Case Compression, Let's Talk About It

[Ronie]

Wow, that's one fast saw.

 

[Deets066]

Wow, that's one fast saw.

It was just a mild woods port, pulls a long bar pretty good too.

 

[Mastermind]

I ran 20 BD on my hybrid.
But you have to take into account what your cylinder is giving you for stock timing and adjust from there. Every hybrid won’t like the same porting because your port timing isn’t starting at the same point.

The 046/460 cylinder has more variations than any cylinder I've mapped.

 

[Ketchup]

Not all port work increases volume.

I’m not convinced it increases the volume of charge either.

 

[Mastermind]

I’m not convinced it increases the volume of charge either.

I'm sure I've posted this a blue million times......but very few people have yet to grasp it.

There really can't be anything moved from the case to the combustion chamber except what the movement of the piston displaces after the intake port closes. Inertia, or a lower pressure inside the chamber area could draw in more.....but in that situation we are likely taking about an engine with a tuned pipe.

So what effect does case volume truly have on transfer action?

 

[Ketchup]

I'm sure I've posted this a blue million times......but very few people have yet to grasp it.

There really can't be anything moved from the case to the combustion chamber except what the movement of the piston displaces after the intake port closes. Inertia, or a lower pressure inside the chamber area could draw in more.....but in that situation we are likely taking about an engine with a tuned pipe.

So what effect does case volume truly have on transfer action?

That is my exact thinking. But I probably think that because you wrote it somewhere earlier.

 

[Stump Shot]

I'm sure I've posted this a blue million times......but very few people have yet to grasp it.

There really can't be anything moved from the case to the combustion chamber except what the movement of the piston displaces after the intake port closes. Inertia, or a lower pressure inside the chamber area could draw in more.....but in that situation we are likely taking about an engine with a tuned pipe.

So what effect does case volume truly have on transfer action?

Velocity or the amount of push I would hazard to say.

 

[Nutball]

I'm attempting to do a 440/460 hybrid and have read that most people go with a short blow down, 15-17, and that got me thinking about the reason short blow down works better in dual ports vs quad ports, because the quad has more velocity and the duels are lazy. wouldn't the smaller case capacity in a hybrid make the dual transfers less lazy and more blown could be used?

Comparing a 395 to a 660, 064, and 9010, I've noticed the 395 quad ports appear to be larger than the dual ports on the other cylinders, so I'm thinking velocity is not higher on the 395 assuming the ports on the other cylinders aren't so small that they slow the flow, but I doubt it considering the 064 makes 9.75hp.

I think reducing blow down narrows the power peak curve, and helps it to peak stronger at a certain rpm.

 

[Ronie]

Comparing a 395 to a 660, 064, and 9010, I've noticed the 395 quad ports appear to be larger than the dual ports on the other cylinders, so I'm thinking velocity is not higher on the 395 assuming the ports on the other cylinders aren't so small that they slow the flow, but I doubt it considering the 064 makes 9.75hp.

I think reducing blow down narrows the power peak curve, and helps it to peak stronger at a certain rpm.

I haven't tried different blow down numbers on the same saw to know how they would react. I was just thinking on this hybrid that with less case capacity the transfers would have more velocity and wouldn't be as lazy as they would be on a 460 case and I might be able to have a little more blow down with good results.

 

[Wonkydonkey]

idk if there’s any other info that disproves Al's thoughts in the link below.

But my thoughts is a higher case compression ratio, thus a few more psi to purge the spent charge. Which was in one of the links I posted earlier, (dragonfly75). But not knowing the case volume difference of the 44\46 it’s hard to calculate it but the swept vol of the two different cylinders 44\46 is the easy bit.

Hyway MS460 big bore on a 044 opinions please

 

[Ketchup]

Comparing a 395 to a 660, 064, and 9010, I've noticed the 395 quad ports appear to be larger than the dual ports on the other cylinders, so I'm thinking velocity is not higher on the 395 assuming the ports on the other cylinders aren't so small that they slow the flow, but I doubt it considering the 064 makes 9.75hp.

I think reducing blow down narrows the power peak curve, and helps it to peak stronger at a certain rpm.

I think it matters where the ports are timed. A saw timed 78/100/120 can run very different from a saw at 78/105/125. Same for 78/100/115 vs 78/105/120. I think it matters more where the ports actually are and less how much space there is between them.

 

[Deets066]

I’m not convinced it increases the volume of charge either.

The intake size and lower trans size can increase volume of charge. But at a cost.

 

[Mastermind]

The intake size and lower trans size can increase volume of charge. But at a cost.

Agreed.

 

[Ketchup]

The intake size and lower trans size can increase volume of charge. But at a cost.

I hear you. I don’t see how that will push more through the transfers. Like Randy said, swept volume remains the same.

 

[Mastermind]

I hear you. I don’t see how that will push more through the transfers. Like Randy said, swept volume remains the same.

Everything is relative to what goal you have in mind for your build. If absolute maximum rpm is the goal, then a big case volume isn't necessarily a bad thing. Less primary compression will allow the engine to rev higher, as will less compression in the combustion chamber. But at lower speeds the engine will usually be lethargic.

 

[Deets066]

I hear you. I don’t see how that will push more through the transfers. Like Randy said, swept volume remains the same.

Swept volume can change. Small differences like angle, shape, size of the entire transfer tract could “let” more charge above the rings. You don’t know what’s actually limiting the charge. Is it time? Volume? Transfer shape?
Lot of unknowns because there’s not really a way to measure amount of actual swept volume.

 

[Deets066]

Even a pop up could possibly change swept volume.
Transfers are opening sooner and case compression remains the same.

 

[Ketchup]

View attachment 325508

Even a pop up could possibly change swept volume.
Transfers are opening sooner and case compression remains the same.

So shiney…


Okay, tell me if I’m way off here. The volume of charge that is transferred to the combustion chamber in a single revolution should be roughly equivalent to the swept volume of the piston from intake close to BDC. So wouldn’t intake timing determine how much volume the transfers can flow? Wouldn’t higher intake result in more swept volume?

I realize inertia, friction, and dwell will have effects.

 

[Deets066]

You aren’t wrong, but there is no right answer. Higher intake has more case comp. but also less time to fill the case. So say they case isn’t full, you’d have less charge and less velocity when compressed.

 

[Mastermind]

there is no right answer

This is the right answer.