Case Compression, Let's Talk About It

[jacob j.]

and then they built the case around it.

I think you're onto something here - one thing I noticed when I started building the 044/046 hybrids many years ago - stock for stock, an 044/046 hybrid still won't quite keep up with an 046. I played around with ignition timing using only the stock cylinders and I could never quite dial in the hybrid to stay with the 046 in bigger wood. Ported vs. ported might be a different story because then the port timing could be idealized for both motors.

 

[Mastermind]

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.

Great post Kevin. Gives a fella something to think about.

 

[huskihl]

I think you're onto something here - one thing I noticed when I started building the 044/046 hybrids many years ago - stock for stock, an 044/046 hybrid still won't quite keep up with an 046. I played around with ignition timing using only the stock cylinders and I could never quite dial in the hybrid to stay with the 046 in bigger wood. Ported vs. ported might be a different story because then the port timing could be idealized for both motors.

But the hybrid probably won with a 20-24” bar because it made more power at higher rpm

 

[jacob j.]

But the hybrid probably won with a 20-24” bar because it made more power at higher rpm

It absolutely did. Even ported, the hybrid was happiest in western softwoods with a 28" or maybe a 30" bar and full skip or semi-skip. The 046 was always happy with a 32" and semi-skip.

 

[huskihl]

It absolutely did. Even ported, the hybrid was happiest in western softwoods with a 28" or maybe a 30" bar and full skip or semi-skip. The 046 was always happy with a 32" and semi-skip.

There’s about 3-4° difference in port heights that will make them run similarly,

 

[Mastermind]

I've run a few hybrids that were wicked strong. Strangely, from guys with total difference build approaches. So many ways to skin a damn cat.

 

[Red97]

Why everyone skinning cats?

And frying fish?

Guess fried cat is a bit wrong

Carry on.

 

[Ketchup]

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.

I think that used to be the approach. The newer saws seem designed for a much tighter case. Lots of stuffers and case material above the crank weights. Maybe it’s more important with shorter strokes? Maybe it helps compensate for the longer transfers?

 

[Ketchup]

Why everyone skinning cats?

And frying fish?

Guess fried cat is a bit wrong

Carry on.

When I was a kid we ate a bobcat my dad shot. It was pretty good. Lots of fried catfish covered in flour and spices too. We skinned both.

 

[Wanab]

When I was a kid we ate a bobcat my dad shot. It was pretty good. Lots of fried catfish covered in flour and spices too. We skinned both.

General Tso's Cat, yummy.

 

[huskihl]

I think that used to be the approach. The newer saws seem designed for a much tighter case. Lots of stuffers and case material above the crank weights. Maybe it’s more important with shorter strokes? Maybe it helps compensate for the longer transfers?

Yep I think it can be attributed to newer technology. The 462 and 500i are stuffed pretty tight from the top with not much more than a slot for the rod to ride in

 

[huskihl]

General Tso's Cat, yummy.

Somewhere between maybe and probably

 

[Ketchup]

General Tso's Cat, yummy.

Close. I think we put it in Kung Pao (not joking). We were really into Chinese food at the time.

 

[pbillyi69]

kung pao kitten

 

[Yukon Stihl]

Why everyone skinning cats?

And frying fish?

Guess fried cat is a bit wrong

Carry on.

Got to skin the cat before frying them
Hard to get rid of the smell of fried hair

 

[Wonkydonkey]

I was looking/calculating the swept volumes the other day, of the 044 & 046 the 46 has 6cm3 more than the 44
So the hybrid has more charge going in to the cylinder . But I guess as Kevin says the 3*\4* difference of port heights plays it’s part on how it works but at the same time trying to stop it being flushed out of the exhaust is something else

 

[Mastermind]

Guess fried cat is a bit wrong

 

[Red97]

My .02$

We are thinking too much into this using the "new" design's.

Saws are built with a performance 3rd or 4th mindset.

1,Emissions, they dont pass they cant sell.
2, Durability, need to meet those emissions for 300 hours
3, Performance
4, Price

Possibly swap 3,4 around.

All the tight cases front transfers that we have seen, I feel are more to meet emissions than anything to do with power.

Same thing with the long stroke engines. Allows more time to control gas flow/mix.

Just like all the random "obstacles" in newer castings. Those odd lips in the transfers, sharp blocks in the corners. All to aim /distribute fresh/spent charge before hitting the sniffer.

Since "emissions " covers a broad range of things C02,N0X, raw fuel etc it is hard to tell what the focus is on.

 

[farminkarman]

My .02$

We are thinking too much into this using the "new" design's.

Saws are built with a performance 3rd or 4th mindset.

1,Emissions, they dont pass they cant sell.
2, Durability, need to meet those emissions for 300 hours
3, Performance
4, Price

Possibly swap 3,4 around.

All the tight cases front transfers that we have seen, I feel are more to meet emissions than anything to do with power.

Same thing with the long stroke engines. Allows more time to control gas flow/mix.

Just like all the random "obstacles" in newer castings. Those odd lips in the transfers, sharp blocks in the corners. All to aim /distribute fresh/spent charge before hitting the sniffer.

Since "emissions " covers a broad range of things C02,N0X, raw fuel etc it is hard to tell what the focus is on.

Joe, I think you are spot on!

 

[rogue60]

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 .

I know nothing about crankcase volume and stuffs never messed with it.
But yeah the crank case will only ever be filled to whatever the atmospheric pressure is sea level being 14.7 PSI.
As it's compressed who knows how much higher the PSI goes over the given atmospheric pressure in the crankcase.
I'm guessing a reed valve engine is more efficient less loses over a piston port ultimately.