Is compression king?

[jmssaws]

Transfers and less compression make rpm
My fastest tuning saws have the lowest exhaust and least intake duration and more bd.
I use to find the opposite of that.
Confusing at times

 

[jmssaws]

What you gettin at Joey? I could make a 20 minute video of the saw cuttin but would get kinda boring to watch

That's what I want to see,a 20 min video of you cutting cookies with a beer break in the middle.

 

[Red97]

Transfers and less compression make rpm
My fastest tuning saws have the lowest exhaust and least intake duration and more bd.
I use to find the opposite of that.
Confusing at times

"Lowest exhaust " as in the jug? Or 98 vs 100?

Curious about the long intake duration tho.

 

[Deets066]

That's what I want to see,a 20 min video of you cutting cookies with a beer break in the middle.

Just stop by any given Sunday

 

[jmssaws]

"Lowest exhaust " as in the jug? Or 98 vs 100?

Curious about the long intake duration tho.

Physically lower in the cylinder
Like 102 instead of 98

 

[Stihlsmoking]

Is compression King ?

Not if your running twin turbo's ?

Wait are we still talking about chainsaws ?

 

[Jon1212]

I'm a believer there is more of a formula to determine what you need.
1.) What is your elevation?
2.) What are you cutting?
3.) What kind of chain you using? Bar length?
4.) How you operate a saw?
5.) What are your physical limitations?

Most saws that are factory hot rods are harder to get gains out of versus the typical pro rated saw. They generally have more cranking compression. In other words, if you tell me you got 200 plus compression and are four stroking at 15k plus, then bring the saw to Mastermind in December and let me run it.

I often relate saw experiences to automotive scenarios. I remember the day of the 5.0 mustangs and lingo and how it changed thru that era. I've been thru the truck scenes too. The moral of this as we all want a truck that pulls like a Mack, rides like a Cadillac but run like a Super charged mustang. We saw enthusiast what a small saw that pull like ole home lite, speed of a race saw but yet handle like limbing saw.

Ok, I may have ranted for a bit but to answer the question of cranking compression, in all my experiences the more compression is not always most powerful.

@Mastermind, I believe @Stihlbro is making fun of your legs, again.......

 

[Terry Syd]

Here's a good reference for all these technical questions. There's heaps of gems to be gleaned from this document -

http://dragonfly75.com/motorbike/2StrokeDesign.pdf

A good reference in regards to compression ratios in two-strokes is -

http://www.bridgestonemotorcycle.com/documents/higher_compression6.pdf

 

[PA Dan]

Funny you should ask. The first ported saws I ever ran was an 038 and an 064 done our esteemed MM. In the above conditions.They ran great but i could tell they would run better in what some drag racers call "righteous air".

I remember that day!

 

[jake wells]

compression isn't king i got 2 cycle engines that spec 80lbs putting out 3-4hp kicking ass in the world everyday yet there is 4 strokes with 150lbs that struggle doing anything.
the 2 strokes i got run a 60mm bore closed port design with transfers going around the entire middle of the cylinder.

 

[drf256]

Transfers and less compression make rpm
My fastest tuning saws have the lowest exhaust and least intake duration and more bd.
I use to find the opposite of that.
Confusing at times

Yeah Jason.

An engine's an air pump. There's gonna be a limit to how fast it will spin based on how much air will flow through it and parasitic losses.

So if a given engine will flow X amount of air based on its design, how does one make it spin faster? The answer is smaller gulps of air, and more of them to equal flow.

I bet you if you throw that full circle crank in one of your latest 066's and raise the intake floor up to 74 with epoxy they tune over 16k. Just a hunch.

It always comes back to the physics of what's actually happening.

 

[drf256]

Heat=Power, that just how it works. Energy can't be destroyed, it just changes form.

We need to think about what compression does. It increases the energy (heat) in the charge so that it can combust quicker. Then the spark sets off the chain reaction and it combusts. Too much compression and the mixture will have too much heat and will combust before the spark (pre-ignition). The pressure in the combustion chamber from the combustion drive the piston down, not the "explosion" that some people think does.

So you really only need "enough" compression to allow for full combustion of the charge at the right time. Ignition timing has an effect, as slower burns from lower compression will need more time to burn completely.

I'd bet money that smaller motors can't maintain the same amount of heat in the combustion chamber as larger motors and that's why they make more power with more compression. Heat, in the right place, isn't the enemy of power some think it is.

Sorry for the technical rant.

 

[drf256]

One things for sure,

Compression sells saws. There's a general misbelief that a ported good runner saw MUST have over 200psi to be strong. It simply isn't true.

Post the same ported saw, one with 175psi and one at 220. The higher number will sell first.

Almost makes you wanna squirt oil in the 175 psi saw, just to help the poor soul.

 

[Mastermind]

I'm so confused.

 

[drf256]

I'm so confused.

Go back and look at what we all wrote 10 months ago on this site.

It will give you a chuckle.

This forum is great. Nowhere else can one question what's been done before and not be met with vitriol.

 

[Sty57]

Shouldn't a smaller bore saw Rev faster. Just from a weight and drag standpoint to begin with.

Everything I know is 4-stroke. But the first thing you do to make them Rev high and live, is lightweight components.

Lightweight pistons, short skirts, thin rings, knife edged cranks......

I relize a 2-stroke is a totally different animal. But they still have to deal with heat and friction.

 

[Mastermind]

Go back and look at what we all wrote 10 months ago on this site.

It will give you a chuckle.

This forum is great. Nowhere else can one question what's been done before and not be met with vitriol.

Can I get an amen?

The real testing all of us have done has really opened my eyes. We talked in theories last winter. Then over the last few months we tested out our theories. Some were spot on, but others have been disproved. I've learned more in the last six months here on OPE than I did in the six years before.

Carbs are my new obsession.

 

[drf256]

Shouldn't a smaller bore saw Rev faster. Just from a weight and drag standpoint to begin with.

Everything I know is 4-stroke. But the first thing you do to make them Rev high and live, is lightweight components.

Lightweight pistons, short skirts, thin rings, knife edged cranks......

I relize a 2-stroke is a totally different animal. But they still have to deal with heat and friction.

Yup. The less friction/gravity/inertia/pumping/compression losses, the faster they should rev.

I wish they made 6 pin 325 sprockets. On a small saw, one needs more rpm to make power.

HP is torque x RPM. So, even if I get a decent amount of torque from a little saw, if it doesn't rev to make power, it will be a flop.

 

[drf256]

Can I get an amen?

The real testing all of us have done has really opened my eyes. We talked in theories last winter. Then over the last few months we tested out our theories. Some were spot on, but others have been disproved. I've learned more in the last six months here on OPE than I did in the six years before.

Carbs are my new obsession.

Amen Brother Randy.

Let's all remember:

Chainsaw Specific Output Formula:

TORQUE x RPM/5250 X 100/saw builders calf +thigh diameter

 

[Redfin]

vitriol.

And the word for the day is...