036 pro cut off head 300psi compression

[Nc grease monkey]

Had an 036 pro piston and cylinder that wasn't scored pretty good on exhaust side. So figured why not experiment and learn a little. So I cut the head off above the squish band and turned it down on a lathe, then turned a cylinder head for it. I figured start combustion chamber off small, it's easier to remove material instead of add it. It fired up on first four or five pulls after reassembly. Tuned it some, and let it run for a bit, then cut some with it. Finally I checked compression, 295psi, checked it with 3 different gauges. I was running 93 octane with about 32:1 mix, no predetonation, saw ran good. It wasn't turning a ton of rpms but it seemed to run good when cutting. What am I missing for this to be possible? I know it's way to much compression, be a short life span for bearings, etc.

 

[Termitebuffet]

Run it some see how it holds up....

Sawhawgz, ruining the internet since 2012...

 

[mdavlee]

Drop compression to 210-220 and pick up rpm. Parasitic losses are slowing it down now.

 

[Nc grease monkey]

Oh yeah, takes so much to turn it that it can't turn the rpms. But I've always thought that for higher compression needed higher octane fuel, or racing fuel. Or am I thinking too much in terms of 4 strokes?

 

[P.M.P.]

That's crazy watch out for shrapnel

 

[wcorey]

Also more compression = more heat

 

[wcorey]

Oh yeah, takes so much to turn it that it can't turn the rpms. But I've always thought that for higher compression needed higher octane fuel, or racing fuel. Or am I thinking too much in terms of 4 strokes?

I think it's more to do with size, these little motors just don't seem to need the octane.
Probably something with the relative size of the squish band and combustion chamber.

Though at 300psi, I dunno.
That would be around 20:1...

 

[Frank bierce]

That’s a lotta comp!!!

 

[Nc grease monkey]

One thought I had was that running compression isn't that high. It has the capability to create that kind of pressure, but if it's not getting sufficient cylinder fill then the running compression wouldn't be as high. I don't remember exactly what carb is on it, but it is definitely undersized. Put a larger carb more suitable for enlarged ports and then I see it being more of an issue. I've seen some it mentioned before about balancing everything out, my theory is this is another example of that in a way. Am I on the right track?

 

[wcorey]

One thought I had was that running compression isn't that high. It has the capability to create that kind of pressure, but if it's not getting sufficient cylinder fill then the running compression wouldn't be as high. I don't remember exactly what carb is on it, but it is definitely undersized. Put a larger carb more suitable for enlarged ports and then I see it being more of an issue. I've seen some it mentioned before about balancing everything out, my theory is this is another example of that in a way. Am I on the right track?

Probably has higher comp running than when you pull it.

I think you're still looking at it in 4 stroke terms, a trap I often fall into.
This one isn't pulling a bunch of vacuum through the whole down stroke even if the throttle plate is closed, when pulling it over it can just fill the cylinder with air through the exhaust if the intake is blocked.
May not run that way but doesn't effect comp.
If that makes sense...

 

[Chainsaw Jim]

I would be very tempted to see if it'll run on diesel.

 

[Al Smith]

It might fire off using K1 kerosene if the combustion chamber were hot .I think what might happen with a heavier fuel like #2 diesel is it would not vaporize and thus you'd be tansfering liquid fuel.Probably hydro lock it . BTW the old John Deeres using kerosene only had about 75 PSI compression .They had to start them on gasoline and wait for the water temp got up to about 180 -190 before they switched fuels

 

[drf256]

Not sure that there is a direct correlation between PSI and CR. I’ve been told that even 250 psi is around 8:1 CR on average. I can’t prove that though.

Also, I’d figure dynamic compression to be less than static. Whatever it is, these small saw motors don’t seem to have preignition problems.

Think about what compression does. It adds heat, converting kinetic energy to potential energy. Energy can never be destroyed, it can only change forms. The compression just preheats charge so that it can ignite and burn fast and evenly. You only need so much. If the chamber and charge are hotter, less comp is needed. This is also why a cold saw will start better without the decomp button pressed.

So you are robbing power from the saw to make power, and the balance is where the secret lies.

Chamber design, plug placement, ignition timing, port timing, fuel, etc will all have an effect on how much compression is optimal for charge burn optimization.

The aluminum chunk that you made the head out of is a big heat sink. I’m not sure what yours looks like, and wether or not you added cooling fins. I’ve always suspected that some power/rpm in a 2 piece head saw comes from keeping heat in the chamber and using less static comp than would be needed in a standard chamber with cooling fins.

Anyway, can we see your head? There’s nothing wrong with a little head...

 

[mdavlee]

Measure the chamber in cc. Thwressoftware out there to figure CR.

 

[Backtroller]

I see you have the flocked filter on there. I’d run it without a filter and see what it does. That flocked filter was a total bottle neck on my ported 036. I Picked up some rpm with a winter mesh one. However, I’m building a higher flowing steel mesh one to see what it will do.

 

[67L36Driver]

Compression ratio is directly proportional to brake thermal efficiency.

Higher the CR, the more work from the same amount of fuel.

 

[wcorey]

14.7 x CR = PSI

 

[Nc grease monkey]

Not sure that there is a direct correlation between PSI and CR. I’ve been told that even 250 psi is around 8:1 CR on average. I can’t prove that though.

Also, I’d figure dynamic compression to be less than static. Whatever it is, these small saw motors don’t seem to have preignition problems.

Think about what compression does. It adds heat, converting kinetic energy to potential energy. Energy can never be destroyed, it can only change forms. The compression just preheats charge so that it can ignite and burn fast and evenly. You only need so much. If the chamber and charge are hotter, less comp is needed. This is also why a cold saw will start better without the decomp button pressed.

So you are robbing power from the saw to make power, and the balance is where the secret lies.

Chamber design, plug placement, ignition timing, port timing, fuel, etc will all have an effect on how much compression is optimal for charge burn optimization.

The aluminum chunk that you made the head out of is a big heat sink. I’m not sure what yours looks like, and wether or not you added cooling fins. I’ve always suspected that some power/rpm in a 2 piece head saw comes from keeping heat in the chamber and using less static comp than would be needed in a standard chamber with cooling fins.

Anyway, can we see your head? There’s nothing wrong with a little head...

At least dinner and a movie first lol

The head that had that much compression has since been modified. It had a 2*taperup to the plug and and then an 80* taper up to the plug threads. 1"thick x 3"round aluminum, no cooling fins, recessed it all about 1/4" to fit snug around the cylinder and prevent it from walking. In trying to lower compression I re angled the combustion chamber and didn't realize the auto feed was set and it gouged the head, making it scrap. If need to I can try to make a drawing of it.

 

[Nc grease monkey]

As far as heat on the outside of the head, used it to cut some wood at thanksgiving, about 50*f pretty day, laser temp gun said 180-190ish on outside of the head. I'd use it for 5-10minutes, cut it off and move wood around for 10minutes and cut some more. Cutting locust 16"at base and some oak that was about the same size

 

[Nc grease monkey]

Pic of piston after running a tank through it
Pic of piston just now after wiping it off. Timing numbers on case, not sure on accuracy of them so they could be a little off, need a better degree wheel.
Ex 95
T 110
In 80