Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Reduced strato timing

I just got back from testing the timing changes to the strato port. I had the 18" bar buried in some very dry hardwood. The saw is holding the torque a bit higher in the range. If I lightened the the pressure on the bar the engine would start to get into light 4-stroking. I ended up leaning out the high speed needle a tad and it would still tend to 4-stroke if I lightened the pressure.

I think this extra bit of leaning out was necessay from the decrease in strato timing. The bias of the strato/intake timing was altered so that less air was being ingested, thus, there was less of a requirement of a richer fuel mixture from the intake to compensate for the strato air. I wouldn't have thought that a two degree change would have made much difference, but it did.

I think this is the original design of Husky and the slight increase in strato timing was to pass emissions testing. I like the present timing and will stay with it.

It would probably be a good idea to start recording the various strato timing figures with all the new saws. I expect that there is a relationship between the intake timing and strato timing. Perhaps the relationship might be something along the line of 6-10 degrees more strato timing to intake timing.

At lower revs, the extra strato timing allows the back-flow of the intake charge to move into the strato port and not the intake port - where it would start increasing the mixture strength. On a conventional two-stroke, when the engine begins to be lugged down the back-flow into the intake starts richening the mixture and the engine power drops off quickly.

However, the broad powerband of the strato may be related to the extra timing on the strato port. The back-flow can move into the strato port and not richen the mixture. Further, on the next induction phase that mixture will be pulled back into engine to be used on the next combustion cycle. In other words, there will be less air inducted and there will be more air/fuel mixture. The result is a broader powerband.

Therefore, to port an engine for higher power with slightly less powerband might require less strato timing. Conversly, an increase in strato timing could broaden the power, but with a slight decrease in higher power.

Perhaps what needs to be done (besides recording the various timing figures) is to test the theory using a slight increase in intake timing. If the intake timing increase moved the powerband up and tightened it, then the theory could be tested by then increasing the strato timing the same amount. If the useable powerband then became longer then the theory would seem to be valid. That testing could be done without porting a jug, it would only require a sacrificial piston to have the intake skirt trimmed, and then later the side of the piston to alter the strato timing.

I won't be doing such testing, but I throw out the idea for anyone else that want's to try it.

Edit: I note an error in a previous post. I can't edit that post so I'll include it here. I stated that the hole in the recess in front of the piston pin would induct a short shot of AIR from the initial strato opening. If you think about it, the transfer port is filled with fuel/air mixture from the previous cycle. Therefore, the hole in the recess will be sucking part of that fuel/air mixture back through the top of the piston. There will be no leaning out of the crankcase mixture by sucking air through the top of the piston, it will be fuel/air mixture going through it. The piston will run cooler because of it.

 

[Terry Syd]

Further work

I have just come back to one of my earlier theories on the bias of the starto charge, that is, the percentage of fuel mixture to the percentage of clean air. I recently tested an increase in intake timing while keeping the strato timing fixed. The saw made more torque/power higher in the powerband. I've trimmed the intake skirt some more so that the intake timing and strato timing are now identical. I expect another increase in power from that change, but I won't be able to test it until next week.

When I am doing these tests I have a few different size restrictors which I place on the strato port to change the amount of air the strato port can ingest. These restrictors allow me to change the bias of charging in the cylinder. The first time I used them I came to the conclusion that the 450 could use some more intake timing.

Here's one of my working theories on this saw. From the amount of soot I am getting in the upper transfers I am confident I am getting a substantial amount of back-flow of exhaust into the transfers (the saw only has 12 degrees of blow-down). However, the long transfer tunnels of the Husky strato design confine the exhaust back-flow on top of the clean strato charge and gets purged without dilution of the intake charge. The extra exhaust gases in the top of the transfer ports may even help establish the initial flow in the cylinder. That intial flow is quite turbulent and mixing does occur, but in this case it is exhaust mixing with exhaust. The clean strato air follows the exhaust as the flow is established and then later the intake charge joins the well established flow.

With only 150 degrees of exhaust timing and 12 degrees of blow-down, this saw still wants to rev (thus my recent increase in intake timing). I intend to continue working with the short exhaust timing to see how far I can push that timing. I'm not trying to build a racer, just a nice running saw for my personal use.

The strato presents some new challenges to two-stroke tuning, but bottom line it is still a two-stroke and the same old concepts still apply. So far, the more intake charge and less strato charge, the more power the saw makes.

Hmm, I should edit that last statement. When I put my restrictors on the strato port it did change the bias, however I also found that the saw didn't want to rev as high, thus it needed that extra flow - but I substituted it with intake timing which helped the charge bias.

Some further information to take into consideration. The strato intake tunnel on this 450 is 13.5mm. I have bumped up the 11mm carb on the intake to a 13.5mm venturi to increase the intake bias. So both of the intakes are about 13.5mm. With the recent increase in intake timing, both the strato and intake are now 152 degrees. Essentially, the saw is a 50/50 bias of intake to strato.

 

[Terry Syd]

Latest test

As background for some of my tuning ideas, I used the carb size (15mm) from the 346NE as a baseline to work from. That 15mm carb gives a total of 176 square millimeters of venturi area.

The 13.5mm venturi carb I put on it gives a total area of 143 mm2. In conjunction with the 13.5mm strato port - that is a total of 286 mm2

I figured if I put a quite large restrictor (plate with a hole in it) on the strato port the saw would still have more area than a 15mm carb. However, even a 10mm orifice on the restrictor plate (reducing total area to 221 mm2) would slightly dampen the ability to rev. A 8mm plate (reducing total area to 193 mm2) tended to choke off the high revs, but then I did pick up a bit of torque (more intake bias).

So, even though I had 193mm of area, which was more than the 346NE's 176mm, the saw was telling me that it wanted to breathe.

Even after bumping up the intake duration from 144 to 148, the restrictors still dampened the top speed. I have now tested the 152 intake duration and the saw has again picked up power higher in the powerband. I didn't have time to try some of the restrictor plates, but the saw is working well enough now that I may not bother (ok, I'll have to try them eventually just to satisfy my couriosity, but I'm happy with the saw now)

Perhaps down the road I may end up with a longer intake duration and a small restrictor on the strato port.

On the transfers, I did take a bit off the edge of the back of the port where the flow off the tunnel insert was directing the flow to hit the edge of the port. All I did was clean up that flow into the cylinder.

I did not lower the intake port, I trimmed the intake side of the piston skirt. The piston is a taper design (for heat expansion) down to about 5mm from the bottom. Then the piston skirt is a uniform diameter. I can trim the skirt to get my timing and not have to worry about ruining a jug.

I prefer to make changes to the piston, like the scallops and the intake skirt, rather than the jug. Pistons are a lot cheaper to replace. If I ever get around to doing further mods to the saw I might take a little bit off the top of the piston to extend the exhaust duration (and increase the blow-down). If I didn't like the extended exhaust duration, I can always purchase a new piston and go back to stock.

Based upon what I have found, a few simple mods to the 450 would help the power. A muffler mod, widening the exhaust port and trimming the intake skirt so that it opened at the same time as the strato port.

The saw has more potential, but those basic mods would realise most of the potential for the least amount of money.

 

[Terry Syd]

450 vs 455

I stopped by the shop and the guys ran the 450 against a 455 (55cc). The 450 cut faster and had less tendancy to loose revs when you placed a load on it.

They thought that the unloaded revs should have been higher with my mods. The saw would only turn 12,700 out of the cut. However, the speed in the cut was what mattered and it was cutting a lot faster than the 455.

Since leaning the carb way out didn't raise the revs much, one of the fellows thought it was the carb feeding too much fuel. Myself, I think it is because of the limited blow-down time.

The engine doesn't rev its guts out, but it does cut faster in the cut and holds the revs under load. To me, that sounds like the engine is hitting the limits of the exhaust porting.

All three of us agreed that it was a good work saw porting.

 

[Terry Syd]

Limited coil

Well, that explains it - this thing has a limited coil. The specs say it is limited at 13,000 and Rick's 445 does limit at 13,000. Why mine only hits 12,700 I don't know. I didn't do the tach test, so maybe it did or maybe it was an operator or equipment error. On the other hand, it is a different coil to Rick's so maybe I got a real dud of a coil.

It looks like I will have to run this saw on the edge of the limiter.

I know I should get a tach - and a new coil - but this chainsaw tuning is starting to get expensive. Thank goodness I still have contracted CAD.

 

[bitzer]

Great thread Terry! A lot of ideas worth thinking about and expanding on! There will be many more stratos lining up in the future to be ported and I think you are pioneering some ideas here. Man, I wish you had some pics of this thing, but then again I have no pics of mine either.


I know now that I NEED to degree my saw and come back with some numbers. I ran my 455 today for the first time in a few months in some 12-18" hickory and I am still impressed with the changes I've made to the saw. It has a lot of torque, just not the revs of the saws that I usually run. It will definetely dig for me! I usually use the owners equipment at work, but its fun to run mine once in a while.

Keep up the good work!

 

[Terry Syd]

Degree wheel

The next time you have the jug off you really should find out what the various timing figures are. On the Husky stratos it is really easy as you can take off a transfer port plate and watch the piston open the strato port and the transfer port.

You may find, as I did, that increasing the bias of the intake port to the strato port increases the power in the upper range. That is easy to do by trimming the intake side of the piston. If the piston taper goes all the way to the bottom of the skirt, then you should take the entire skirt off horizonally so that the skirt seals the intake port. On my saw the last 5mm or so is straight and I just trimmed off the area that was blocking the port.

I did most of the trimming to the intake skirt with the jug in my lap and moving the piston up and down. Since I finally ended up with matching times for the strato and intake I could sit there and move the piston and keep trimming the skirt until they matched - easy mod.

I'm already at the edge of the coil limiter, so I won't be raising the skirt any further. However, if you aren't hitting the limiter you may want to try adding a bit more intake timing than strato timing. I'd check it with matched ports first, then make a decision if you want to add a bit more intake timing.

Also, check that the transfer port covers are sitting in the right position. Mine were way off and I had to enlarge the screw holes so that I could get the covers all the way back and at the top of the port. I expect that this could be a common problem for all the Husky stratos. Since you can take both covers off you can look at the postion of each cover from the other open side, again it's and easy mod. However, it takes some time as you need to keep putting in the various screws and tightening them down to see if the cover shifts. If the thread of the screw touches the side of the hole it probably won't align properly.

My engine also has some 'tabs' on the covers that were probably designed to be used to lever the covers into position while you tightened the screws down. If you don't stick a slot screwdriver in there, then twist the screwdriver to hold the covers in postition - the covers will shift when you tighten down the screws - you need to keep the pressure on the covers as you tighten the screws.

 

[bitzer]

I am fairly new to porting saws only having heard of it just 6 months or so ago, but I learn very quickly and I have a good background in trig, physics, and chemistry. Before I knew about porting, I had been trying to figure out a way to give the 455 a little more power and I knew it had something to do with limiting the amount of strato air coming in. That is when I first undid the strato butterfly and realized the results and that the strato air is needed it is just finding out how much air to produce optimum results. Now that you have found that matching the port timing gives good results and that increasing the intake timing over the strato may give better results I need to do some numbers like you have suggested. I have ported 5 saws now with good success, this one is the only strato of that bunch. I will hopefully get to some numbers tomorrow.

 

[Terry Syd]

Diminishing returns

Like you I disconnected the strato and found out it needed that extra air. Obviously there is some mixing going on in the cylinder where the pure air is getting used in the burn.

There is probably a cross over point of diminishing returns where there isn't much power increase compared to the amount of fuel mixture being ingested. Perhaps it may be 50/50 or 80/20 - nobody knows at this point.

The impression I got from my saw was the longer strato timing was setting the top of the rev range (it was an impression, it may not be correct). It was revving well, but not making good power at those higher revs. However the saw could be loaded and as the revs dropped the saw would gain torque. The torque curve was likely set by the port figures.

By matching the intake and strato timing the power increased in the upper region, but I still retained the torque provided by the porting. Increasing the intake timing didn't seem to decrease the lower rpm torque.

It may be that with the matched timing figures, which results in more flow, that a small restrictor can be bolted over the strato port to decrease the strato flow and increase the cylinder charging bias, but without cutting off too much airflow so that the saw looses the ability to rev.

I'd like to play around with my saw some more and see where I can take it, but this limited coil problem sits like a brickwall in front of further development.

I'll do some searching on other potential coils. I get a kick out of working on engines and this saw is a lot cheaper to play around with than a bike or car.

 

[bitzer]

This is the first saw I have degreed so the numbers may be a little off. I am pretty sure the exhaust and transfers are right, just not sure about the intake and strato. When measuring the intake do I start with the port closed then moving toward TDC then end the measurement when the port closes again? I am measuring the time the port is open. I did the strato port the same way. I see my intake and strato numbers are a ways off from yours. The intake number is also not stock though.

Exhaust-164/2-180=98
Transfer-140/2-180= 110
Intake-160/2-180=100
Strato-164/2-180=98
Blowdown-12


I know transfers were originally at 132 or so. When I raised them I angled them up from the exhaust side to the intake side. So the exhaust side of the transfer is still stock, but on the side closet to the intake it is at 140. What will shortening the blowdown do in terms of power? I know it will get more fresh mix into the chamber faster, but what am I robbing by doing so? I will keep the saw apart for now just in case I do have these wrong.

 

[Terry Syd]

Timing

For the exhaust, measure BBDC to ABDC - it looks like your figures may be 82 degrees BBDC to 82 ABDC to get 164 degrees of duration.

Transfers are the same, measure BBDC to ABDC - that is a lot of transfer port timing at 140 degrees

For intake and strato, measure BTDC to ATDC - it looks like your figures may be 82-80 degrees BTDC to ATDC

What other ports did you modify? Did you raise the exhaust, lower the intake and lower the bottom of the transfer port?

 

[bitzer]

It looks like I did degree it correctly then according to what you said. The exhaust port I started it closed and set the wheel at zero, I then rotated it down (BBDC) and back up (ABDC)to close again. The transfers the same. The intake I started closed (BTDC) and set the wheel at zero, rotated up and then back down (ATDC) to close. The strato the same. I did widen the exhaust port to 65% and ended up raising it close to 1mm. I did not intend to do this, but my marker moved up at some point and I went higher than I wanted too. I generally have not raised the exhaust in other saws unless it was to blend in the widening. I am not trying to build a racer and I don't want to lose compression. The transfers I raised and did not lower at all. I lowered the intake and widened as much as possible. I did not make all of these changes at once. This happened gradually over the course of 3 tear downs and I saw increasingly better results. I did do port maps originally, but I could not find them today. I did nothing to the strato ports. I did the measurements at least three times. I also compared them to several other sets of numbers found on this site. It seemed like they were high but in the ball park. I will check them again tomorrow. Thanks for the clarification.


My thought behind the angled and widend transfers was to get as much fresh mix into the chamber as possible and increase turbulence at the back of the cylinder by aiming the transfers up and back. The saw has a lot more torque than it did.

 

[Terry Syd]

Timing

I wonder what the intake timing was before you increased it. It may have been similar to mine with 10 degrees difference in timing between strato and intake. In any case, you may get a small improvement by nipping up the piston skirt so that the intake opens at the same time the strato port does. If you take a transfer port cover off, you will be able to watch the strato port open on the piston and then rotate the jug to see where the intake skirt is.

As far as the transfer angles, they are a complex art. If you look at a four port jug you may see that the two side ports may be of different heights, different roof angles, different angles on the side closest to the exhaust and a different angle on the piston side. A five-port will add an additional complexity to the flow.

The ports will not be aimed at the back cylinder wall, rather they are aimed at an area on the piston crown near the back wall (which will vary from engine to engine). The various heights and angles are to shape and direct the various flows into the cylinder flow. The joining together of the various flows creates the 'loop' scavenging in the cylinder.

Husqvarna has been building saw engines for a long time and they have a lot of experience and testing equipment to arrive at a port arrangement. It is risky to deviate too far from a factory designed port arrangement on a well developed engine. If your were porting a communter motorcycle into a racer, then yes, there will be some big changes made to the porting. However, maintaining the stock port angles is a good starting point to a port job.

My impression of the porting mod you made to the transfer port did not significantly increase the port flow, but did change the direction and decrease the blow-down time. The decrease in blow-down time without the attendent increase in port flow may be affecting the top-end power. If you had just raised the exhaust and left the transfers alone, I would expect that your engine would be running stronger in the upper range.

The next time you want to increase the transfer timing, try nipping a bit off the side of the piston crown adjacent to the port window and see how it affects the power. If you don't like the change in timing, you can always just get a new piston.

So where to go from here. Matching the strato and intake timing would likely be a risk free mod. However, the higher transfer port timing really throws a curve ball into the mix. Your exhaust port duration is already long enough, so increasing it to increase blow-down is out. You could try shaping the piston crown to the shape of the top of the exhaust port, but you will have to take your time if you do the mod right. If you try hurrying the mod, you could end up increasing the exhaust duration even further.

It looks like just a small increase in intake timing, and then just run the saw the way it is. Perhaps someone else on the forum might have some other ideas.

 

[bitzer]

Thanks for the info! My job on the transfers probably looks like a hack job to a pro and It may have robbed some power, I am not really sure tho. I think the strato air is enough to blow the exhaust out hence the shorter blowdown on these saws. I may have distrupted the flow and it may not be looping like it should, I don't know. This saw was only rated a 3.4hp which is pretty low on a 56cc saw in my opinion so the changes I have made let it run with classic two strokes in the 60cc range. This saw is not a work saw and does more sitting than running so it is more for play than anything.

The probelm is that I did not make all of these changes at once and did not have a single log to make timed runs in. I did do some timed runs, but without one single consistant log its hardly a scientific comparison. I know that it seems to be running stronger now than it did before I made the changes to the transfers. My thought behind the transfers was that getting the strato air out onto the piston crown and sweeping the exhaust out faster would allow the fresh mix in faster and would give me more power by allowing more time for the combustion chamber to fill. I also rasied the transfers at the bottom near the cylinder base where the fresh mix is taken up from the crankcase. I raised that by 1mm and smoothed that whole area out in order to increase the timeing. The transfers on this saw are really long as you know and by shortening them in any way I thought I would be able to get more velocity.

I agree on the intake. I have the intake manifold and carb off right now and I can see the strato ports opening before the intake. Another interesting thing with this saw is that before I did any porting I modded the muffler and did some timed cuts. The muffler mod did increase the timed cuts by 15% or so, but as I made the saw leaner the times got faster and the drop off point was much leaner than the stock drop off point. I mean that between 2 and 1.75 turns out on the stock muffler it made its best times. On the modded muffler its best times where between 1.5 and 1 turns out, which was obviously too lean and it finally dropped off from there. I added a second port to the front of the muffler, kind of a vertical slit type opening like a dual port cover on a Stihl 660. The combined muffler port areas where 80% the exhaust port opening at the time and still are.

I also noticed that trying to do timed cuts to figure out tuning did not work. The saw just kept getting faster until I knew I was way too lean. I did contact the manufacturer of my coil asking if it is limited, but they have not contacted me yet. These stratos are a whole different animal and it may take some sacrificial lambs to find out where the optimum levels are. I just wish that I would have documented what I did to this saw with video and numbers and such. I will check the numbers one more time before closing the saw up again just to be sure. I will also figure out a function of degrees vs. millimeters to see what kind of timing change I made to the bottom of the transfers on the crankcase side. Thanks again for your input!

Oh yeah, the plastic intake manifold after the air filter and before the carb has a removable sleeve that directs the flow into the strato butterfly. This sleeve also restricts some flow to the carb. I am going to see if removing this sleeve does anything. If perhaps the air will bypass more to the carb or not. I wish I would have done more testing with this saw stock and learned to do the numbers first, but too late now. Thanks again!

 

[bitzer]

I lowered the intake another 2 degrees. I think I did get more power out of it, but it pools at idle. It will die when I pick it up after idleing for 30 seconds or so. I generally don't let it sit at idle when I use it so I don't think it will bother me too much. I messed around the the L and idle screws for an hour or so and I have it set about as good as it will get. I should have mentioned that the bottom of the intake skirt does not clear the top of the port so the actual duration is 77 degrees now. I would have to lower the intake several milimeters for the strato port to open at the same time that the intake opens. The saw seems to make a whistling noise while WOT in the cut now. Kind of strange... Thats all I have for now.

 

[Terry Syd]

Intake

I don't understand your intake reference to 77 degrees duration. If the port is opening at 77 degrees before TDC then you would have 154 degrees of duration (add another 77 degrees after TDC).

Did you measure the strato port opening at the transfer port window? With the cover off you can see the cutaway on the side of the piston begin to peek over the bottom of the transfer port window.

As far as the pooling, yes your intake timing is on the long side. I suspect that somewhere along the way you must have extended the strato timing out to 164 degrees.

You work on this 455 jug reminds me of a lot of earlier work I did, it provided me with lessons for later work. The most significant one being 'more is not always better'. Jeeze, I did some butcher work when I first started out. A far cry from my work on the transfers of this 450 - I used a lady's sandpaper nailfile to clean up the 450's transfers, when I started out I would have stuck a rotary bit in there.

 

[bitzer]

I am not sure how I would have extend the strato timing. I did not touch it. The 77 degree figure is one way so yes it would be 154 if I only count the amout the intake opens and not BTDC and ATDC. I wish I had degreed the saw originally, but too late now. I agree with you on a little more than a lot. The other saws I have done I did not take to the extreme and I liked the results. That is also why I did this over the course of now 4 tear downs. With the strato the game is different tho and I was looking for the optimum point. Well, too far now.


I recieved an email about my coil, the manufacturer is SEM AB out of Sweden I believe and they said only their blue coils are limited. Mine is black. I am glad to know that. I will put a tach on the saw tomorrow and time it against a Stihl 310 I have also ported.

The intake pooling is annoying, but it is holding its rpms well in the cut. True I have a lot to learn, but this is the first one I have gotten overzealous with. Like you said live and learn.

 

[bitzer]

Why would you want to measure the strato ports from the transfer windows? Wouldn't you want to measure them from the intake? I guess I don't get why. I had thought about lowering the intake more, but I hadn't planned on touching it until you suggested it. I guess I should have gone with my first instinct. Its very difficult to judge anything without pictures I guess. You didn't know how far it was and I didn't think it would have made that big of a difference. I have heard some saws can be taken to as much as 160 degrees, but they are larger displacement saws. I will play around with tuning more tomorrow and a tach and see what I can come up with.

 

[Terry Syd]

Strato timing

At least on my Husky, and I expect your Husky, the timing of the strato is done by the cutaway on the SIDE of the piston. The area adjacent to the strato intake (where the butterfly is) is already open when the other end of the cutaway starts to peek over the edge of the transfer port.

The strato charge goes into the top of the transfer port. The cutaway moving above the bottom of the transfer port starts the flow into the transfer port.

You may remember in the thread where I filled in the lower part of the transfer port and changed the timing from 154 degrees to 152 degrees.

Yeah Bob, live and learn. I bet your next strato port job goes a lot better.

 

[bitzer]

Ok that makes sense. The strato air does not come into play until it hits the transfers. I got what you mean now. That is why you measure there. I guess I'm a little slow on the up take. The saw will run and it seemed to be running well today, but the pooling may become a PITA. I have seen used cylinders on ebay for $80 or so from time to time. I may keep my eye out for one to try this again. I will do some timed cuts tomorrow against the 310 and put a tach on it just to see where I'm at. Thanks again for the info.