Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

I did some calculations on the width of the intake plus the two strato port windows (transfer port window width) to see what the combined intake and strato ports gave as a percentage of bore diameter. I knew that the intake and strato ports together would be a good size gulp, but I wasn't ready for the actual number.

The 450 has a piston diameter of 44mm. The intake port is 15mm and the two transfer ports (strato intake) have a combined width of 27mm.

The combined width of the intake and strato ports is 42mm.

That gives a total intake cycle opening of 95% of bore!

Not only is it 95% of bore, but the openings are flat, there is no curvature to the opening like a conventional two-stroke.

Don't even worry about the small size of an intake on a strato, with the combination of the strato intake these engines can gulp air a lot quicker than any conventional two-stroke. As such, the strato should also be able to use less intake timing than a conventional two-stroke.

Now that I have run the numbers, I can see that a strato has more potential for power than a conventional two-stroke.

 

[hunter h]

coil

I'm assuming all limited coils are blue? Mine is black.

Some are green.
hunter h

 

[Terry Syd]

Update

I should add some more observations I've had about the Husky stratos.

I never liked the color I got from my sparkplug (even stock). The center electrode was always showing that it was running very hot, and yet the deposits in the combustion chamber and top of the piston indicated that the engine was running rich.

This problem continued to worsen as I increased the power of the engine.

The light finally came on after some discussion in another thread where Timberwolf was commenting on the 'air injection' system of pre-filtering the air. Although the discussion was primarily about how the flywheel pre-heats the air by about 10 degrees Farinheight (which decreases the density of the air), it also occurred to me that the 'air injection' system was sucking air out of the cooling system.

The more power (and thus HEAT) that I produced from the engine, the less air circulation I had available to cool the engine. - The more flow I created inside the engine, the less flow I had over the cooling fins.

I blocked off the 'air injection' system and did some further work on the cooling fins. I modified a few areas on the jug to increase the flow of air deeper into the fins. I drilled a couple of holes at the back of the top cover to allow enough air into the 'air box'.

The result was spectacular. I kept the same jetting as before, then got the engine hot and did a 'plug chop' with the 18" bar buried in a log. The plug came out looking like a plug should that was running rich. Instead of the center and outer electrodes being cooked white, they were now a chocolate brown.

If you're going to mod one of these engines that has some sort of centrifical pre-filter off the flywheel fan, you better watch the temperature.

As it was, the combustion chamber had been showing rich, but the underside of the piston was showing dark carbon buildup. Worse yet, the taper on the piston was being used up by the crown expansion to where the entire piston had expanded all the way to the top and I was getting wear the full circumfrence of the area above the top ring. - Now the piston is getting a layer of carbon built up in the area above the top ring.

I thought I might have to put a 'clearance band' on the piston, but all I really needed to do was to give the jug a good flow of air across it.

 

[Terry Syd]

I read back through the thread a bit and saw that I didn't mention anything about the crankcase compression results. Here's a thread where I mention what happened and some further explanation of what increased crankcase compression does. There are also some links that help explain the concept.

http://www.arboristsite.com/chainsaw/141737.htm

I could only stuff the crankpin, sealed (shield) bearings weren't available in my size. The use of the styrofoam was a mistake, it broke down and dissolved. I cleaned out the pin and completely filled it with the Ozzie equivalent of JB Weld.

 

[cheshire cat]

If nobody is interested in this stuff I'll quit posting it. It seems like I am talking to myself.

I'm listening albeit 2 years on!!:msp_wink:

 

[Terry Syd]

Cheshire Cat, every now and then someone posts on this thread and I get an update. Which then prompts me to add another update.

I found that I wasn't getting full flow through the strato ports. What was happening was that at TDC there were portions of the piston skirt blocking the strato port openings. I had to reshape the piston skirt so that the strato ports remained full open. I also took a bit off the end of the pin bosses to aid the flow of air through the piston 'cutaways'. Essentially, all my porting of the strato system was done on the piston, I never touched the cylinder.

Since the strato system is part of the total intake system, you want to maximise both the intake flow and the strato flow.

I never could get the stock flocked air filter to flow enough. The filter was built for a stock 445, not a modded 450. I purchased a nylon mess filter (yellow frame) through Baileys and had to open the High speed needle another 1/8 turn. I also purchased a black frame nylon filter, which is the winter filter. I might use that if I find an appropriate venue - like racing my mates.

After modding the muffler to get it to flow, I found the 'fish gill' vents I put on the sides were allowing a sonic wave to escape directly out which gave the exhaust sound a 'crack' to it. I took the muffler apart and brazed in a number of baffles so that the sonic wave has to reflect off of at least two surfaces before exiting any of the openings. This mod got rid of the excess noise and produced a deeper tone to the exhaust. At idle, you can hear the waves reflecting around in the can and coming out at different tones from the five different exits to the muffler. Once you hit the throttle everything blends together. While I was in there I found that I needed to enlarge the openings in the tubes through the muffler. I also reworked the back of the stock exit. In other words, although I quieted down the can muffler, I also increased the flow through it.

I'm now experimenting with an 8-pin on the clutch. It works well with the 15 and 18" bars, but I am having trouble getting the 20" bar on. I will have to wait a while and 'stretch' the 20" chains enough so that I can get the bar on with the 8-pin. I was surprised that I could still use the 5 degree cutting angle with the 18" bar while using the 8-pin.

I had previously taken the cutting angle of the chain out to 6 degrees and the engine pulled it just fine, however it was way too aggressive for limbing. I settled on 5 degrees as a compromise for general usage. Then I thought about jumping up to an 8-pin. I could increase the chain speed 14% and decrease the cutting angle. Essentially I could have a very smooth limbing saw that would still cut well while bucking up big logs. It looks like it will work, I just have to stretch the 20" chains some more so I can run the 8-pin with all the bars.

20" bar with an 8-pin sprocket, not bad for a 10.8lb powerhead.

 

[cheshire cat]

strato exhaust port sizing

Whilst I had pretty much made up my mind to only "attack" the piston rather than major port mod's now I've actually measured it, it seems to be approx 47% of bore with stacks( well approx <1/2") of room to expand before hitting the transfer port it's very tempting to get cutting:eek2: any pointers gratefully recieved. p.s it's a ryobi strato(china div'):msp_smile:

 

[SawGarage]

I was surprised that I could still use the 5 degree cutting angle with the 18" bar while using the 8-pin.

I had previously taken the cutting angle of the chain out to 6 degrees and the engine pulled it just fine, however it was way too aggressive for limbing. I settled on 5 degrees as a compromise for general usage. Then I thought about jumping up to an 8-pin. I could increase the chain speed 14% and decrease the cutting angle. Essentially I could have a very smooth limbing saw that would still cut well while bucking up big logs. It looks like it will work, I just have to stretch the 20" chains some more so I can run the 8-pin with all the bars.

20" bar with an 8-pin sprocket, not bad for a 10.8lb powerhead.

Terry,

would you be so kind to explain which cutting angle your referring to? the angle cut of the tooth from the top, or side?

thanks,

J

 

[Terry Syd]

Cheshire, what limited the exhaust port width on my saw was the bolt holes for the muffler. I took the width out to about 63% of bore and left it there. I could have gone all the way to 70%, but I would have had to fill the ends of the bolt holes with GB Weld. As this is a day to day wood saw, not a race saw, I didn't want to chance problems in the future.

Another technique to widen exhaust ports is that they can be give a generous radius into the port. Since the flow is INTO the port, the radius can effectively 'widen' the port where there are mechanical issues preventing a full port to that width. In other words, I could have slightly 'bell mouthed' the port around the ends of the bolt holes.

However, if the exhaust port was taken out to 70%, you would need to increase the size of the exhaust port into the muffler. The size of that opening would be determined how you rebuilt your muffler and the opening into it. There is very little you can do laterally, but you could increase the opening vertically a small amount.

On my saw, the port opening at the cylinder is almost the same area as the port opening at the muffler. Going any further with the port width creates other problems that I don't want to deal with on a day to day work saw.


Saw Garage, I tried to find BobL's excellant thread on cutting angles and progressive filing of the rakers, but the search function is acting up.

Essentially, it is the 'cutting angle' that determines the bite of the cutter - not the 'raker depth'. The cutting angle is a trigometric function of the width of the gullet (tip of cutter to contact point on raker) as the hypotenuse of the triangle and the raker depth as the base of the triangle.

I really need to find that thread as there were heaps of pictures and drawings to explain it all. A number of people on this forum considered the thread as the most important thread on chain sharpening.

 

[Stumpys Customs]

subscribed:hmm3grin2orange:

 

[Terry Syd]

I found BobL's thread on cutting angles - http://www.arboristsite.com/chainsaw/114624.htm

 

[husq2100]

I found BobL's thread on cutting angles - http://www.arboristsite.com/chainsaw/114624.htm

ask Rick in the aussie drible thread

 

[Terry Syd]

My post about BobL's thread was a reply to SawGarage's question about cutting angles. I found BobL's thread using the 'Google' search engine. The search engine on this site just doesn't work very well.

In fact, it was using the Google search engine that I found the references to Pop Off Pressure on this site. I suggest that if anyone wants to research this site, that they use the Google format. Here's an example of looking for 'pop off' -

pop off site:arboristsite.com

That will bring up a bunch of references to 'pop off' on the Arboristsite that you won't be able to get with the Arboristsite search engine.

 

[Terry Syd]

In regards to carb tuning, I made this compilation of posts from the EC Birt forum. I just took the good parts and left out the waffle. Each 'paragraph' is the essence from each of the threads -

We like it closer to 9 lbs pop, usually no lower and when we go to high altitude we raise it 2 to 4 lbs.
At your altitude you should be higher on pop than if you were at sea level because of the air density differences. On gas a .085 arm height and about 12 to 13 pop should do it.

It should work now, .065 arm height gets the needle open sooner than .075 as the demand diaphram has less distance to move to open the needle and creates more fuel volume in the wet side. If it bogs bring that low side needle out till it quits bogging and pulls hard. After you get it fattened on the low side you may have to lean the top some to get it lean enough to fly on top.

Is this a gas carb? Gas pop-off should be at least 10 lbs maybe a little more. If it was low it would make it rich. Go with 11 lbs pop with a .075 arm height if gas and 8 to 9 pop if alky with a .065 arm height. Then you can tune with the needles to lean it out. Jon

It likes fairly high pop-off , because of the small venturi. Try around 14lbs, if it acts lean, and wants the needles to be way out then drop a couple of pounds.

the lower the pop-off is set the richer the fuel curve is at the bottom.

the pressure of the spring controls pop-off. So ,if you go from a low pressure pop to a higher one, spring pressure must increase. Cooler air is more dense, so pop-off should be lower in the winter, and raised some for higher temps.

We run right at 11 lbs pop-off for our Leapords here in the States, and this is for tracks from sea level to 1500 or so. Higher altitude would require slightly higher pop-off.

it's all about the spring . You can cut a coil off on some springs and still have a full flat coil on the end, if not you should go to a softer or shorter spring to lower. To raise pop, you should either change to a stiffer or longer spring or if your Tec allows we have some .010 shims that go under the spring to raise at about 1/2 lb at a time.

We have a complete assortment of springs that we carry, but there's a lot of them you wouldn't use if you run just gas or just alky. We can pick you a spring that will hit close to what you want then either shim up or cut down to get it right where you want it.

18 lb pop-off is pretty typical for the UK, because of conditions and the way the carbs are built for UK tec. Higher pop-off would take some more fuel away and would only be productive if you were too rich at 18 lbs. As far as arm height , I prefer .085, which is leaner than .075 and will make a big difference in fuel flow. You just have to try some different settings, to see what will work best for you. Go with .085 arm 16lb pop-off, tune it about 1 3/4 low and 1/2 high or whatever on high to get engine heat where you want it. If you find your needle settings want to be way less than this, then raise the pop-off a pound or two.

you need a .085 arm height and about 10 to 12 pop-off pressure, that will get you where you can tune it with the needles. If you were at high altitude you would want it higher. We measure arm height from the carb base down to the top of the arm, and thats with no gasket

You just have to optimize the tune on each individual carb, the main difference will be the 166 having a smaller venturi will have a stronger fuel signal than the 334 which will make the 334 even more touchy about having the correct pop-off. And when you are in the right range of pop-off then you can tune it out with the High and Low neddles. If carbs are on the same motor then 166 needs higher pop than the 334. The 166 12 to 14 at sea level going up as high as 18 for real bad air or altitude. The 334 needs about 10 on a small motor and then go up from there with heat and or altitude.

 

[Terry Syd]

Here's an interesting article with a formula for computing the size of your carburetor.

http://webspace.webring.com/people/bf/flphg/carb.html

Using the formula, the size of the carb for my 50cc saw (at 12,000 rpm) would be between 16 and 22mm. The combined area of my 570 carb and the strato port is equivalent to a 19mm carb.

I suspect that the lower POPs on karts may have a lot to do with the larger carbs relative to the engine displacement. The lower POPs are necessary on the karts because the signal in the venturi is lower than on a small chainsaw carb.

 

[Terry Syd]

Here's a thread from 2008 that discusses POPs. You will note the concern with the high POPs as compared to the kart engines.

http://www.arboristsite.com/arborist-101/67662.htm

Here is a post from another thread -

"OilHead, yes I have tested about all the saws I can get my hands on. From what I have found, if the carb makers are paying any attention at all to the pop-off pressure on their carbs for stock saws, they must be on a secret mission to conceal their knowledge...ie, the pop-off pressures are all over the board. A 242xp that I tested today, had a pop-off pressure of 34psi! More often than not, I'm finding stock pressures above 20psi. Don't know if you saw the email I posted from Walbro on the subject.... Basicly, a guy there said it doesn't matter on stock equipment, but did go on to say that in racing engines, like in karting, the pop-off is more critical. I guess that they think it is out of their jurisdiction to have to respond informatively to any question that goes beyond stock engine requirements. Unfortunately, they act totally vague on the subject. I still haven't exactly settled on what I find is best for my modded saws, but I doubt it is 34psi."

I note that these fellows were finding a POP of 15-19 psi was working for their carbs (whatever they were) on their saws (whatever they were). However, as the formula for computing the optimum size venturi for a two-stroke shows, CHAINSAW CARBS ARE SMALLER THAN THE OPTIMUM. That means that they will need higher POPs than the kart engines.

 

[Terry Syd]

After reading the threads on the EC Birt forum, I was fairly well convinced I needed to drop my POP. In particular the statement "the lower the pop-off is set the richer the fuel curve is at the bottom." The lower POP would allow the carb to flow more fuel at lower velocities through the venturi - and then the top-end RPM would be limited by the fixed orifice of the High speed needle. - That is the 'fuel curve' information I was looking for.

So far I have cut off two of the collapsed loops from my metering spring and the saw is picking up more torque down low.

I'm still uncertain about lifting the metering lever (reducing the distance from the diaphragm to the lever). It appears that it may result in a linear (as opposed to a curve) increase in fuel flow. I know that when I did raise the lever on the old stock carb it did increase L and H speed fuel flow and I had to lean both out, but I would have to get some more experience to say definitively.

Other considerations to get optimum metering:

If you put a bigger carb on your saw, you may have to reduce the POP.

If you put a big bore kit on your saw, you may have to increase the POP.

If you substantially increase your RPM and use the stock carb, you may have to increase the POP.

EDIT: The above guidelines only apply under the assumption that the stock POP was dialed in correctly in the first place. I would expect that with the EPA influence on emissions, that the POP may be biased to being too high in the first place.

If the POP was higher than the optimum for a saw, a big bore kit may put the carb POP nearer to the optimum setting. In which case, some of the increased torque that is experienced from big bore kits could be related to the more uniform metering of the carb over the powerband.

 

[Terry Syd]

As to why I thought dropping my POP would improve my low-end, here is a bit more of an explanation.

I'll limit this discussion to the main high speed circuit and leave the low speed circuit out.

If we made up a diagram/chart of fuel flow to air velocity through the carb, we could have fuel flow being the vertical axis and the CFP of air flow the horizonal axis. A properly designed carb would have a uniform fuel flow (uniform mixture strength) that matched the increase in air flow. In other words, perfect 'jetting' all through the rev range.

Of course, the air flow through the venturi creates a depression (the 'signal') that rises expotentially as the velocity increases. That would normally create an excessive strength (too rich) as the air velocity rose. However, we also have the fixed orifice of the High speed needle that limits the flow of fuel expotentially as the fuel flow increases. A properly set up carb would have the two systems (air flow and fuel) cancel out their expotential functions and deliver a uniform fuel/air mixture.

If you dropped the POP on a carb, you would increase the fuel flow at all velocities - except that you could limit the top end of the curve with the High speed needle orifice. The result would be a carb that would be richer down low, but with the same high speed fuel flow.

Conversly, if you increased the POP you would decrease the fuel flow at all velocities - except that you could richen the High speed needle. The result would be a carb that would be leaner down low, but with the same high speed fuel flow.

 

[Terry Syd]

Rather than just start cutting up a metering spring with a set of toenail clippers, I think I have figured a way to test a saw to see if the POP is correct for its powerband. Since there are two 'peaks' to consider (peak horsepower and peak torque), you tune for each of them.

Peak horsepower will be where the saw is doing the most work. Take the saw and put it in a log and tweak the High speed needle to get the best peak horsepower. Then put a load on the saw and pull the revs down to peak torque.

Now, check the settings at peak torque and see if a change in the High speed needle setting will increase the torque.

If you find the engine needs a bit more fuel at peak torque - then you need to decrease the POP.

If you find the engine needs to be leaned out a bit - then you need to increase the POP.

After changing the POP, you will need to re-test the same way. For example, if you decreased the POP you will likely have to lean out the High speed needle a bit to get the peak horsepower back, then check peak torque again. Rinse and repeat until you get the carb the way you want it.

 

[drkptt]

Here is a post from another thread -

"OilHead, yes I have tested about all the saws I can get my hands on. From what I have found, if the carb makers are paying any attention at all to the pop-off pressure on their carbs for stock saws, they must be on a secret mission to conceal their knowledge...ie, the pop-off pressures are all over the board. A 242xp that I tested today, had a pop-off pressure of 34psi! More often than not, I'm finding stock pressures above 20psi. Don't know if you saw the email I posted from Walbro on the subject.... Basicly, a guy there said it doesn't matter on stock equipment, but did go on to say that in racing engines, like in karting, the pop-off is more critical. I guess that they think it is out of their jurisdiction to have to respond informatively to any question that goes beyond stock engine requirements. Unfortunately, they act totally vague on the subject. I still haven't exactly settled on what I find is best for my modded saws, but I doubt it is 34psi."...

I've designed small engine fuel systems for the last 10 years and can assure you that the manufacturers don't pay attention to pop-off pressures. They may fiddle with spring rates to fix positional issues (idle speed change when turned upside down). Measuring pop-off pressure is a goofy way of checking the spring preload but the magnitude of the number has nothing to do with carburetor operation.

Look at the metering circuit as a pressure regulator that maintains the pressure in the metering chamber at some value below atmospheric. That pressure difference is dependent on diaphragm area, lever dimensions, and spring force (and slightly on diaphragm stiffness). Pop-off pressure removes the diaphragm from consideration and adds the seat area of the metering (needle) valve. For the same carburetor a higher pop-off pressure should translate to a lower metering chamber pressure, but the value of the pop-off pressure has no correlation to the value of the metering pressure.

Clipping the spring to reduce the pop-off pressure is not a logical procedure. It reduces the free length of the spring and increases the spring rate. So the initial preload is reduced and the metering valve initially opens at a lower pressure. But for the metering valve to open more at a higher fuel flow a greater pressure difference (between atmosphere and chamber) is required because of the stiffer spring.

That's all great for steady-state pressure control, but now throw in the very significant dynamic conditions. Mechanical vibration of the spring/lever/diaphragm system is driven by the shaking of the carburetor, which is hugely dependent on the stiffness and damping of its mounting. Gas dynamics--pressure waves bouncing around in the airbox and carb throat--are very much speed dependent and typically have the greatest affect on AFR at max torque and max power.

If you want to pursue a study of metering spring weights I'd recommend obtaining a series of springs from Walbro or Zama and seeing if you can measure reproducible performance differences between them. Measuring pop-off pressure is a waste of time, imo.