Chainsaw Porting Theory

[Mastermind]

If I got it right.... It's in the transfers???

Can I go to the front of the class? Lol!

So.......you lowered the transfers .030"........unless they are bottom fed, and the roofs are untouched......you didn't decrease volume. If they are fed from the bore.....they are still the same size. And....if you raise the roof.....you just added the volume back.

 

[awol]

There are more factors to consider when talking blow back, than just in port timing and case capacity. The biggest may be the size, shape, and length of the intake tract.

 

[drf256]

But that would only be true if you didn't raise them back up.
Damn, I was seeing it how Al does til Randy had to throw in the whole piston moves the same amount fact.
Since the entire base is sitting on the case, nothing more protrudes into the case to reduce volume except the top of the transfers are lower until raised back up.
Unless the cylinder has case extensions that don't need to be trimmed...then it would reduce volume a bit.
Do I get a sticker now?


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I'm with you. It matters and it doesn't at the same time

A hoosky jug with case opened transfers will decrease case volume when cut.

A dual port Stihl, with jug placed lowers, likely not at all.

 

[mdavlee]

I must interject.....

How can cutting the base decrease case volume?

The piston will still be traveling the same height, so where is the decease?

Teach me.

It would be tiny but the amount you drop the base would be lower with the piston covering more up in the cylinder bore. I don't think that amount would be measureable. Now on a 7900 with a spacer to run a 272 piston you've increased by a good amount. You have to run .100" or thicker spacer to clear if I remember right.

 

[paragonbuilder]

It would be tiny but the amount you drop the base would be lower with the piston covering more up in the cylinder bore. I don't think that amount would be measureable. Now on a 7900 with a spacer to run a 272 piston you've increased by a good amount. You have to run .100" or thicker spacer to clear if I remember right.

Even on the 7900 it may not add volume because the stroke is the same. The only difference would be the amount of mass on the piston. You could actually decrease case volume doing this if the piston has more mass below the wrist pin. The only reason it's raised is because the top of piston is taller from the wrist pin.

Think of the cylinder as a closed tube with no ports and a 1" thick base. The piston goes up and down and you have a certain volume below it. Cut a half inch off the base and bolt it back and volume hasn't changed at all. The base is just smaller.

Now I know I get a sticker. I want a star. ️

Unless I'm wrong...[emoji12]


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[Mastermind]

What sets what we have here apart from other forums I've been a part of is that none of us are dead set on being right.

I'm just fine with being wrong.....

Nothing humbles a guy better than traveling a long distance in the wrong direction. lol

 

[jmssaws]

Dropping the cylinder decreases volume above the piston but not below it?

 

[jmssaws]

I like this,I honestly have no idea how this crank is going to affect port timing but I'm excited to find out with you guys.

 

[jmssaws]

I'm getting a set of cases ready to put it in so I can throw my tank and p&c on it.

The p&c is 101 118 78 fingers at 116 230lb
I'm not going to change anything just put a full circle under it and try it,if it doesn't work I got a cylinder and a 288 piston to port for it.

 

[paragonbuilder]

I'm getting a set of cases ready to put it in so I can throw my tank and p&c on it.

The p&c is 101 118 78 fingers at 116 230lb
I'm not going to change anything just put a full circle under it and try it,if it doesn't work I got a cylinder and a 288 piston to port for it.

It'll be interesting to see if the reduced case volume reduces or gains compression in the combustion chamber...


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[Mastermind]

I think it will need a much shorter transfer duration than what we usually see in the 066.

 

[jmssaws]

It has to change things so I'm using a good running cylinder so I can tell which way it has went. A 66 loves velocity so I'm fixing to give one some.

 

[idiotwithasaw]

I have trouble seeing that the case compression is actually lower when the jug is lowered I like the closed tube argument because it illustrates this perfectly.

However I do believe there is part of this discussion that is being ignored of forgoten about and that's volume above the piston. That areas volume is obviously being reduced, especially if the squishband or a popup is cut. So to me it seems that the case volume is staying the same while you are lowering the combustion areas volume therefore lowering the amount of fuel and air mix that can effectively be transferred from the bottom to the top.

I'm curious how many cc we are actually losing when we lower a jug. I mean think about it the whole reason we cut a squishband or turn a popup is to effectively lower the volume in the combustion chamber to make a more violent and effective burn. I could be wrong, I'm here to learn like everyone else.

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[jmssaws]

I think it will need a much shorter transfer duration than what we usually see in the 066.

I'm at 128 now,think they need to be higher?

 

[jmssaws]

I have trouble seeing that the case compression is actually lower when the jug is lowered I like the closed tube argument because it illustrates this perfectly.

However I do believe there is part of this discussion that is being ignored of forgoten about and that's volume above the piston. That areas volume is obviously being reduced, especially if the squishband or a popup is cut. So to me it seems that the case volume is staying the same while you are lowering the combustion areas volume therefore lowering the amount of fuel and air mix that can effectively be transferred from the bottom to the top.

I'm curious how many cc we are actually losing when we lower a jug. I mean think about it the whole reason we cut a squishband or turn a popup is to effectively lower the volume in the combustion chamber to make a more violent and effective burn. I could be wrong, I'm here to learn like everyone else.

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Smaller volume on top of the piston but relatively the same below it.

 

[paragonbuilder]

I have trouble seeing that the case compression is actually lower when the jug is lowered I like the closed tube argument because it illustrates this perfectly.

However I do believe there is part of this discussion that is being ignored of forgoten about and that's volume above the piston. That areas volume is obviously being reduced, especially if the squishband or a popup is cut. So to me it seems that the case volume is staying the same while you are lowering the combustion areas volume therefore lowering the amount of fuel and air mix that can effectively be transferred from the bottom to the top.

I'm curious how many cc we are actually losing when we lower a jug. I mean think about it the whole reason we cut a squishband or turn a popup is to effectively lower the volume in the combustion chamber to make a more violent and effective burn. I could be wrong, I'm here to learn like everyone else.

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You sound like my wife.[emoji23] it's not an argument, it's a discussion! [emoji2]
An hour ago I thought case volume decreased when we cut the base. Now I "know" it stays the same. Without Randy's interjection I would still "know" it decreased.
What will I know in another hour![emoji122]


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[jmssaws]

I know it's hard to hunt shed antlers and be on here at the same time.

 

[wigglesworth]

But that would only be true if you didn't raise them back up.


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Right. If U raise the trans back up, the tiny bit you lost on volume is gained back.

 

[idiotwithasaw]

Smaller volume on top of the piston but relatively the same below it.

That's what I was getting at, sorry if I wasn't too clear. Makes me think back to school where we were taught that things want to go from areas of lower volume to higher volume. Or higher pressure to lower pressure. So in order to overcome a having a lower combustion area volume we would need to have higher pressure in the crankcase. I could be completely talking out of my butt here but it makes sense in my head.

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[awol]

I'm at 128 now,think they need to be higher?

Nope, lower. Maybe. I think.