getting some revs out of the 041

[blsnelling]

just to be clear, i havent altered anything on this saw yet, still gathering my thoughts. My bad.

i'll be bringing the squish from .057" or so down to around .020" which is considerable .. this will retard the transfer opening a degree or two (Probably about 3°. Each .010" is about 1°) and give me as much additional blowdown from the jug as i can get, and raise static compression. i doubt it would run on pump gas WITHOUT raising the exhaust quite a bit after that much squish clearance coming out. I think you'll be fine on pump gass and doubt you'll see 200 PSI. think about the exhaust port closing point. most saws close at around 90* before TDC and then begin compressing. Most are 100°-105°. I've never seen one at 90°. That's piped saw stuff. with the current port roof my 041 is closing at 109* before TDC, so its compressing for nearly 10 degrees longer. i dont think i have a choice but to raise the roof if this thing is gonna stay below 200psi cranking compression. even if i were to grind the roof .180" upward it would still start compressing at 95* BTDC which is on the slightly early side. I would put it at 105°. i guess im illustrating how misplaced the 041's exhaust port is in comparison to modern performance timing. I just timed a NE 346XP and the exhaust was at 108°. There's more to it than exhaust duration. yeah, if it was a modern 90* BTDC exhaust close and i went up 3/16.. the saw would be junk.

just have to decide how much blowdown to go for.

If you do nothing but widen the ports to skirt limits, you're likely to see a 1000-1500 RPM increase. No way would I raise the exhaust to 95° BTDC. You'll kill the torque. It's no just the loss of compression. Even when compression is maintaned, torque still seems to suffer when exhaust duration is increased significantly. There's not as much time for energy to be put into the crank. Your plan of .020" squish sounds good. Start with 105° BTDC on the exhaust, or even simply just return it to stock. You can always go back in if you don't get what you want. Widen both the intake and exhaust to skirt limits. I don't recal how the transfers are on the 041, so no comments there.

BTW, here's some more factory exhaust port opening numbers. As you can see, none have the exhaust duration you're talking about.

MS362 - 103
MS441 - 99
Solo 681 - 105
7900 - 102
5100 - 106
NE 346XP - 108
372XP - 101

 

[Terry Syd]

Percentage increase

MCB, here is another approach to looking at the increase in blowdown time, look at it in terms of percentages.

Since you have 11 degrees now, if you took it out to 13 degrees, that would be an 18% increase in blowdown time. However, the pressure drop is not linear, it is much greater during the early part of the blowdown period when the pressure is highest. Thus, a small increase in timing gives a more effective decrease in pressure than the percentage figure indicates.

Couple that timing increase with the increase in area from widening the port and the pressure will drop considerably more by the time the transfers open.

How much do you want to raise the cutting speed by? Is it 10%? 18%?

This is one of those areas where 'more is not always better' - and caution should be the rule.

 

[mcb]

that list is the most helpful thing for me so far, and i appreciate it sincerely brad. finding reliable/accurate info has been the most difficult task on my transition and most of the substantive reading ive found pertained to dirtbikes. some info suggested that the timing numbers were comparable, but i was mistaken. red ink on my numbers is great, little mental shift and im back on track. this is what i hoped the thread would accomplish. a little black marker on the jug wont hurt it! :newbie:

no matter how good or bad this one comes out, im gonna learn something and thats the value to me here. every cylinder head i ever sacrificed to the bandsaw was a benefit to every head thereafter, so this saw might just be the sacrificial jug to save my 2150, 385, 395, buddies 7900, etc.

 

[mcb]

MCB, the reason I mentioned cutting the piston crown to increase blowdown is that blowdown is related to the dynamics of the gas flow. Once you have enough for the engine speed, it's enough. You would be able to find that point much easier trimming the piston and running the engine, than relying upon a linear measurment such as compression which isn't related to gas dynamics.

You could easily go past the point of sufficient blowdown (for your engine) if you are aiming at a certain compression figure. In that case, you would end up with a 'peaker' engine and with less compression than you could have had.

Of course, if you don't have a piston to sacrifice, then slowly raising the exhaust is your only option.

terry, your posts have been great and i wanted to specifically respond this morning but im working like a dog lately.

im not rebutting anything you said, im in agreement and appreciate your input. im gonna set the squish and check compression without any port mods first, then adjust for dynamic compression (within reason) because its gotta run on 93 octane or less without knock, it cant snap pull cords every day and my father has to be able to start it. i will always go for the maximum cylinder pressure that the guidelines allow but it really is going to come down to the pullcord for pops.

 

[mcb]

using mr. snelling's generous data:




exhaust valve opening, in order of saw displacement.

346xp 50.1cc 108 ATDC
5100 50.4cc 106 ATDC
ms362 59cc 103 ATDC
372xp 70.7cc 101 ATDC
ms441 70.7cc 99 ATDC
7900 79cc 102 ATDC
681 80.7cc 105 ATDC



there appears to be a trend toward late exhaust opening on the smaller displacement saws. could this be to assist the smaller saws in retaining some torque that the larger ones make up for with displacement?

 

[mcb]

Widen both the intake and exhaust to skirt limits.

is there a generally accepted guideline for the margin between port and skirt width that still seals ? for example, a skirt of 1.500" width - .050 margin per side = 1.4" max port width ?

 

[ironman_gq]

I think .10 from the skirt edge is generally considered the max and still have a reliable seal. Some push it but it gets risky. Brad can tell you if Im off or not

 

[hotshot]

041 Super numbers

MCB,
I finally got all the ice storm damage cleared out here, & had time to dial up the Super 041. These transfers are plus/minus a few degees as are very hard to measure.

Exhaust port opens- 105° ATDC, thus 150° duration
Crankcase (Carb Intake) opens- 64° BTDC, thus 128° duration
Transfer ports open - 120° ATDC, thus 120° duration

Blowdown is also short, like yours, at roughly 15°

It had a new OEM "flat topped" piston that someone literally put on a belt sander, but ignorant or not, they did not hurt the squish band. They also drilled two holes on the sides above the wrist pin, but the piston is already slab slided for closed tranfers. Seeing this modified stuff it is bliss sometimes. Cranking compression was low on it. Would have to mill/drop the cylinder base down a good .025" to get some of the compression back, but there goes the port timing issues again....don't buy saws from Flea-Bay.

 

[blsnelling]

Exhaust port opens- 105° ATDC, thus 150° duration
Crankcase (Carb Intake) opens- 64° BTDC, thus 128° duration
Transfer ports open - 120° ATDC, thus 120° duration

Blowdown is also short, like yours, at roughly 15°

The exhaust and transfers look similiar to a current saw, but the intake has way less duration. Most new saws are close to 150° duration. I'd lower the intake to about 75° BTDC and widen all of the ports, including the rear transfers.

 

[mcb]

im going to say that my 041av and your 041 Super pretty much have the exact same timing, though my numbers posted on here might differ by a degree or two. it aggravates me to no end to constantly have my readings change every time i check. as opposed to 4 strokes where i can land on the nail 100x in a row.

if i have issues getting enough compression with base turn and gasket removal, ill probably clay the hot tub shape out on an old piston and weld a bump on the new one.

so whats the difference between the AV and the super then? is it just piston and jug?

 

[Brmorgan]

The Super is a 48mm bore and 72cc, the standard 041 bore is 44mm and 61cc. Also, the Super cylinder has a different fin design and is secured to the case with long studs rather than bolts. The ports all looked the same otherwise to me, with the exception of the lower transfers being a bit more smoothed out in the Super cylinder by my eyes.

 

[hotshot]

Brad M is right, when you put the two side by side there is little difference at first glance.

Brad S makes a good point about the crankcase fill timing duration & advance for the carb being so low, but this is where the skirt trimming or notching would apply. There is plenty of skirt left to cover the exhaust port bottom at TDC on these long slugs.

The problem is that my 041 series saws already spit out their charge if the bore or intake side skirt is even a little worn. From looking at the degree wheel map I made, it looks to be starting at 15-20 ATDC, but I'm only guessing. Probably where the rammed intake air (inertia driven) stops flowing.
A higher rise intake manifold would have made these alot more powerful.

 

[mcb]

theres a NOS stihl piston and cylinder kit on ebay for $200 missing a ring. probably make a big difference but not nearly worth it to me.

 

[SpaayDawg]

backwards intake spacer

On the last 041av I purchased, someone had previously took the intake spacer and "flipped" it backwards. Then to compensate for the carb being closer to the rear handle the air filter cover was notched to fit around the av handle, and the linkage rod was bent for the trigger to carb.
Original design of the spacer has the air/fuel charge pointing "up" toward the spark plug, whereas the backwards spacer then pointed the charge "down" toward the crankcase. Would there be realistic performance benefits from doing this that could be felt or measured? Otherwise I'm going to put everything back together as from the factory.

 

[Brmorgan]

Brad M is right, when you put the two side by side there is little difference at first glance.

Brad S makes a good point about the crankcase fill timing duration & advance for the carb being so low, but this is where the skirt trimming or notching would apply. There is plenty of skirt left to cover the exhaust port bottom at TDC on these long slugs.

The problem is that my 041 series saws already spit out their charge if the bore or intake side skirt is even a little worn. From looking at the degree wheel map I made, it looks to be starting at 15-20 ATDC, but I'm only guessing. Probably where the rammed intake air (inertia driven) stops flowing.
A higher rise intake manifold would have made these alot more powerful.

Well there really isn't much stopping a guy from doing that - the top cover is mounted to the carb alone, so you don't have to worry about the mount not lining up if you raise the intake assy. with a spacer block. Any good machinist should be able to build you something out of aluminum. All you'd have to worry about is making the throttle linkage line up, but unless the mod is drastic that shouldn't be a huge issue.

 

[Brmorgan]

Brad M is right, when you put the two side by side there is little difference at first glance.

Brad S makes a good point about the crankcase fill timing duration & advance for the carb being so low, but this is where the skirt trimming or notching would apply. There is plenty of skirt left to cover the exhaust port bottom at TDC on these long slugs.

The problem is that my 041 series saws already spit out their charge if the bore or intake side skirt is even a little worn. From looking at the degree wheel map I made, it looks to be starting at 15-20 ATDC, but I'm only guessing. Probably where the rammed intake air (inertia driven) stops flowing.
A higher rise intake manifold would have made these alot more powerful.

Well there really isn't much stopping a guy from doing that - the top cover is mounted to the carb alone, so you don't have to worry about the mount not lining up if you raise the intake assy. with a spacer block. Any good machinist should be able to build you something out of aluminum. All you'd have to worry about is making the throttle linkage line up, but unless the mod is drastic that shouldn't be a huge issue.

This would be a lot easier to do IMO than reversing the block as Jeff described, though a combo of both might have an interesting effect...

 

[nate cutter]

9 pin sprocket any one:monkey:

 

[mcb]

i would actually like to go up on the sprocket.. any idea where i could find one for this saw? im not too familiar with the usual places other than baileys who doesnt seem to have it.

where would you guys get your parts for an 041? i need to order up pretty soon.

 

[Brmorgan]

You can get a rim drive clutch drum from Bailey's to fit this saw, then you can run whatever sprocket you like. It's the same as the 031 drum, found here:

http://www.baileysonline.com/itemdetail.asp?item=ORF+18200X&catID=

I can't imagine a 61cc 041 being able to pull a 9-pin 3/8" rim in more than maybe 8" wood. A ported Super could perhaps make it work, but even then I don't know. The geardrive I just got though,

 

[mcb]

thanks brad