Case Compression, Let's Talk About It

[Wonkydonkey]

Good poast,

 

[Mastermind]

Depends on size of engine and port shape. Wider ports need less duration.

For 90cc saws I like Intake in the 158-164 area, Exhaust around 156-160, Transfers somewhere near 112-120.

On the other end. For 25cc I like less intake (136-142), less exhaust (144-150) and similar transfer (110-116).

Some of the new Stratos have strange transfer durations and it’s important to note the cold air duration vs fuel mix duration on the intake side. The fresh air may open at the port at very different time than it feeds the transfers. For example I just timed a 550mk2 at fuel mix duration 148, Fresh air at intake 154, piston to transfer 140.
Figuring out what I want with those is specific to my changes and the individual model. Mostly I just leave them alone.

I feel Intake duration is the most significant for case pressure. Width doesn’t matter as much as timing because the real issue is piston position when the intake closes. The earlier the case is filled and the intake closes, the better. I like short intake duration and a wide port.

Transfer height effects pressure, but will still move nearly the same volume, maybe more at longer durations as rpms increase. In general, longer transfer durations seem beneficial regardless of effect on case pressure.

For exhaust a low roof (shorter duration) adds force to the down stroke and add a slight bump in pressure right before the transfers crack. That translates to higher transfer velocity, improved scavenging, and saw with some extra torque. I like shorter exhaust durations with very wide ports. But if there is too little duration the saw will just be a slow grinder. I try to set my exhaust at a happy balance. Usually with slightly more duration than stock.

Every model has its peculiarities. I probably go against my own habits 30% of the time.


…


Not sure why I wrote all that. I get carried away sometimes.

Glad to have you here getting carried away. The "thrill" of all this has faded for many of us.....so we need you.

 

[Al Smith]

Another thought .A well known builder of hot saws especially reworked 3120 Husky's is of the opinion the fan blast helps the performance .Fact he was sent a new one to rework somebody had shaved the fan and sent it right back to him .Who am I to debate that with somebody who has forgotten more than I know or may ever know .
However based on that theory if the air box was tight and the fan blast was enough it could raise the air pressure going into the carb .In effect acting some what like a turbo .That's only a guess on my part .

 

[Dieselshawn]

My buddy did that with his Yamaha Apex 4 stroke snowmobile.

He added some fan kit to the engine That goes to the intake.

Made 2 psi at 10,000 rpms.

I drove the sled. It has power.

 

[Al Smith]

There have been many method through the years for both boosted compression plus scavenging air .Aside from the normal turbo chargers of course the positive displacement Root type blowers .Some early slow rpm oil field engines used a single piston compressor .Cooper Bessemer that built huge pipe line compressors similar to the Coreless steam engine design used one piston for every two power cylinders .Some of those old antiques are still in service after 70 -80 years .All those however were for scavenging air not boost .

 

[Wanab]

Another thought .A well known builder of hot saws especially reworked 3120 Husky's is of the opinion the fan blast helps the performance .Fact he was sent a new one to rework somebody had shaved the fan and sent it right back to him .Who am I to debate that with somebody who has forgotten more than I know or may ever know .
However based on that theory if the air box was tight and the fan blast was enough it could raise the air pressure going into the carb .In effect acting some what like a turbo .That's only a guess on my part .

An interesting concept. I don't know how much boost you could actually see but an undeserved air box could benefit.

 

[Ketchup]

They are stout. But the "old" versions will make more power after both are modified.

To me a lot of the pro saws that really count seem pretty close in ported old or new. And the new ones have other functional advantages.

If anyone does, you have the data to say for sure. What ported old models beat out their ported newer versions? I could see 372oe beating a 572, maybe 346ne over 550mk2? 046 probably has more power than a 462 (though the displacement makes 044/462 a more fair comparison). I could believe a 192 is stronger than 193 but probably not 194. 200t won’t beat a 201tV3, 026 vs 261v2 is close. Interested to hear your specific thoughts.

 

[Al Smith]

I've never seen a modified 201T but can say in that displacement area it would take a lot to out run a 200T .None of the stock newer low emission saws I've ever seen even came close the older designs they replaced .Then again I've never seen any that had been reworked .I might add the only brand I'm talking about is Stihl .I have no idea about Husqvarna as they are not a popular brand in this area

 

[Ketchup]

Ask @sawmikaze which top handle wins out. Having run both quite a lot, I agree with him. I’d say of all the comparisons that’s the one I’m the most certain of.

Since we’re talking case pressure, I’m willing to bet the 201 has lower case volume and higher peak transfer velocity in the cut. The variation probably isn’t large but with saws that small minor changes make a big difference. 200t/201t is a really good illustration of shift in design theory and the effect of EPA regulation. Outside looks about the same, inside is very different. The first 201t showed the engineers hadn’t quite figured it out. We’ve seen that with lots of other models. 550mk1 and 661v1 (really all the V1 computer carbs) come to mind. There are still issues with delicate components, manufacturing flaws and overly complicated engineering (ever put a ms150/151 back together?). But by and large the latest versions are powerful, reliable and user friendly. Hopefully we’re through the worst of it. To me it looks like all the latest models address case compression compared to their predecessors.

As far as the 201/200 debate, the dyno doesn’t lie. I really need to send some hot top handles to Joe.

 

[Mastermind]

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.

 

[srcarr52]

My buddy did that with his Yamaha Apex 4 stroke snowmobile.

He added some fan kit to the engine That goes to the intake.

Made 2 psi at 10,000 rpms.

I drove the sled. It has power.

I had an 800cc Polaris sled with an aftermarket whirligig that stuffed more air in it, at only 7psi worth… it had powa!

Now I have a KTM 450sxf snowbike with a Garret GT20 spiny thing under the seat. It makes 110hp somehow.

 

[farminkarman]

I had an 800cc Polaris sled with an aftermarket whirligig that stuffed more air in it, at only 7psi worth… it had powa!

Now I have a KTM 450sxf snowbike with a Garret GT20 spiny thing under the seat. It makes 110hp somehow.

The second one you mentioned just sounds scary, lol!

 

[Dieselshawn]

I had an 800cc Polaris sled with an aftermarket whirligig that stuffed more air in it, at only 7psi worth… it had powa!

Now I have a KTM 450sxf snowbike with a Garret GT20 spiny thing under the seat. It makes 110hp somehow.

7 psi from a spinning thing? That’s awesome.
I need one on my 600...

Would you happen to know the name of said fiddle spinner?

 

[Dieselshawn]

The second one you mentioned just sounds scary, lol!

I drove a 50cc dirt bike that I put a new piston kit in for neighbour.

That little thing was peppy. It didn’t like my 260 lb ass though. Kept stalling or bucking me off.

 

[Bigmac]

I had an 800cc Polaris sled with an aftermarket whirligig that stuffed more air in it, at only 7psi worth… it had powa!

Now I have a KTM 450sxf snowbike with a Garret GT20 spiny thing under the seat. It makes 110hp somehow.

How much case compression is that getting?
That a metric s word of power! Lol
Super cool, but then again it’s a snow bike….it’s already cold, I mean cool

 

[Al Smith]

7 PSI would one and one half atmospheres if going through a reed valve check would really make a boost if you think about it .

 

[Dolkitafreak]

Lots of talk about forced induction here, let’s think of other ways to add oxygen to the charge…nitrous and nitromethane come to mind. Would a turbocharger make more power on a saw?

 

[srcarr52]

7 psi from a spinning thing? That’s awesome.
I need one on my 600...

Would you happen to know the name of said fiddle spinner?

It had an aerocharger 66 turbo with an air to water intercooler. I only ran 7-8 psi at 10k altitude so I could run pump gas. It was supposed to be 200hp. You could run pure race fuel and bump them up to 14-16psi, upwards of 300hp. 7-8psi was plenty, 70mph track speeds when hill climbing with a 2.6” 163” track.

 

[Wonkydonkey]

I just wanna add something about weight, the 201 is heavier than the 200. It may ripass. but Now every one is raving about the super light 2511 up in the tree,
Lightness up tree really counts

but your right about case compression, it would be interesting to know the volume difference between the 200\201. I know it won’t be a lot but it will be significant.

 

[Dieselshawn]

Lots of talk about forced induction here, let’s think of other ways to add oxygen to the charge…nitrous and nitromethane come to mind. Would a turbocharger make more power on a saw?

That would be true, but those are temporary power adders.