from the head volume, if this ends up being too much I can remove a little from the head while giving it a better angle for mixing/combustion.

 

[Rockfarmer]

The piston is fine, it's has one slight score down it and by my best guess is that I'd blown something back into the cylinder when cleaning the saw with compressed air.



It's already with an engineering firm, 1.5mm coming off it which will leave me with about 1mm to "play" with when sorting out gaskets to suit.
1.5mm amounts to 5.1cc removed from the head volume, if this ends up being too much I can remove a little from the head while giving it a better angle for mixing/combustion.

Hey someday we love to see some pics of your milling/090/Ireland!! , Brenainn

 

[flotek]

keep in mind bells book is very outdated compared to what we now know about 2 strokes engines but many of these principles apply today especially the squish clearance ..the dome shape volume angle width and other parameters are just as important i would shoot more for the compression than the squish clearance becuase unless it was rechmabered you will likely push the static compression well over the useful limit to attain a .030 clearance if its at .080 over now.there is very little modern 2 cycle technology used in todays chainsaws like most people think

 

[Dibbs]

Pinkey and Perky!

Stihl 090, new rings, pop top piston, decked cylinder, 0.9mm squish, no decomp, 210 psi cold, considering deeper gasket and Avgas!

It's so hard to start that when it kicked back half way through a stroke, it ripped the black plastic parts of the elastostart rope and took the cord back into the housing!

I did get it to start now and then but it's pinking badly (strancely enough!) on 98 RON petrol (Premium Gas to the Yanks!), it also hurt my arm and fingers a number of times.
Is the compression likely to rise much more as the new rings seat?

Sorry there are no pics, I was so intent on geting it together I wouldn't take the time to get the camera from the house!
I'll post a few pics when I take her down again for the new gasket trials.

 

[Nitroman]

I think you'll be better off in the long run to pull the jug and re-install the decomp.

 

[harrygrey382]

Stihl 090, new rings, pop top piston, decked cylinder, 0.9mm squish, no decomp, 210 psi cold, considering deeper gasket and Avgas!

It's so hard to start that when it kicked back half way through a stroke, it ripped the black plastic parts of the elastostart rope and took the cord back into the housing!

I did get it to start now and then but it's pinking badly (strancely enough!) on 98 RON petrol (Premium Gas to the Yanks!), it also hurt my arm and fingers a number of times.
Is the compression likely to rise much more as the new rings seat?

Sorry there are no pics, I was so intent on geting it together I wouldn't take the time to get the camera from the house!
I'll post a few pics when I take her down again for the new gasket trials.

If only I could rep you...
So when it's done and perfected you gonna time it with a 72" bar burried?

 

[Dibbs]

I have another decomp on the spare 070 cylinder which can be used and I intend to use it often!

I'd have no use for a 72" bar!
I barely have need of the 36" one on it either!

I'm figureing 180-190psi would be sufficient so I'll make different thickness gaskets untill I find which suits.

 

[William Balaska]

If you don't run race gas in it I garuntee you at 210 psi it will come apart due to preignition, anything over 180 is going to need it

 

[rupedoggy]

I don't think it is pre-ignition that causes the trouble. I would call it knock or pinging. That would be the uneven flame front travel across the piston. Pre-ignition, to me, is lighting off before it should, such as when it gets hot and a spot ignites the mixture before the plug fires.

 

[parrisw]

If you don't run race gas in it I garuntee you at 210 psi it will come apart due to preignition, anything over 180 is going to need it

I don't know about that?? But, over 180 you should be ok on pump gas. I got a 372 that gets 185 hot. Its fine on pump gas.