Case Compression, Let's Talk About It

[Wanab]

What’s the intake timing? Have you removed the coils? And is the muffler off both saws?

Timing is 78/80, not sure why you guys think it's the coil I am feeling but yes it does it without the coil or muffler. Give me a little credit please.

 

[Wanab]

Define “popping harder”. Runs better? Higher cutting rpm?

More rotation resistance and a louder audible pop/rush of air when the transfers open. Yes the saw is stronger but that has more to do with the transfer design IMO. It does have about 1 1/2 degrees more intake.

 

[Bigmac]

Timing is 78/80, not sure why you guys think it's the coil I am feeling but yes it does it without the coil or muffler. Give me a little credit please.

Asking questions, what your opinion on it? Better ring seal?

 

[wcorey]

More rotation resistance and a louder audible pop/rush of air when the transfers open. Yes the saw is stronger but that has more to do with the transfer design IMO. It does have about 1 1/2 degrees more intake.

Could be rings are sealing better on one than the other.

I've long suspected that how efficiently rings seal/seat from one p&c to another (assuming similar comp readings) has a significant effect on performance, all other things being fairly equal. I have to wonder what dynamics are happening/changing when the piston is moving a thousand times faster than pullover speeds were the comp is measured. Particularly with 'cleaned up' cylinders, which is what I'm typically porting...

Are they being pushed into the bore with compression behind them in the grooves or bouncing/floating off of slight variances in the bore plain, or...?

 

[Ketchup]

Working with the stock components, the piston sweeps a set volume.

Case pressure builds most on the down stroke from the time the intake closes to when the transfers crack. More importantly, the volume available to be pushed through the transfers is the remainder of the stroke when the intake closes. The intake being open past a certain degree is bad because it shortens the duration and volume trapped in the case.

Volume is most important. The lower the intake, the smaller the volume that will be pushed through the transfers. Shorter stroke saws have less swept volume per degree and will be more affected by a lower intake.

Case pressure is still significant. It effects transfer velocity. Higher velocity transfers will have a better scavenging effect. Velocity is determined by the ratio of swept volume to total case volume. Yep, more volume. In my experience, the more swept volume you have, the better.

RPM is also important. More RPM means less actual time for pressure to equalize in the case. Pressure will be higher under the piston relative to the extremities of the case (along the piston walls and in the bottom of the crank chamber). That means less will get pushed out the intake at higher rpm.

In general I try not to let too much out the intake or increase case volume a lot. Getting more technical than that hasn’t gained me much. I would like to experiment with epoxy and longer strokes, but haven’t gotten there yet.

I should be out in the shop instead of writing on this forum…

 

[Mastermind]

I would like to experiment with epoxy

There's a whole nuther kettle of fish to be fried. LOL

 

[Bigmac]

Working with the stock components, the piston sweeps a set volume.

I was thinking the piston casting itself, a poorly cast piston with more mass would also raise case pressure

 

[Al Smith]

It's a puzzle .If you use a full circle crankshaft and a tight stuffer it will raise the crankcase pressure but reduce the volume obviously .Then it goes back to transfer port time area open and thus at sustain high RPM's you need more because the time opened is decreasing . It's got to get out before you stuff any more in so you about have to work backwards regarding port timing ,over lap etc. It takes fuel to make power .

 

[Mastermind]

It's a puzzle

And that is what keeps it interesting.

 

[Bigmac]

If you really want to play with case volume, get an old Mac

 

[CR888]

I wish MAC was still around today with the same intent or pursuit of innovation that they used too. They were really doing some cool stuff in their day.

 

[Wanab]

Asking questions, what your opinion on it? Better ring seal?

I don't think that's the case because it was an out of the box piston and rings. Now that I have run the saw nothing has changed. I do agree 100% ring break in is crucial for full performance.

 

[pbillyi69]

when i read threads like this where people who actually know what they are talking about go into detail about how changing something in one place affects the overall result makes me feel like i am a child staring at the stars i wonder. thank you to all of the saw builders that post willingly sharing their results of trial and error. it just proves to me the thing i think i know and understand are so much less than the reality of it all.

 

[Stump Shot]

There's a whole nuther kettle of fish to be fried. LOL

I like a good fish fry.

 

[Al Smith]

There really are not any actual secrets because at some time or another it's all been tried .It's more or less revisited from time to time .However saying that there are many ways to skin that same cat .

 

[Michpatriot]

when i read threads like this where people who actually know what they are talking about go into detail about how changing something in one place affects the overall result makes me feel like i am a child staring at the stars i wonder. thank you to all of the saw builders that post willingly sharing their results of trial and error. it just proves to me the thing i think i know and understand are so much less than the reality of it all.

This is why I just stick to tracking down all the unicorn pieces from the ends of the earth and sending them to Kevin to sprinkle the magic shavings..I enjoy the whole process but trust those who do it for a living..ill stick to magic with welding.

 

[pbillyi69]

me too im going to send one to him in the near future and one to tree monkey then i can just reap the rewards of their hard work

 

[Ketchup]

I was thinking the piston casting itself, a poorly cast piston with more mass would also raise case pressure

Extra piston will reduce total volume but swept volume is the same. Extra weight and roughness on the piston will cost far more than it gains. That’s why stuffers are on the crank, not the piston.

It's a puzzle .If you use a full circle crankshaft and a tight stuffer it will raise the crankcase pressure but reduce the volume obviously .Then it goes back to transfer port time area open and thus at sustain high RPM's you need more because the time opened is decreasing . It's got to get out before you stuff any more in so you about have to work backwards regarding port timing ,over lap etc. It takes fuel to make power .

At high RPM you may lose a margin of volume through the transfers but I doubt much at all. The case has to go back to negative pressure on the upstroke. The case may have residual pressure ABDC, but I think it’s neutral long before the transfers close and certainly by the time the intake opens. The intake operates on negative pressure and whatever inertia remains in the intake tract when the intake port opens. I don’t think there is a strong stuffing effect.

I like a good fish fry.

Me too. Makes me think back to all those channel cats we caught as kids.

Now I’m hungry.

 

[Al Smith]

You can the kick the can forever about crankcase pressure etc ,However things like Vee stacks in effect only kick in gear at high RPM's you've got aprox 14.7 PSI air pressure on one side and slightly negative on the crankcase side .In the port over lap of the transfers and exhaust closing there will be some degree of blow back . How much I have no idea .As I've mentioned Art Martin had reed valves of some sort in the transfers of that hot rod 090 Stihl from years ago .I am only assuming that was to prevent the blow back .That example has been the only one I've ever heard of .Never the less the whole thing works on differential pressures .Starts in atmospheric pressure and ends on same .

 

[huskihl]

Back to a question early on…about crankcases all being built as tight as could be or not…
I think for the most part, manufacturers design a crankshaft that is strong enough to last and weights that have enough mass to offset the piston for vibration, and they build a crankcase around it. Different companies have different ideas on how big or small it should be. I don’t know of any cases that are exceptionally larger than the crankshaft that they are designed for. A Dolmar 7900 is considered to have a tight case, but if you look at it, the crank weights aren’t spread very far because it has a thin rod, and the weights aren’t a large diameter because of the lightweight piston. It’s what they deemed was strong enough, and then they built the case around it.