Ported 026 Squish Check

[blsnelling]

Nearly every saw I've seen has a taper in the squishband, where it is tightest on the edges. This is an important factor that AM cylinders often miss.

Perhaps even more important than taper is squishband width. A wide squishband consumes power and is best for low RPM operation. A more narrow squishband is ideal for sustained, high RPM applications. This is one reason that I choose not to cut the squishband unless required for other build criteria. A pop-up piston does not alter the built in taper and width of the factory head. To widen the squishband would be to go in the wrong direction, from my point of view. A chainsaw does its work at WOT, at sustained high RPMs, relying on chain speed to do its job. Unless you're going to build a custom 2-piece head, I find it more efficient to leave the head stock and to use a pop-up piston for compression increase. There are more reasons, but this one is most pertinent to this discussion.

 

[drf255]

I'm pretty sure that tapered or untapered, the squish volume only has one way to go.

 

[blsnelling]

My atv heads had tapered squish and plug in the center of a perfect round dome ,but they ran pipes and longer stroke ,and slide carbs etc.
Had interchangeable domes in a billet head to change compression ratios also ,but they also produced close to 60 hp ,not 5 to 8 like a saw .

The science is the same.

I'm pretty sure that tapered or untapered, the squish volume only has one way to go.

It's not just directing the charge. There's a whole science behind what's happening with efficiency, boundary layer cooling, acceleration and velocity of the charge towards to CC, etc.

 

[Red97]

While we are on the topic of squish band and angle what not.

Just some vintage food for thought.

Saw had over .040 squish and 175 comp haven't touched the jug yet...

 

[blsnelling]

250cc? .040 would be very tight for a 250cc MX engine.

 

[Definitive Dave]

While we are on the topic of squish band and angle what not.

Just some vintage food for thought.

Saw had over .040 squish and 175 comp haven't touched the jug yet...

curious fool wants to know what saw that be ??
Dave

 

[Red97]

100cc

Homie

<<<<<<Top left in avatar pic.

 

[windthrown]

I need another compression tester. Mine all suck.

I'm pulling 175psi with one and 200 with the other. Mine eat schrader valves.

The one that's reading 200 has read low by 50psi on occasion.

This engine feels really tight, like 250 psi tight.

Yah, compression testers can throw the readings like solder can squish. But 250 PSI? Holy shirt! If the saws are say, 175 PSI to start, and you push them to 250, that is a 43% boost. *cough* An Rx for bearing or rod failure? Or grenading clutch bearing?

 

[jmssaws]

Yah, compression testers can throw the readings like solder can squish. But 250 PSI? Holy shirt! If the saws are say, 175 PSI to start, and you push them to 250, that is a 43% boost. *cough*

Have you started a 90cc saw with that much compression? You need to set your purse down to do it. Lol

 

[windthrown]

Have you started a 90cc saw with that much compression? You need to set your purse down to do it. Lol

Uh, no. I cheat and use a decomp... and my 660 is no where near 250 PSI. More like 160 PSI.

 

[jmssaws]

It's a battle I usually win unless it's cold out then I find a different saw. One can be injured in such battles.

 

[windthrown]

Even a no-decomp Stihl 310 can snap your wrist badly if you start them w/o a firm grip. My first light ported red light 066 did not have a decomp. Unfortunately (or fortunately for my arm) the clutch side bearing grenaded and I parted out that saw.

 

[jmssaws]

I have 2 red-light saws one of them is 240lb and I have a 394 that's over 240, my 064 is the light weight at 225, it's my daily runner well when I run one which isn't daily. Lol

 

[blsnelling]

100cc

Homie

<<<<<<Top left in avatar pic.

Wow. Now that I take a closer look at that, it doesn't look like that cylinder has a combustion chamber, rather just a pocket for the spark plug. "You've come a long ways, baby!"

 

[Chris-PA]

The squish band must be tapered because the intent is to create a constant gas velocity, and the diameter at the outside is greater than the diameter at the edge of the combustion chamber. If you try to squeeze the gas from the outside through a smaller aperture then the velocity, pressure and temperatures must increase. Therefore the height of the squish band must increase as you move closer to the combustion chamber to keep the aperture area constant.

The energy that goes into increasing the velocity, pressure and temperature of the gas comes from the fuel and is not converted directly into mechanical energy, so it is waste except to the extent that it might improve combustion efficiencies.

 

[blsnelling]

Yah, compression testers can throw the readings like solder can squish. But 250 PSI? Holy shirt! If the saws are say, 175 PSI to start, and you push them to 250, that is a 43% boost. *cough* An Rx for bearing or rod failure? Or grenading clutch bearing?

I'm with you. I've never seen a need for that much compression, at least not in a work saw. It's become a fad around here, a measuring stick of sorts, but it's not required for a very strong saw. Stock appearing or race saw? I have no idea. I don't build those.

 

[drf255]

I'm not very experienced. Not at all, and I fully admit that.

I've built several 026 at this point. The flat cut band you see in this thread with 225+ compression will make alot more torque than a stock squishband 180psi saw. With the same timing numbers.

 

[Andyshine77]

I'm with you. I've never seen a need for that much compression, at least not in a work saw. It's become a fad around here, a measuring stick of sorts, but it's not required for a very strong saw. Stock appearing or race saw? I have no idea. I don't build those.

I'm not very experienced. Not at all, and I fully admit that.

I've built several 026 at this point. The flat cut band you see in this thread with 225+ compression will make alot more torque than a stock squishband 180psi saw. With the same timing numbers.

But does that translates into more power. Torque means nothing without rpm.

Sent from my HTC One_M8 using Tapatalk

 

[jmssaws]

I guarantee you 2 saws with the same numbers but one with 185lbs and the other with 220 the 185 saw won't be anywhere close to the other.
Tourqe, power,rpm,hp, whatever you want to call it compression is power plain and simple.

 

[redfin]

The squish band must be tapered because the intent is to create a constant gas velocity, and the diameter at the outside is greater than the diameter at the edge of the combustion chamber. If you try to squeeze the gas from the outside through a smaller aperture then the velocity, pressure and temperatures must increase. Therefore the height of the squish band must increase as you move closer to the combustion chamber to keep the aperture area constant.

The energy that goes into increasing the velocity, pressure and temperature of the gas comes from the fuel and is not converted directly into mechanical energy, so it is waste except to the extent that it might improve combustion efficiencies.

Good to read something from you Chris. Havent seen you much lately. How have you been?