Part 8: The Stratified Chainsaw

[Ketchup]

If you increase cold air intake duration, why can’t you compensate with a fatter carb tune?

I have done exactly that on 201t with good gains.

 

[Ketchup]

Here’s three strato pistons:



372xt, 41mm Poulan, ms201t.

The 372 and 201 have included the wrist pin race. Maybe this helps cool the bearing?

All three have that second window. Two open into the transfer upper, one into the intake. They might help lighten and balance the piston. Maybe they help cool the piston as well. I think they also have an effect on the flow of the cold air track. Could they help switch the direction of transfer flow?

Just looking at the cold air windows they look to be roughly half the volume of the transfers they feed. How much volume makes it into the transfers, how much gets pushed back into the windows, and how much actually goes out the exhaust is harder to know. I guess fuel efficiency improvements indicate a rough percentage.

 

[Nutball]

Do strato saws have smaller carbs because they waste less gas out the exhaust, and so they don't need as much fuel/air mix?

I used to think that maybe they pulled an extra rich mix through the intake which later leaned out with the left over fresh air in the cylinder, and this may be true to a small extent.

 

[wcorey]

Do strato saws have smaller carbs because they waste less gas out the exhaust, and so they don't need as much fuel/air mix?

I used to think that maybe they pulled an extra rich mix through the intake which later leaned out with the left over fresh air in the cylinder, and this may be true to a small extent.

I believe you're overthinking this backwards, you more or less had it right the first time...

The smaller strato carbs do supply a richer mix that when combined with the strato air intakes ends up equal to the mix supplied to the combustion chamber in a non strato saw. So since you don't need a bigger carb just to pull enough fuel and with strato less air needed through the carb = smaller carb.
Strato is potentially more fuel efficient but I doubt that has a significant effect on the size of the carb.

 

[Ketchup]

Anybody got any info on jet sizing in strato carbs? Say RWJ compared to HD12. Same size jet?

 

[drf256]

Carb size would be related to strato design. On some saws, like the 362/441, there is a totally different dedicated strato butterfly and throttle body to feed the strato’s. I’d imagine those carbs might be smaller (but smaller carbs actually need a relatively smaller jet because they have more Venturi effect-its back asswards). On the newer and more ergonomic designs like the 261/462, the carb bore is the appropriate size with the bore and boot fully divided for strato and intake port at WOT via the use of dividers. The two systems are fully divided at WOT, but not fully at idle and mid throttle.

It doesn’t matter how much one lowers the jug by base cutting, the stratos won’t move any differently than the intake, even though they are in different parts of the bore and piston speed differs. The physical distance of the openings on the slug and piston are the same, it won’t change unless you grind on these areas as well.

Biggest issue is the balance, as Shawn mentions. If you drop the intake floor, you’ll get less case vacuum to pull strato air into the engine backwards through the uppers. If you open the stratos sooner, you’ll get less vacuum when the intake opens to pull fuel mix in. The factory figured out the balance, and part of why these little wonders respond so well to porting is the fact that we are increasing durations in both by lowering the jug. Granted, their goal for balance is different than ours.

I remember AWOL saying he “matched the stratos” on his legendary 562. He would grind in the intake floor and then match the piston strato divider to open with it. It was possible to visually see the stratos opening in the 562 because of its removable transfer covers. I’d like to try it on a 261/462 when used jugs become available some day at reasonable prices. I already get a fair amount of spitback in these saws without touching anything. A 261 will probably where I will start playing with this, using the reasonably priced oem piston alone.

Just another port and timing event to think about, but the factory did put another way to get more air in there which is a good thing.

Sorry for the long winded entry.

 

[huskyboy]

Anybody got any info on jet sizing in strato carbs? Say RWJ compared to HD12. Same size jet?

HD12 larger jet. The carb on a 441 has the same size jet and a similar sized venturi as a hd12. This is because the split design venturi of the rwj4 is smaller (when the divider is in it) and doesn’t need as large of a jet to fuel the saw properly. Think of it it as putting the choke on partially in your saw... richens the air/fuel mixture. Smaller venturi does the same thing. Now strato carbs like the 441/572 with the “separate” carb and strato venturi’s have the potential to flow more air than the “combined” setups like the 372xtorq and 462 use. The cross sectional area is greater in the strato and carb intake tracts/bores on “separate” strato setups. The reason they use the “combined” setups is to save space/weight and greatly simplify the design. As to what design is truly better... well I still think a old style traditional non strato saw is better for all out performance potential. But we don’t really have that option anymore with the emissions requirements now.

 

[Mastermind]

Excellent discussion men.

 

[Ketchup]

I think tunnel volume and case volume dictate a lot relative to optimal intake timing. Who’s to say having cold air advanced 2 degrees is bad? Or having charge intake advanced a few degrees? Each model of saw is a little different.

 

[Mastermind]

Who’s to say having cold air advanced 2 degrees is bad? Or having charge intake advanced a few degrees?

You just have to test.....then test more. To arrive at any of the "recipes" I have saved in my notes, I started with several different saws of the same model, and used slightly different durations from unit to unit. This takes a lot of time.

 

[timberjack90]

On my 362 I noticed the strato butterfly doesn't open till like half throttle, while most others seem combined and open at the same time? Could that be used as variable port timing? If the thought has any merit I can try it this spring, while I wait for the woods to dry up.

( I'm thinking that the closed butterfly will negate the lost vacum from an aggressive port timing)

 

[Ketchup]

362 behaves funny on cold start. Maybe that’s why.

I think it’s worth experimenting with. It might do good things to the power band. 362 is kind of a peaky creature.

 

[huskyboy]

362 behaves funny on cold start. Maybe that’s why.

I think it’s worth experimenting with. It might do good things to the power band. 362 is kind of a peaky creature.

It does affect off idle torque, throttle response and also “cold bloodedness” messing with the strato linkage. You can make those 3 things better or worse. At least in my findings with a 441 and 572. There’s a tiny adjustment set screw on the 441 and on the 572 you just bend the linkage to alter it. Imo the factory settings are close enough to optimal to where I don’t feel the need to mess with it usually.

 

[Lightning Performance]

On my 362 I noticed the strato butterfly doesn't open till like half throttle, while most others seem combined and open at the same time? Could that be used as variable port timing? If the thought has any merit I can try it this spring, while I wait for the woods to dry up.

( I'm thinking that the closed butterfly will negate the lost vacuum from an aggressive port timing)

I been scratching my head on that one for a quite a while thinking it limits spit back some and improves mid throttle grunt. It is one distinct change you can feel on the trigger compared to any carb on my 361 ported or not. The 362 carb saw is getting ported first, over done possibly, to see just how far one can go before cutting the base, the losses come or severely diminishing returns with down low losses.

The other question I found more interesting is this... asked earlier on page one.

How far can you go killing off base case time before the pressure losses start showing up at mid throttle on a 362 carb saw? I'd bet the Mtron saw covers up more than we think at part throttle to compensate for these "mistakes" if you will. The carb is not very forgiving on most saws. The Mtron is something I'm still learning to work with but the question remains the same. How low to go on base case time without losing too much pressure to feed this huge intake system compared to a non strato quad port?


Plenty of the stuff applied to other bigger bore saws limits your doings like dual ports are easier to fill on larger case volumes so less case time is possible with less losses to the transfers imo. Also the larger bores have a limit on compression more so than these smaller under 48mm bores. Heat becomes the enemy on the larger stuff.


Let me simplify that because we all know how well my thoughts come out in print :/
Edited at the witching hour
I want to drive the transfers up to the point of no returns then start filling in the intake floor to limit timing even farther but open up the the exhaust ports on the muffler and widen the exhaust without raising it. Then the squish goes at 15 base gasket delete only 51813 sealer with a matched case. Advanced ignition timing. Then start playing with the second throttle opening and the strato ports only but leave the rest alone at that point.

This was how I figured out the factory 361 complete intake system and air filter is a joke. Am still running all OEM parts on my saw nothing is aftermarket. The 362 HD filter seems more restricted than the old style. Both are terrible but not worse than a 361 filter on a ported saw with a stock carb.

 

[Lightning Performance]

It does affect off idle torque, throttle response and also “cold bloodedness” messing with the strato linkage. You can make those 3 things better or worse. At least in my findings with a 441 and 572. There’s a tiny adjustment set screw on the 441 and on the 572 you just bend the linkage to alter it. Imo the factory settings are close enough to optimal to where I don’t feel the need to mess with it usually.

The 362 linkage on the carb will be a nightmare to alter imo.

Hows work going in the mud?

 

[huskyboy]

The 362 linkage on the carb will be a nightmare to alter imo.

Hows work going in the mud?

About to freeze up again tomorrow morning

 

[Lightning Performance]

About to freeze up again tomorrow morning

Yup same here.

Mark said it was blowin like an... a never mind

Stay Safe out there

 

[Ketchup]

How far can you go killing off base case time before the pressure losses start showing up at mid throttle on a 362 carb saw? I'd bet the Mtron saw covers up more than we think at part throttle to compensate for these "mistakes" if you will. The carb is not very forgiving on most saws. The Mtron is something I'm still learning to work with but the question remains the same. How low to go on base case time without losing too much pressure to feed this huge intake system compared to a non strato quad port?

Shortening intake duration is an interesting idea. I’m not sure how much a fatter tune in the carb could compensate (another possible use of a larger jet) but transfer velocity should improve.
There’s not much room to widen the mixed intake on a strato. So all you have is height and carb tune to get enough gas in there.

 

[huskyboy]

Can anyone tell what I did here to this 572 airbox... I figured even if it didn’t gain, more of the spitback would get into the stratos. Give that side of the piston a little more lube while keeping the strato completely functional still. I ran the saw today cutting 30 or so ash trees. It seems to have a little more low end torque now and if anything the fuel mileage is a little better. I’m not sure if it actually made the saw measurably faster though. Or if it was worth the effort to grind that baffle out. I did notice the saw is a little louder now as well. The intake baffle must have had a silencer effect on intake noise.

 

[huskyboy]

I thought this was relevant to this thread because I have noticed some strato intake setups on some saws allow spitback to get in the strato intake. Like the ms441 for example. Some have baffles purposely to keep spitback out of the strato. Makes me wonder why...