High Compression??????

[jmssaws]

So your saying by lowering the cylinder you increase the compression but sacrifice rpm or speed?

I haven't noticed that so much with a stihl. A big husky needs the exhaust raised after machine work.

 

[drf255]

Compression doesn't make torque, and torque doesn't turn big sprockets

Increasing compression makes more power from the same charge, up to a point.

Then there's the parasitic drag that compressing the charge puts on an engine. So when the drag exceeds the power increase, you've reached the point of diminishing return and you're just producing more heat and putting more wear on the motor.

If HP is increased, than so is relative torque. HP is torque at a given RPM. HP is mathematical. Torque is what is measured. The ability to turn a lever arm.

 

[jmssaws]

I haven't ran a saw yet didn't run better with more compression. I don't know how far u have to go till your going the wrong way but it must b a bunch. Although the absolute strongest saw I've ever ran was a 066 that was 190 lb but it was just one of those magical saws u run across now and then

 

[smokey7]

The saws i have modded have gained rpm and power with large increases in compression. The most noticeable ones were my 630 and poulan 3314. Those went from 150ish to over 200. The 630 went from running good at 9500 free rev to 12400, and the 3314 went from 9000 to 15200. I dont think ive hit that wall yet.

 

[drf255]

You're effectively creating a larger displacement engine by lowering the exhaust port as well. More swept volume into same chamber. The length of the cylinder when the exhaust closes is simply longer by whatever you took from the band.

So taking 1mm from the band and not touching the exhaust height should be the equivalent of a 2mm stroker crank.

Compression is just one of the parts of making power. All have to act in concert together to make it "right".

But compression sells saws. Port a saw and put it up for sale. Try to explain that 160psi is the sweet spot for that model. The guys saw that's pulling 220psi will sell first.

 

[jmssaws]

I think a 660 is strongest below 200 for most purposes but a husky seems to want all u can give it. The biggest gain is compression and ignition advance, I do a bunch of 660's and I went through the hog it all out stage years ago, after machine work I raise the transfers and depending on what it's going to b doing it may raise the intake others than clipping the lip on the lowers I don't touch it,not the intake or exhaust at all, I use to square up the intake but I don't anymore, I can guarantee u there stronger now. Lot of saws that don't work but on that one it does. U can prune and primp the ruff spots and polish here and there, then do one without touching the exhaust or intake and I bet u won't know one from the other. Flow does gain but not like compression and having the ports at the correct hight. 230lb is the highest I have and it's bn from 180 to 230 and all in between, it will turn along chain but with a shorter bar it was better at 210. 230lb is hard on parts also.

 

[Deets066]

I think treemonkey built a saw that had over 300 psi once. If I remeber correctly it self destructed in a few minutes

 

[drf255]

I think treemonkey built a saw that had over 300 psi once. If I remeber correctly it self destructed in a few minutes

I believe you're dead on. IIRC it was a removable head saw.

 

[Moparmyway]

You're effectively creating a larger displacement engine by lowering the exhaust port as well. More swept volume into same chamber.

Only problem I see in this is that to lower the exhaust, most cylinders need the squish cut. This reduces chamber volume, effectively reducing displacement.
Additionally developing a wider squish band = reduced chamber volume

One would need to cc the chamber after machining to produce actual numbers.
In other words, lowering the exhaust might add 3cc while reducing the squish could reduce 1cc

**EDIT; The underlined displacement should have said trapped chamber volume**

 

[Deets066]

Displacement is dictated by bore and stroke, if you don't change either of these then you've effectively changed nothing

 

[Moparmyway]

Displacement is dictated by bore and stroke, if you don't change either of these then you've effectively changed nothing

The bore in the upper chamber is being changed ............ ever so slightly, but it does alter when you start changing when the exhaust closes, and how much volume is in the combustion area

 

[smokey7]

I agree completely. If you remove volume from the head, raise or lower exhaust you are atleast changing the trapped volume. Which as i see it is changing the displacement. A beaker would prove this out in a second.

 

[Moparmyway]

Although Deets point is that the stroke isnt changing, so the piston will still displace or move the same volume

 

[smokey7]

I get that about stroke not changing. With your point is the chamber/trapped volume. As far as modding we are more concerned with the trapped volume and timing.

 

[Deets066]

Let's say you open up lower transfers or change intake or even change tune and are able to get more fuel transferred. Would you still think displacement has changed because there is a bigger charge in the combustion chamber?

 

[Red97]

So if I put a 76cc head on a chevy 350 that makes it a 351? Or if I swap to 64 cc head it is a 348?

Bore X stroke determine the displacement of an engine.

Yes the exhaust height or intake height may change the volume of "charge" in the engine. But doesn't make it a larger cc saw.

 

[Deets066]

I agree completely. If you remove volume from the head, raise or lower exhaust you are atleast changing the trapped volume. Which as i see it is changing the displacement. A beaker would prove this out in a second.

Trapped volume is how much you trap in the crankcase, transferred volume is in the top end

 

[Chainsaw Jim]

I see where squarefile is going with this. I too have wondered if compression can be high enough for the pressure to slow the piston down right before tdc. But then again it takes several pumps for a comp tester to reach full capacity, so there might not ever be enough pressure on one stroke to cause that.

 

[jmssaws]

There's nothing that will change displacement no matter where the ports are. A 92cc 660 is always gonna be unless the bore or stroke is changed. It's harder for a motor to squeeze more compression but it also comes down harder, does it always equal each other? I don't no. But a supercharger cost power to run, the more boost the more power it takes to turn it but there's no point where one will cancel the other.

 

[Chainsaw Jim]

There's nothing that will change displacement no matter where the ports are. A 92cc 660 is always gonna be unless the bore or stroke is changed. It's harder for a motor to squeeze more compression but it also comes down harder, does it always equal each other? I don't no. But a supercharger cost power to run, the more boost the more power it takes to turn it but there's no point where one will cancel the other.

What about lowering the jug and cutting squish band? You add displacement if you don't raise the exhaust to match the distance the jug lowered.