Case Compression, Let's Talk About It

[Mastermind]

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

I think about case compression in degrees of crankshaft rotation rather than in psi.

 

[huskihl]

I think pressure is only +-4-5 psi. Likely can’t be measured accurately at running rpm

 

[Ketchup]

I think about case compression in degrees of crankshaft rotation rather than in psi.

I do too.

So what is the optimal number of degrees for optimal transfer velocity? Depends on a lot of variables. But a case pressure ideal would be relatively the same across many models, port geometries, etc. Then we tweak the variables to get there.

I think pressure is only +-4-5 psi. Likely can’t be measured accurately at running rpm

Doesn’t 500i sense case pressure? Anyone know 500i stock case compression duration?

But we still don’t have a way to measure the PSI while running.

Better questions:

Do we want to maximize case pressure?

What would we do if we didn’t have to change port timing to raise case pressure?

How many degrees of transfer duration do we get before case pressure goes to ambient?

 

[Mastermind]

I look at 40° of case compression and think....that'll do.

 

[ManiacalMark]

I do too.

So what is the optimal number of degrees for optimal transfer velocity? Depends on a lot of variables. But a case pressure ideal would be relatively the same across many models, port geometries, etc. Then we tweak the variables to get there.



Doesn’t 500i sense case pressure? Anyone know 500i stock case compression duration?

But we still don’t have a way to measure the PSI while running.

Better questions:

Do we want to maximize case pressure?

What would we do if we didn’t have to change port timing to raise case pressure?

How many degrees of transfer duration do we get before case pressure goes to ambient?

I would think the 500 measures fresh air coming in more than pressure it’s holding towards the injector.

 

[Bigmac]

I do too.

So what is the optimal number of degrees for optimal transfer velocity? Depends on a lot of variables. But a case pressure ideal would be relatively the same across many models, port geometries, etc. Then we tweak the variables to get there.



Doesn’t 500i sense case pressure? Anyone know 500i stock case compression duration?

But we still don’t have a way to measure the PSI while running.

Better questions:

Do we want to maximize case pressure?

What would we do if we didn’t have to change port timing to raise case pressure?

How many degrees of transfer duration do we get before case pressure goes to ambient?

On the piston port, ambient pressure would be variable ratio between intake and transfer timing.
Lower transfer timing would alow more intake timing, or more transfer timing would need less intake to create the same pressure, per given case volume. That’s given to an oem platform, and still doesn’t answer any questions! And a spacer and longer rod, case pressure would drop? Dose that change the formula?

 

[Ketchup]

I would think the 500 measures fresh air coming in more than pressure it’s holding towards the injector.

That’s what I originally thought but as I have learned more about fuel injection I have begun to think the 500i senses barometric pressure, not Oxygen.
But I still haven’t looked into it that deeply. Please explain your thoughts. I’m happy to be wrong.

 

[Mastermind]

That’s what I originally thought but as I have learned more about fuel injection I have begun to think the 500i senses barometric pressure, not Oxygen.
But I still haven’t looked into it that deeply. Please explain your thoughts. I’m happy to be wrong.

I thought that sensor was a crank trigger for the spark and ignition. I was wrong...

Next time I hook one up I'll post some shots from the program.

 

[isaaccarlson]

Case pressure would be relative anyway, because it still has to go through the transfers. More exhaust vacuum and better transfer flow would make case pressure peaky. A lazy exhaust and less flow would make it flatter and wider.

How much air can you pack in there effectively AND put in the cylinder without losing it out the exhaust? Little changes can do a lot.

 

[Kitslot]

The biggest unaddressed issue with the MS201 is the poor fit of the transfer covers. After a little squish cut, a tossed out base gasket, and some fine tuning of the ports.....the little saw becomes a completely different animal.

Hello this is an old thread but can you or anyone help w/ porting and helping ms201t

 

[Mastermind]

Hello this is an old thread but can you or anyone help w/ porting and helping ms201t

Yep.

 

[Kitslot]

Ok great Ty I will be contacting you. Is there any more info on porting the ms201t. I would like to educate myself on wat benefits it delivers.

 

[Mastermind]

Ok great Ty I will be contacting you. Is there any more info on porting the ms201t. I would like to educate myself on wat benefits it delivers.

Pretty much like any other saw we port. We add some extra compression, massage the ports, tweak the timing, carb, and muffler. The benefits are more rpm under load.....and a saw that is more fun to use.

 

[Ketchup]

For me, less is more on 201t. Open the muffler. Tighten the squish to 0.018”, raise exhaust (just raise, don’t widen) and uppers 2 degrees. Cut that cap junk out of the uppers. Blend any sharp or rough edges. Timing advance depends on what version of 201 you have. The current model is the best and I don’t advance it.

This recipe reduces case compression by 3 degrees, but it doesn’t seem to matter.