Case Compression, Let's Talk About It

[Maintenance Chief]

Yes.

 

[Stump Shot]

Yes.

Yes indeed... little buggers are good!

 

[Bjorn]

How about 2 Big Bores?

 

[Bjorn]

Hello. Here is a perplexing problem with saw cranks, the counterweights I'm guessing are about 1/3 of a circle so lower crankcase compression ratio. But take the piston and cylinder off your saw and turn the crank to BDC so it's at the bottom of the downstroke- when the transfer ports are open... Now look at the crank,it's counterweights are at the top of the crankcase and a large part of your fuel/air charge(about 2/3 of the crankcase volume) is trapped in the bottom of the crankcase with the counterweights above blocking your fuel charge from getting up past them to the transfer ports,considering the short amount of time the transfers are open.

 

[Stump Shot]

Hello. Here is a perplexing problem with saw cranks, the counterweights I'm guessing are about 1/3 of a circle so lower crankcase compression ratio. But take the piston and cylinder off your saw and turn the crank to BDC so it's at the bottom of the downstroke- when the transfer ports are open... Now look at the crank,it's counterweights are at the top of the crankcase and a large part of your fuel/air charge(about 2/3 of the crankcase volume) is trapped in the bottom of the crankcase with the counterweights above blocking your fuel charge from getting up past them to the transfer ports,considering the short amount of time the transfers are open.

Which could just possibly go to show that what goes out the transfers is what was between the crank and the piston for the most part at that particular moment.
Where a stuffed crank can't get filled in the bottom in the first place and keeps more up top.
A lot of things going on at the same time to ponder that's for sure.

 

[Ketchup]

Hello. Here is a perplexing problem with saw cranks, the counterweights I'm guessing are about 1/3 of a circle so lower crankcase compression ratio. But take the piston and cylinder off your saw and turn the crank to BDC so it's at the bottom of the downstroke- when the transfer ports are open... Now look at the crank,it's counterweights are at the top of the crankcase and a large part of your fuel/air charge(about 2/3 of the crankcase volume) is trapped in the bottom of the crankcase with the counterweights above blocking your fuel charge from getting up past them to the transfer ports,considering the short amount of time the transfers are open.

I think of the charge as having momentum. The downstroke of the piston forces charge down and the leading side of the crank weights push charge up into the downstroke. As the piston reaches bottom the charge is swept down into the bottom of the case by the weights. I think of the down stroke as creating a pulse of high pressure than continues down through the case and into the transfers, aided by the rotation of the crank weights. That’s why I like bottom fed transfers. I think they flow for longer after bottom dead center than side fed transfers.

 

[huskihl]

Just my opinion, obviously. I think the crank is spinning so fast that the crank lobes contribute very little as to where they are at any given time and how much of them is in the way or not. They aren’t the pump. The piston on the down stroke is the pump. You can’t look at it like case capacity being only the case. The underside of the piston contains way more charge than what is ever moved through the transfers in 1 downstroke

 

[Mastermind]

The spinning crank pulls the mix across the top of it for the most part I believe. That's why most manufacturers are picking up the transfer flow from under the exhaust port where the pressure is highest.

 

[Stevetheboatguy]

Just my opinion, obviously. I think the crank is spinning so fast that the crank lobes contribute very little as to where they are at any given time and how much of them is in the way or not. They aren’t the pump. The piston on the down stroke is the pump. You can’t look at it like case capacity being only the case. The underside of the piston contains way more charge than what is ever moved through the transfers in 1 downstroke

The spinning crank pulls the mix across the top of it for the most part I believe. That's why most manufacturers are picking up the transfer flow from under the exhaust port where the pressure is highest.

Think both you guyz are correct. Ever noticed how much less volume the new strato pisons have on the underside? Probably helps with the added volume of moving the transfers to under the exh port.

Kind of like filling the backside of a piston for a two cycle drag motor. Kind of thing.

Steven

 

[Bjorn]

Everywere where it is posible I like to use a Windowed piston. For example in the 372,it has large modern transfer ports but the stock full-skirt non-windowed piston completely blocks off the fuel charge in the crankcase and under the piston from getting to those excellent ports at BDC,at high rpms the transfer ports are only open for about 1 thousandths of a second so you want nothing impeding the flow of those gases. A large pocket of the fuel/air charge is trapped under the piston,a windowed piston allows it straight out into the transfer ports unimpeded.

 

[Al Smith]

Like I've always maintained there is more than one way to skin the same cat and they all work to a certain extent .Regarding case compression you have the option of a full circle crankshaft decreasing volume which would increase the pressure .That's only part of it, what are you going to do with increased transfer flow ? You have to put it someplace rather than blow it out of the exhaust port .Sure you can get somebody like Falicon to make a crankshaft to do what ? Try and shave a half a second on three cuts on a 9" popular cant . That's a very limited participation I would say .

 

[Bjorn]

Like I've always maintained there is more than one way to skin the same cat and they all work to a certain extent .Regarding case compression you have the option of a full circle crankshaft decreasing volume which would increase the pressure .That's only part of it, what are you going to do with increased transfer flow ? You have to put it someplace rather than blow it out of the exhaust port .Sure you can get somebody like Falicon to make a crankshaft to do what ? Try and shave a half a second on three cuts on a 9" popular cant . That's a very limited participation I would say .

If you like to put the charge in the exhaust port, is totally up to you. But I prefer the combustion chamber, more power that way.

 

[Stump Shot]

If you like to put the charge in the exhaust port, is totally up to you. But I prefer the combustion chamber, more power that way.

Fortunately for the rest of us, there's still bold people tinkering on an engine design that's been tinkered with since shortly after its inception over a century ago and no end in sight either going into the future.

 

[Ketchup]

Just my opinion, obviously. I think the crank is spinning so fast that the crank lobes contribute very little as to where they are at any given time and how much of them is in the way or not. They aren’t the pump. The piston on the down stroke is the pump. You can’t look at it like case capacity being only the case. The underside of the piston contains way more charge than what is ever moved through the transfers in 1 downstroke

I used to think exactly that about the cranks weights. Something you said about temperature boundaries in strato transfers got me thinking about the colder charge coming into the case and that the crank weight breaks up that boundary as it spins around. Now I imagine the crank weight as having a warmer and higher pressure charge in front and a colder and lower pressure charge trailing behind. It could still be that the cranks is just stirring everything so fast that none of that matters though.

I always assumed the volume of charge under the piston was about the same as the displacement of the saw. I should measure that.

Think both you guyz are correct. Ever noticed how much less volume the new strato pisons have on the underside? Probably helps with the added volume of moving the transfers to under the exh port.

Kind of like filling the backside of a piston for a two cycle drag motor. Kind of thing.

Steven

I hadn’t considered the shape of the newer pistons. The skirt is much shorter on the exhaust side. Is it an accident that most new transfer lowers are under the exhaust? Randy said that was the area of highest pressure. Maybe there’s more to strato piston shape than just the long, complex intake.

 

[Al Smith]

If you like to put the charge in the exhaust port, is totally up to you. But I prefer the combustion chamber, more power that way.

You missed the entire point --with that I'm out of the conversation .

 

[Stevetheboatguy]

I used to think exactly that about the cranks weights. Something you said about temperature boundaries in strato transfers got me thinking about the colder charge coming into the case and that the crank weight breaks up that boundary as it spins around. Now I imagine the crank weight as having a warmer and higher pressure charge in front and a colder and lower pressure charge trailing behind. It could still be that the cranks is just stirring everything so fast that none of that matters though.

I always assumed the volume of charge under the piston was about the same as the displacement of the saw. I should measure that.



I hadn’t considered the shape of the newer pistons. The skirt is much shorter on the exhaust side. Is it an accident that most new transfer lowers are under the exhaust? Randy said that was the area of highest pressure. Maybe there’s more to strato piston shape than just the long, complex intake.

I'd say there is a lot more going on than most can imagine with the strato port layout and the piston shape. Including the underside. I mean do they really need to make them as full as they are now...... They weigh a ton compared to a regular piston. Guess thats why the ms400 piston is mag. Remove some of the weight from all the filler they added.

Tripple piped drag sleds people did some crazy stuff. Ceramic pistons that were killer on the strip but wouldn't make it 10mi down the trail. Filling the underside of the piston with jb weld and styrofoam pellets to reduce case volume. Counter/taper bored light weight wrist pins with caps. Truing and tig welding crank pins to keep them from twisting out of time.

Can really go of the deep end when you get to the competitive nature of heads up run what you brought racing. Always hope you brought enough. 1/10 of a second could mean a payday or being S.O.L.



Steven

 

[Bjorn]

You missed the entire point --with that I'm out of the conversation .

I dident miss it. If you Can get 10% more charge in the combustion chamber, and 5% is lost in the exhaust port, you still have 5% more charge that make power. I have very hard to belive that all of the extra 10% is lost. Just an example.

 

[Ketchup]

A couple thoughts on the modern transfer feed designs.

Both Husky and Stihl have gone to feeding the transfers below the exhaust. Husky seems dedicated to full circle cranks, Stihl is using just weights. Husky appears to have a larger volume in the case lowers.

Several Stihl saws now have a diverter flange on the intake side of the case. On the 462, the diverter has windows that allow intake to flow directly in line with the crank weights and a scoop that may direct some cold mix up inside the piston.

Has anyone had luck removing the stuffers in 5-series Husky?

On the 462, the diverter windows look jagged and a bit small. Has anyone gotten after those with the foredom? Anybody try fabricating stuffers for a modern Stihl crank?

 

[isaaccarlson]

Subbed.

I have been thinking about this for years. Full circle cranks allow the charge to flow in without pulling it under the crank, and the under exhaust transfers are in a high pressure area because air has a lot of momentum.

I was using a shop vac and had about 20 ft of hose on it. I was vacuuming dirt, rocks, chunks of wood, etc... my son was at the shopvac to remove clogs. Turns out he was removing a clog about every 15 seconds, but I didn't know because the hose kept sucking up dirt. That's how much momentum air has. It kept flowing even when the hose was disconnected for 2-3 seconds. He said the stuff was still flying out the end too.

The air in the carb never stops flowing at high rpm. It compresses against the piston skirt. That's why I like a wider intake port for the charge to collect in. If you can tune it right, there are gains to be made. Carb spacers add momentum and allow more charge to compress against the skirt. It's like a built in supercharger. I like how the engineers are thinking with full circle cranks, under exhaust transfers, strato, and computerized fuel systems, but the older saws are lighter and easier to work on. A full circle crank adds a lot of weight to a saw, but that's one of the tradeoffs.

I have seen some of the newer saws after porting and they are impressive in both forms, but they really wake up after porting. It seems the engineers are onto something, but they are a bit behind what the good porters and builders have known all along.

 

[Ketchup]

What do you guys think is the ideal PSI for peak case compression?