Let's Talk 262 Timing and Performance

[tickbitintn]

Will dibz work here! [emoji12]

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[exSW]

Will dibz work here! [emoji12]

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Nope,someone(s) else are way ahead of you.

 

[Mastermind]

Waaaaaaaayyyyy ahead.

 

[tickbitintn]

Lol, yeah I figured... Thought it would be good for a laugh though.
I missed out on one a few months ago and have been kicking myself for it....

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[drf256]

Toad's jug is done

And they can pry this from my cold dead hands



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[Deets066]

Al... When you guys get done dissecting the 262 we need to look at the 394.

Great info here fellas

 

[paragonbuilder]

Al... When you guys get done dissecting the 262 we need to look at the 394.

Great info here fellas

Husky convert already? [emoji23]


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[MustangMike]

Hey, I want initials on my jug! (I'll have to bring it back when I cut that Oak Tree)

 

[PA Dan]

Damn huskies

I know right? And I have two of them! One with the 120/mahle and one with the 87/KS! Can't wait to try the 87 against the 120.

 

[drf256]

I'm jonesing for an 87 if someone wants to give one up.

I think this saw wants even more than an 87 can provide.

 

[drf256]

I'm thinking that this saw would like more intake duration.

My next jug, which I'm starting this weekend, will be 103/119/75

C'mon guys, post up pics of your saws, mufflers, flywheels, jugs, etc...

No jug too ugly, we can all learn from each other.

Don't be bashful.

 

[Moparmyway]

Toad's jug is done

Doc, you did a fantastic job on what would have been trash !

 

[drf256]

Thank you Kevin.

 

[Deets066]

Nice sig Al

 

[Mattyo]

now I want the docs initial on my jugs! .....

wait a minute....

 

[Onan18]

I have two, one genuine with a non deco Mahle jug and 120 carb and one I built from a 257 chassis with a non deco Mahle jug. That one is wearing the stock 257 carb for now, cant remember the number currently.

Joe

 

[shorthunter]

So we have small case volume, a nice high intake and very low factory blowdown.

Why was this done? With the high transfer duration, one would expect the loop scavenged charge to be pushed right out of the cylinder.

Without reading each and every reply, here goes my theory

I bet you would notice a fair bit of carbon buildup on the transfers in a 262xp. As I mentioned in your other thread, many two strokes will back flow into the transfers before they ever flow up into the cylinder. Pressures are far greater in the upper cylinder after combustion than they are in the crankcase. This difference in pressures combined with a short period for blowdown tells me that Husky used the short blowdown period to increase crankcase pressure and ultimately charge velocity. Another thing to keep in mind here is that a shorter blowdown period will allow less time for fuel to escape out the exhaust.

With back flow the crankcase pressure will eventually exceed that of the upper half of the cylinder and the transfers will begin to flow fresh fuel up toward the combustion chamber. With the 262xp, the crankcase pressure will likely be lower than the cylinder pressure until somewhere near bottom dead center. This means that in 16-20° of crankshaft rotation, the piston will have trapped all of the fresh fuel above the exhaust port.

 

[drf256]

Without reading each and every reply, here goes my theory

I bet you would notice a fair bit of carbon buildup on the transfers in a 262xp. As I mentioned in your other thread, many two strokes will back flow into the transfers before they ever flow up into the cylinder. Pressures are far greater in the upper cylinder after combustion than they are in the crankcase. This difference in pressures combined with a short period for blowdown tells me that Husky used the short blowdown period to increase crankcase pressure and ultimately charge velocity. Another thing to keep in mind here is that a shorter blowdown period will allow less time for fuel to escape out the exhaust.

With back flow the crankcase pressure will eventually exceed that of the upper half of the cylinder and the transfers will begin to flow fresh fuel up toward the combustion chamber. With the 262xp, the crankcase pressure will likely be lower than the cylinder pressure until somewhere near bottom dead center. This means that in 16-20° of crankshaft rotation, the piston will have trapped all of the fresh fuel above the exhaust port.

Very interesting.

I'm having a tough time believing that the charge in the transfers is reversing itself 250 times a second. I think charge inertia will overcome some amount of combustion pressure up to a point.

But, I think-I don't know. I may well be completely wrong. Thats the point of the thread.

 

[drf256]

What stops the charge in the transfer from combusting if there's backflow of exhaust down into them?

Is the charge always fully combusted by that point under all circumstances?

I'm just thinking out loud for proof that there is indeed backflow, and its not just a burn of residual accumulated oil during deceleration.

 

[Redfin]

This difference in pressures combined with a short period for blowdown tells me that Husky used the short blowdown period to increase crankcase pressure and ultimately charge velocity. Another thing to keep in mind here is that a shorter blowdown period will allow less time for fuel to escape out the exhaust.

I believe I am misinterpreting the meaning of what you wrote. Using the stock numbers Al posted of 103/115/69 I was under the assumpition to lower case comp you would either have to decrease blowdown or add intake?

Also, wouldn't a longer blowdown give more time for ex to escape and not be mixed with incoming charge?