Stihl 090 Max Squish?

[Brmorgan]

Dibbs, have you taken the time to create a personal will? As much as I would LOVE to see a video of this thing in action, I fear for its safety and yours. Personally I'm not sure that any performance benefits gained over about 180 lbs would outweigh the extra wear and tear on the engine in the long run. It would be fun to run one done like this for a little while though!

 

[Dibbs]

Dibbs, have you taken the time to create a personal will? As much as I would LOVE to see a video of this thing in action, I fear for its safety and yours. Personally I'm not sure that any performance benefits gained over about 180 lbs would outweigh the extra wear and tear on the engine in the long run. It would be fun to run one done like this for a little while though!

No will at this point, my wife gets it all automatically, if anyone wants to call "dibbs" on any of the saws I'm sure she'll be only too happy to let you have them, infact, she would be happy if you already had them rather than sitting in the garage!

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 agree with the both of you on the 180psi+ being of no real benefit with regards to power/torque and actually severely lessening lifespan (of it and me!), heat issues, preignition and octane rating.

My reason for posting the (CRAZY) numbers was to let you all know the project was commencing, I'm not finished yet!

 

[Crofter]

Rupedoggy is correct on this. Basically I believe most guys that use the word "pre ignition" are thinking of detonation. They are entirely different things though often related. If anything where preignition actually occurs it might blow the crankcase seals out. Detonation happens toward the end of the spark ignited combustion event and usually is triggered by cylinder temperature, lean mix, high pressures and lugging.

There really is too much difference in operating rpm and physical size of the cylinders to equate an acceptable compression ration in a 372, to what will fly in an 090. If you want to take the physical relationship to extremes it makes it easier to understand. The old stationary engines that operated at 600 or so rpm had to reduce compression ratio to around 5 to one to prevent detonation knocking. The flame propagation rate of gasoline and air, is a constant but the total time of combustion, chamber dimensions etc. varies greatly with size of engine, rpm and piston speed. If a small engine had only to run at a fixed rpm of 15,000, probably a fuel of 50 octane rating would not detonate at 250 psi static compression but you surely cannot get by with it in a larger cylinder at lower rpms.

The compression pressures kicked around on the forums the last few years for the new high rpm saws cant be used on the thirty year old design thumpers.

 

[William Balaska]

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.

Ok I'll call it compression ignition, however the excessive compression causes the mixture to ignitie before it should thus this is one type (not all types of) pre-ignition as the other is quoted above anyway it will come apart without race gas in my opinion.:bang:

 

[rupedoggy]

Excuse me William. How much compression does it take to self ignite gasoline? If you did in fact make a saw that did this you could not shut it off with the switch. You would have to take away the fuel. I have built some stout compression ratios in saws. The only ones that kept running without the switch, in the on position, were fed with nitro methane. I would challenge you to try and build a machine that does what you say, auto ignite gasoline mixture. BTW just for information, I have an 090 that has tremendous compression (removable head) and .010 squish and it has not blow apart yet. It does get hot quickly and takes a man with good technique to start.

 

[Dibbs]

Relax guys!
The 210psi build was never going to stay!
Squish is now at 1.1mm and compression at 190psi and it has a working decomp valve again.
I think I'll keep it like this for a while and keep an eye on compression readings over time and use.

 

[Nitroman]

Relax guys!
The 210psi build was never going to stay!
Squish is now at 1.1mm and compression at 190psi and it has a working decomp valve again.
I think I'll keep it like this for a while and keep an eye on compression readings over time and use.

Dibbs, you now have a choice:

1. An action flick showing some serious flying chips...or
2. A quick hangin'

Which one?

 

[Brmorgan]

Excuse me William. How much compression does it take to self ignite gasoline? If you did in fact make a saw that did this you could not shut it off with the switch. You would have to take away the fuel. I have built some stout compression ratios in saws. The only ones that kept running without the switch, in the on position, were fed with nitro methane. I would challenge you to try and build a machine that does what you say, auto ignite gasoline mixture. BTW just for information, I have an 090 that has tremendous compression (removable head) and .010 squish and it has not blow apart yet. It does get hot quickly and takes a man with good technique to start.

Isn't compression-induced combustion in gasoline engines often referred to as "dieseling"? I've had a couple small engines run-on for a few seconds like this after the kill switch is thrown, but I've never seen one run indefinitely until the fuel supply is gone. Not to say it couldn't happen, but like you say it's unlikely with gasoline without unreal compression numbers. My brother's RC truck's tiny engine runs on something like 75/25 nitro mix, and it uses a little glow plug and runs on compression ignition. But, it also needs a little electric starter to buzz it up a couple thousand RPM before it'll fire up.

By the way, I think you should take a few pics and ideally a video of that removable-head 090. I've only ever seen two tiny pics of Wayne Sutton's specimen. Is yours one of the 10 c.i. engines or the regular displacement but with a removable head?

 

[Dibbs]

Dibbs, you now have a choice:

1. An action flick showing some serious flying chips...or
2. A quick hangin'

Which one?

It won't be number 1 as i've built it for power/torque rather than RPMs
and the translation of number 2 has got lost somewhere over the Atlantic!