Is compression king?

[Mastermind]

Meh.....too much trouble. Plus, where would we put those posts?

 

[jmssaws]

Meh.....too much trouble. Plus, where would we put those posts?

I got a idea where they should go.
Swinging 40 golf clubs

 

[MustangMike]

Randy, do you remember what compression you used to go for when you ported the 362? Just curious as that seems to be about my easiest to start saw.

 

[MustangMike]

Aren't you glad I did not post about disc compression???

Hey, the guy obviously had an attitude even if you said nice things to him, the derail was just and excuse to be himself.

 

[Mastermind]

Randy, do you remember what compression you used to go for when you ported the 362? Just curious as that seems to be about my easiest to start saw.

Not really Mike. Over the last three years or so I've been using a little less compression each year it seems. I've sorta given up on compression gauges, and just keep good notes on what works.

 

[Stihlbro]

Here is one compression question that gets me. Can you increase cranking compression without changing squish clearance? Here is an example, if your filling the crankcase more from the intake, does it put more up top to compress? If you lessen intake duration and that increases crankcase compression, again you are putting more to the combustion chamber, does that make more cranking compression?

A lot of variables with different saw manufactures but here is what I don't understand, screw a gauge in the saw, first pull what does the gauge read? Does 90 psi indicate great cylinder fill? Does 60 psi mean your leaving something on the table? I only base this from my experiences, and it's just one of those things that is foggy in my mind.

For discussion......

 

[Stump Shot]

Thought I was kind of alone on this, but I only break out the gauge when I think an engine may have low compression as a diagnostic. When I'm setting squish etc. in a rebuild, it never really crossed my mind to check as I knew there was enough compression. Maybe I'm too easy going in this respect?

 

[Time's Standing Stihl]

Here is one compression question that gets me. Can you increase cranking compression without changing squish clearance?

For discussion......

Won't a pop-up achieve that?

 

[Stihlbro]

A pop up would increase squish but alter porting.

Edit: I'm talking squish clearance. Not squish band.

 

[Time's Standing Stihl]

Roger! Had me confused for a min!

 

[Red97]

Here is one compression question that gets me. Can you increase cranking compression without changing squish clearance? Here is an example, if your filling the crankcase more from the intake, does it put more up top to compress? If you lessen intake duration and that increases crankcase compression, again you are putting more to the combustion chamber, does that make more cranking compression?

A lot of variables with different saw manufactures but here is what I don't understand, screw a gauge in the saw, first pull what does the gauge read? Does 90 psi indicate great cylinder fill? Does 60 psi mean your leaving something on the table? I only base this from my experiences, and it's just one of those things that is foggy in my mind.

For discussion......

Believe that would be dynamic compression.

I think the main variables effecting cranking (static) compression are ring seal, and total chamber volume above the piston.

Never paid much attention to first pull fill. My tester has 2ft of hose. That adds a good bit of "volume " to a 1 pull reading on 50cc saws. But might be something to look at.

 

[Sty57]

I have cases but they need split unless you're talking 365 poulan

I might have a set split if you don't mike.

 

[Stihlbro]

I Agree, it's just thoughts that go thru my mind at times. Using static is a reference in correlation to running dynamic. I don't know all the answers but I do think about stuff. You made a post the other day in regards port timing and relating that to how a saw performs to help under stand. I get it and am right there with you.

 

[MustangMike]

We have no way to measure dynamic (would need to be measured at WOT), but the more air / fuel that comes in the cylinder, the higher it will be. So a good porting job (or even deleting the base gasket to increase intake duration) should improve dynamic compression.

Just like a SC or Turbo engine need lower compression ratios than a naturally aspirated engine.

 

[Brush Ape]

Cash is King, but compression is ok. Just don't pack the fudge too tight.

 

[Brush Ape]

How about we'll just call ya, "Ski."

 

[David Young]

550xp has the nicest factory combustion chamber I've seen, IMO. Its factory machined. Small chatter marks can be seen. Tho I'm no a fan of the small spark plug. My 50 screnches don't fit.

View attachment 35857

So here are a couple benefits to this style

Charge trapped in the squish (end gasses) are largely Unburned. They subtract from the fill of the cylinder. The charge available to do work.

The wide shallow combustion chamber is easier to completely clean out during scavenging. Imagine the charge hoes up the back wall and swoops through the chamber before hitting the exhaust. If the combustion chamber was smaller in diameter but deeper it would not be as easily cleaned out.

On the flip side we know the benefits of higher squish velocity.

 

[Red97]

So here are a couple benefits to this style

Charge trapped in the squish (end gasses) are largely Unburned. They subtract from the fill of the cylinder. The charge available to do work.

The wide shallow combustion chamber is easier to completely clean out during scavenging. Imagine the charge hoes up the back wall and swoops through the chamber before hitting the exhaust. If the combustion chamber was smaller in diameter but deeper it would not be as easily cleaned out.

On the flip side we know the benefits of higher squish velocity.

I would imagine squish velocity has an ideal range too..

More is not always a better thing.

 

[Red97]

We have no way to measure dynamic (would need to be measured at WOT), but the more air / fuel that comes in the cylinder, the higher it will be. So a good porting job (or even deleting the base gasket to increase intake duration) should improve dynamic compression.

Just like a SC or Turbo engine need lower compression ratios than a naturally aspirated engine.

Dynamic compression is calculated not measured.

But needs a few exact measures taken from the jug/saw

 

[junkman]

.one of my 046 jugs has a real wide squish band ,it washes the edge of the piston ,but makes more power than any other 70-80cc i have so i do not think it hurts performance any .