Case Compression, Let's Talk About It

[Wanab]

I guess I never realized how much resistance can be felt. Got me to thinking is more or less better.

 

[Dieselshawn]

This should be interesting.

I know for sure the filter and muffler tune has a huge impact in how the crank case gets the charge in the bottom end and into the chamber

 

[Dieselshawn]

While there is a little bit of compression under the piston, it’s more a vacuum to pull the air and fuel through the carb into the bottom end.

A well tuned muffler or pipe will help pull the charge through the transfer ports better thus creating a little more vacuum than a stock saw can.

That extra vacuum created will pull more air and fuel when the intake port opens.

Then the cycle repeats.

 

[Bigmac]

There is a sweet spot for every engine. And can be tuned, on some engines easier than others. Some engines will rev better with more case compression, and some piped applications can make more power with more case volume, I guess it all depends. And obviously at extremes, the engine won’t run

 

[Lee H]

Case compression is needed as well as case volume, I think it all depends on RPM and at what
RPM you want your power curve to be. Back in the day and probably today as well Mcculloch
kart motors as well as others you could get aftermarket stuffers which fill the case more which
takes away volume. You could also have custom full circle cranks that would take away volume
but increase compression. I'm no expert and i'm sure others may chime in to give a better explanation.

 

[Wanab]

Well there is a good bit of compression. Spinning the engine slowly by hand there is well defined resistance and then a pop/rush of air when the transfers open.

 

[huskihl]

Well there is a good bit of compression. Spinning the engine slowly by hand there is well defined resistance and then a pop/rush of air when the transfers open.

Are you sure it’s not the flywheel magnets you’re feeling? There’s only maybe 4-5 psi in the bottom before the transfers open

 

[cus_deluxe]

@ayoungdipshit would probably love this thread.

 

[Wanab]

Are you sure it’s not the flywheel magnets you’re feeling? There’s only maybe 4-5 psi in the bottom before the transfers open

Not unless they are firing @ about 120 ATDC and don't work when there is no spark plug. I will see if I can capture it on video. PSI I have no idea. A rough estimate is 30% of the resistance felt @ TDC.

 

[Bjorn]

crankcase compression only reaches like 10 psi. If you think about the engine running at 50-250 revolutions per second or 3000-15000 rpm, think about how long the transfers are open for each stroke. If you have an air hose pressurized to 10 psi, and could open the gate for 1/50th of a second, and that is a full stroke of the engine at idle, how much air is coming out of that hose?
The only way these engines can pump efficiently is from the energy produced by the burning mix.
At first the expanding gases push the piston down, then as the exhaust opens and the gases hit the muffler can and expand, it created a pulling effect on the rest of the cylinder, vacuum. And a lot of it. So the transfers want to open just as the pressure is changing to a vacuum for the most efficiency.

After pondering this for a bit I see this as the only way to get enough transfer to run the engine. The crankcase compression combined with a vacuum on the other side means the transfer port sees a much greater differential than just 10 psi to atmosphere

 

[Wonkydonkey]

Case volume is an interesting subject, somewhere I remember reading about the different cases off the 064\066\660 being filled and volumes being accurately recorded.
I think this was debated as too which ones ran better and why.
I will see if I can find a link to the thread.
Here’s the link from Definitive Dave

https://opeforum.com/threads/064-066-ms650-ms660-info-thread.696/page-2#post-131475

And you could look into the 44/46 hybrid, as well as the 10mm to 12mm crank swap. This gives a change in the case volume and the psi of charge pushed through.

however on an other note, many yrs ago a friend had a Suzuki x7. I remember a few things about it being ported, apart from it running very well afterwards . The intake was tuned, there was something about pistons being filed as it gave a different result to just lowering the intake. Iirc it was to do with case compression and spit back or something like this. I,m sure there are others that know a lot more than me about the whole subject.

 

[Mastermind]

Mcculloch kart motors as well as others you could get aftermarket stuffers which fill the case more which takes away volume. You could also have custom full circle cranks that would take away volume but increase compression.

You touched on something here that a lot of people miss Lee. The amount of pumping action, or displacement, is the same regardless of case volume. Take away case volume, and case compression increases. Air stretches, and it compresses. So taking volume away will give it more movement up though the transfer passages because less time is wasted compressing mixture that will never leave the crankcase anyhow.

 

[Wanab]

You touched on something here that a lot of people miss Lee. The amount of pumping action, or displacement, is the same regardless of case volume. Take away case volume, and case compression increases. Air stretches, and it compresses. So taking volume away will give it more movement up though the transfer passages because less time is wasted compressing mixture that will never leave the crankcase anyhow.

There is something more at work here. If there was not more to it every case would be as tight as possible. I guess every saw is different. My example is you may have a bore/stroke/timing/case combo that creates more pressure than one with smaller volume. Understandably less case on any same saw in theory should create more pressure. Pretty sure most motorcycle cranks get stuffed for more low-end response at the expense of top end, at least according to Eric Gore they do.

 

[Bigmac]

There is something more at work here. If there was not more to it every case would be as tight as possible. I guess every saw is different. My example is you may have a bore/stroke/timing/case combo that creates more pressure than one with smaller volume. Understandably less case on any same saw in theory should create more pressure. Pretty sure most motorcycle cranks get stuffed for more low-end response at the expense of top end, at least according to Eric Gore they do.

There are a lot of differences between reed valve and piston port motors and add a pipe into the mix and everything changes again! The mx 250 motors have changed a lot over the years, and usable power was always the goal, they could have built more power if they wanted too, the guys moding motors for shifter karts were always on the edge. Have heard a lot of modded 125’s making mid 40+ hp nearly matching a 250 stock.road race motors would also be more max efficient type builds, so the 125 is vary similar. I have played with case volume a fare amount and sometimes get mixed results, it doesn’t always work like the book says! Lol then you have reed spacers on the 250’s they clam it’s for low end power, saying adding volume adds low end power

 

[Wanab]

I did a test with the 2 660's I have sitting here. One has a Hyway cylinder and one has a Huztl jug. The one with the higher transfers is popping harder. There must be more to it that just port timing.

 

[Bigmac]

I did a test with the 2 660's I have sitting here. One has a Hyway cylinder and one has a Huztl jug. The one with the higher transfers is popping harder. There must be more to it that just port timing.

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

 

[Mastermind]

I did a test with the 2 660's I have sitting here. One has a Hyway cylinder and one has a Huztl jug. The one with the higher transfers is popping harder. There must be more to it that just port timing.

Both jugs are just Chinese junk.....so I doubt it means anything.

Truth.

 

[huskihl]

I did a test with the 2 660's I have sitting here. One has a Hyway cylinder and one has a Huztl jug. The one with the higher transfers is popping harder. There must be more to it that just port timing.

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

 

[Ketchup]

I did a test with the 2 660's I have sitting here. One has a Hyway cylinder and one has a Huztl jug. The one with the higher transfers is popping harder. There must be more to it that just port timing.

On two of the same saw with the same case volume, what else would effect case pressure other than port timing?

 

[Bigmac]

On two of the same saw with the same case volume, what else would effect case pressure other than port timing?

The piston itself, and port volume, they are China
Probably intake port timing thou