Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Transfers

If I had taken it too far, a new piston would fix it right up - I have been trimming the crown of the piston in front of the exhaust port. Actually, this 158 degrees exhaust timing (from 12 degrees blowdown to 16 degrees, a 33% increase) feels about right. There is a noticeable torque increase in the upper revs where the saw likes to sit and cut.

To properly raise the transfers on these Husky stratos would be a real job. On these Huskys there is a transfer cover that has an insert into the port that directs the flow. Essentially, you can't raise the transfer without stuffing up the flow characteristics. The transfers are already sitting quite high (126 degrees duration) and it is the small blowdown time that limits the power.

For someone who doesn't want to take a Dremil to their jug, just nipping two degrees off the crown next to the exhaust port and trimming the intake skirt to 150-154 degrees duration would probably be an easy performance mod that anyone could do. It would also still retain the stock cylinder liner at the top of the exhaust port for long cylinder life. The purists on the board would consider it a hack job, but for someone who just wants some more power out of their saw it would be a quick and easy mod that they could easily reverse by purchasing a new piston.

I think I will find someone to build me a dome on a new piston. The 4 degrees I added to the blowdown has taken a bit more off the front of the piston than I wanted and is trapping some mixture under the squish band that I could be using for better power. The first trimming of 2 degrees was very small and likely not trapping much mixture (or perhaps just air as it is a strato) and I would consider it a negligible loss - especially since the extra blowdown time made such a significant difference.

 

[AUSSIE1]

Yep, all makes sense. If you can't remove from the transfers, from the piston is the option. Playing with the piston is a good way to get a feel for what is happening without taking a cyl to far for sure. Widening the exhaust (area) increases blowdown also and helps with tight blowdown no's.

Like to see a video of your saw sometime Terry.

 

[Terry Syd]

Pictures

I finally broke down and purchased a new camera (now it is a sure thing to find the old camera cable). When I get the new piston set up I'll take some pictures of the cut piston, dome piston, ports, etc. and post them up in a 450 Husky strato thread. I'll also see if I can get another saw, say a NE 346, to run it against in some wood for a video. If I used a well known saw to run it against then it would give a better perspective of the effectiveness of the mods to this plastic homeowner saw.

 

[bitzer]

Terry,

The "strato at transfers" numbers are with the covers off and when the strato cut out of the piston opens up.

I wish I knew someone who could do a piston dome for cheap, but I don't and I have limited funds right now. I think a bump in compression would really help the saw, but thats the problem with the damn clamshells.


The crown of the piston actually ducks below the floor of the exhaust so I can drop it there and I think I may raise the exhaust only by 2 degrees or so. I will do this when I am widening the port. Hopefully the slight increase in blowdown will help like with what you found out. The intake I might take out to 145 at first to see what happens. I will also test between the stock muffler and dual ported. I am still contemplating shortening the transfer tunnels at the base of the cylinder like I did with the 455. The problem is, I don't know what effect it had, because I did this originally before I did any good timed tests. I will not be raising the transfers at all on this one. There is so little blowdown as it is. I wish I would have degreed the 455 origianlly, but you live and learn.

Thanks again for the tips Terry.

 

[Terry Syd]

Port volumes

I bought a new piston to try some further experiments. I'm going to port the jug for the extra 2 degrees of blowdown and then work up on the intake timing. I know the engine needs the extra blowdown, but I want to check out my theory on having the strato ports at a longer duration to the intake port - it might provide a broader powerband.

I pulled the transfer port covers off to look at them and discovered that the insides not only had soot halfway down the ports, but the area near the ports was black from the exhaust gases shooting into the transfer ports. This was most likely left over from when I had the stock blowdown of 12 degrees.

I went ahead and ccd the tunnels with some oil and a syringe and came up with 7.8cc for each tunnel. That is 15.6cc for both tunnels combined. I then assumed a pumping efficiency (similar to volumetric efficiency in 4-strokes) for this engine of 50%, which gave me 25cc of mixture/air for each revolution.

If the strato is 50/50 biased between mixture and air, then the tunnels have to hold 12.5cc of air (half of 25cc). With the exhaust pushing into the tunnels the air has to shift downward into the crankcase. If the exhaust was pushing into the tunnels halfway, then the tunnels have to have a volume capable of holding and extra 7.8cc in order for the air not to be pushed into the mixture in the crankcase.

Add the 12.5cc of air and the additional 7.8cc of exhaust gases and the tunnels have to have a volume of 20.3cc. I only had the 15.6cc volume because of the window I put in the rib between the bolt holes. - I was 4.7cc short of volume without that area.

I then filled in the window I put in the rib with liquid metal and did the volume check again. The 'reservior' under the tunnels holds 4.2cc. In other words, the stock configuration has a volume of 19.8cc - pretty close to my rough calculations.

Of course, the stock engine is not 50/50 biased. The stock carb is 11mm and the strato port is 13.5mm, plus the stock intake timing is 144 and the strato is 154 degrees. The biasing looks like it may be closer to 35/65 on this engine, which means even more air in the tunnel, like about 16.25cc worth. Basically it is probably impossible to retain all the air in the tunnel structure with the stock blowdown.

With the window I had in the rib I was likely getting a lot of mixing of the intake mixture and strato air. I would not suggest anyone cutting a window in that rib or to shorten it any significant amount. I did some measuring with a calipers in the cases and the area of flow past the rib is just enough to allow enough flow for the tunnel structure. As a precaution to ensure enough flow, I did nip about .5mm off the bottom of the rib and shaped the rib so that the mixture could more easily access the tunnel structure.

When I get all the testing of the lower intake timing figures, I post the results.

 

[Terry Syd]

New piston

I was impressed with the new piston. The crown was machined flat to the edge and didn't have the corner trimmed off. Then I started to do some measuring.

It was a different cast and a different machine that produced it. The strato port cutaway is a bit higher so the strato port is now back to 154 degrees.

The skirt was a bit shorter so it was 147 degrees of intake duration rather than 144.

The deck height was a tad lower so the transfer and exhaust durations were extended - and the compression was a bit lower. The transfer duration went from 126 to 127. If anything, I would have prefered the deck height being higher to give me more compression and a shorter transfer timing.

Now the biggie, the pin offset is different. Whereas the first piston had an offset of .02mm, this one is .16mm. The first piston essentially split the duration evenly, but this one creates a 1.5-2 degree difference in the opening and closing times of the ports due to the rod angularity.

All these differences can mean a big difference in the performance between two stock saw engines - all it takes is to use a different piston.

Caveat: If a person is going to add two degrees of extra blowdown, they must insure that they are rotating the crank the way the engine runs when they measure the timing. If they accidently reverse the rotation the timing figures will be off. Further, if you change pistons on a ported jug you may end up with very different timing figures. I had made timing changes to the piston so there isn't any problem. However, if I had dropped the intake port down to get that initial 152 degrees duration (instead of trimming the piston skirt) my intake duration would now be sitting at 155 degrees.

 

[bitzer]

Terry,

The 460 is ported now. Widened the exhaust to 70% of the bore and raised slighlty for blending. Lowered the exhaust and widened as much as possbile. Extended the transfers back towards the intake. I did not mess with the transfers at the crankcase side.

Here are the numbers now:

Durations:

Exhaust: 162
Intake: 147
Transfers: 130
Blowdown: 15

I re-did the numbers again before porting and my transfer figures were off. The 130 figure is stock, I did not raise them and don't intend to.

The saw averaged 19.5 secs in the same black cherry log I tested it in the last time. So I achieved an 11% increase. I was hoping for better, but maybe there is something to raising the bottom of the transfer tunnels. I would say the saw retained its torque and I was able to put a good amount of pressure on it. I will also try it again with a stock muffler on it. I could also nip the skirt on the intake side to see if any more duration would help.

I am thinking about taking the 455 jug and opening the tunnels all the way to where the skirt reaches BDC just like in a non-clamshell design where it is just a jug on a case. I see what you are saying about the exhaust blowing down the tunnels. There was a lot of evidence for that on the 460 jug when I got it. I would image with better exhausting this would not be so prevalent. When I have time I will try it and see what happens to the times.

Where did you put the windows in the tunnel? Just before the tunnels split?

 

[Terry Syd]

Tunnels

The window in the web that I put in (and have now filled in) was located at the mouths of the two upper transfer tunnels. It allowed the crankcase to feed directly into those tunnels. I really should get some pictures of this thing the next time I have it apart. It appears that the transfer tunnels may be slightly different between models. I note the picture from Rick's 445 that his tunnel entrances were different to mine. The 455/460 may be another variation in design.

What is the intake/strato 'biasing' of your 460? The reason I ask as the more intake biasing the more likely that the 'window' concept may work, especially since you have now increased the blowdown timing.

On this 450 of mine, the engine is so strato biased that it really needs to retain all the volume of the tunnels it can.

I tried the new piston with an extra 2 degrees of blowdown cut into jug. The wood was a lot softer than what I had been working on before, but I did get the carb tuned and could feel where the saw made the best power. It looks like 14 degrees of blowdown is a good figure for the 450 if a person wants a faster cutting, but broad power band working saw. Simply doing a 'woods port' (widening the exhaust) isn't enough, the engine needs not only the extra area, but also the extra time to get the pressure down in the cylinder before the transfers open.

When I can get it into a good strong log I'll get a better idea how the intake timing is working. The strato port is closing 4 degrees later than the intake (with this new piston) and I want to see if this will help the lugging ability of the engine.

 

[bitzer]

Terry,

I meant to say "lowered the intake and widened as much as possible" not exhaust. Sorry about that. The original intake duration was 140 so I increased it to 147. I would say that the strato ports are much more biased on the 460 than on the 455. On the 460 they are open longer than on the 455. So I could lower the intake more, but as far as exhaust, upper transfer, and intake I've gone about as far as I could go. Others have had success with widening the strato ports as well. I will have to play more with my 455 jug. The 460 jug may be the cleanest port job I have done out of the 8 saws I have ported so I'm going to try to keep it that way until further testing proves anything. Man its tough to put a decent champfer on the exhaust with these damn clamshells!

 

[Terry Syd]

Chamfer

Bob, the easiest way to put a champfer back on a port you have modified is to use a bit of emery cloth on the end of your finger. If the cloth/paper slips too much on your finger, put a drop of contact cement on your finger to hold the sandpaper.

With just a small piece of sandpaper on the tip of your finger, you can move it around to the areas you want and take off only the amount that needs to be removed. Of course, more in the center of the port and tapering off the champfer to the sides of the port - just like the wear pattern in a used cylinder.

Your finger will also be a soft backing to the sandpaper and the champfer will get a more curved surface than if you tried to use a tool.

So are you saying you picked up 11% cutting speed over your modified 455 jug?

 

[bitzer]

The 11% number is over stock on the 460 jug. I just wanted to start out with a classic woods port to see what happened. I did not keep very good records on the 455 and had no baseline for comparison so it was difficult to see what the changes did. It may be a while before I can experiment further, but I still have about 6 feet of the same log left to keep testing to a somewhat accurate degree. I would also guess that my chain could have been sharper on this last test.

I actually start out with 220 grit sand paper and then move to 600 grit to finish the champfer. I must have short fingers because it can be a PITA.

 

[Terry Syd]

Carburetion

I've got long fingers, great for playing the piano, but really, really bad for fistfights.

11% increase in cutting speed and still retaining the torque of the 460 - that sounds like a success to me.

If you start increasing the intake duration you might run into a phenomena that some of the other fellows porting stratos have run into (notably the 441). The mixture gets richer as the revs build, then starts leaning out and the revs increase.

I expect that it is a carburetion problem with modified stratos. The carbs on these engines are very basic, the high speed screw is just a set orifice with no compensation circuits. When the demand gets to a certain level the set orifice becomes restrictive. With the older saws that wasn't a problem, however with the stratos, the extra strato port keeps sucking straight air as the revs rise. The combination of the reduced fuel flow and the strato continuing to feed straight air gives a leaning condition as the revs rise.

I may have avoided this phenomena by putting on the larger carb. If I had retained the stock 11mm carb I might have run into it when I had the longer exhaust and intake duration.

If you run into it there are three things I think could fix it. One, is a bigger carb. Second, you could use a slightly longer discharge nozzle for the main jet. By placing the outlet of the nozzle further into the venturi you can lean out the mid-range of the carb.

The third way is the way I would address the problem. I would slightly increase the size of the venturi of the stock carb, say .25mm. Not only would this give a slight larger carb, but it would also move the outlet further into the venturi. You could open the carb up a bit at a time and see how it affected the mixture strength through the powercurve.

A set of needle files is all you need to do the work. You just have to maintain the choke point of the venturi at the point of the fuel outlet.

 

[keith811]

thanks

thanks again terry for aiming me at this thread when I start the port job on my 455 I will keep you posted and more than likely be bugging you for advise.
I think that one of my first purchases is going to be a bigger carb though. It seems that the carb is a major choking factor in the intake system.

 

[Terry Syd]

Venturi

What size carb do you have on the 455?

 

[keith811]

What size carb do you have on the 455?

I'm not sure what size it is I only get to work on saws on the weekends.
I have no where to work on them at my house so I must go to my cutting partners house. Since he lives 20 miles away and my truck only gets 11 MPG I can only afford 1 or 2 trips down there a week.
I will measure the carb and get a bunch of pics for you as soon as I start on this build. right now I have a homelite xl925 and a super 2 on the bench so as soon as I have those running I will get started on my husky

 

[Terry Syd]

Port timing

Well, I went ahead and tried to see if running a longer strato timing to intake timing would improve the low-end lugging ability. Yes it does, but it also kills the power through the mid-range and higher.

I am now back to running identical strato and intake timing. Since I had raised the exhaust port 2 degrees before I increased the intake timing, the change in performance was dramatic.

I also have made a further muffler mod. The two tubes extending through the muffler are now vents for the muffler. I read where some fellows had taken a Dremel to the inside of the muffler and opened up a hole in the tube. This made the muffler loud as the hole took a direct hit from the exhaust wave coming out of the exhaust port. I took a different approach, I drilled in from the sides of the muffler to about the first 1/3 of the tube length and then brazed a patch on the outside of the muffler.

The exhaust wave must travel around the tube before it can enter the hole in the tube, then as it comes in it is split with half the wave going out the front and the other half going back, bouncing off the mounting bolt and then up and out the tube. This drops the amplitude of the exiting wave. The mod didn't seem to increase the noise of the muffler.

Another nice feature is that the patched holes through the sides of the muffler are behind the seam of the muffler and can't be seen with the muffler mounted on the saw - essentially it is a stealth muffler mod.

The tubes can be blocked off with a wooden plug, so I am going to try some backpressure experiments. Whereas the original testing by Timberwolf of the muffler opening to 85% of the exhaust port area was on a conventional two-stroke with adequate blowdown, the limited blowdown of the Husky strato may present a situation that requires a greater area of flow.

 

[Terry Syd]

Scollops and transfers

When I put the engine back together with the 2 extra degrees of blowdown, I didn't put the two scollops on the piston crown to match the crown to the top of the exhaust port. The power wasn't as good as with the scollops, so I decided to put them back on to maximise the blowdown.

The blowdown is still very short at the new 14 degrees and it was evidenced in the top of the transfers with some carbon build up. The scollops should help relieve some of that back flow by increasing the initial area of the exhaust port.

As I was cleaning the transfer port covers, I decided to check the cross section of the transfer port. I made up some 'play dough' from some flour and made a casting in the transfer port. The cross section showed the port window at 12mm and the height at 7.4mm - that gave 88 square millimeters of area.

I then did some measurements of the port tunnels. The smallest area was 8.6X8.7mm for a total of 74.8mm. In other words, the transfer tunnels are too small for the transfer port window at BDC.

I expect the small size is because the 450 is based upon the smaller 445. The cylinder looks like it is identical with the same transfer covers, further, the case has the same inlet to the transfers as the 445.

After looking at the transfers, it appears the only logical way to open up the tunnels is to make the tunnel deeper in the covers. This will mean grinding out the back of covers and building up the cover with liquid metal. I'll then have to match up the extra flow of the tunnels with the case.

The bigger tunnels and further porting will drop the base compression, but since this engine is based upon the smaller 445 I doubt that it will be a problem.

Engines, the tweaking never ends.....

 

[Terry Syd]

Update

I've finally finished working on the 450.

The engine has insufficient blowdown as stock, a woods port job of just widening the exhaust is not enough. It needs another 2 degrees (for 14 degrees total blowdown) in order for the saw to rev. It also needed the top of the piston shaped to the top of the exhaust port to increase the blowdown. The shaping of the piston crown feels like another two degrees of blowdown have been added, but the saw retains more of the torque than if you raised the exhaust two more degrees.

You can see a picture of my test piston. I took it out to 14 degrees, then to 16 degrees. You can also see how the crown was curved. After I raised the exhaust port two degrees I tried the stock flat piston crown and was disappointed. I then put in the two small scallops on the crown to shape it and the power jumped up.

Take a look at the transfer tunnel covers. You can see the amount of blowback into the transfers on the unmodified cover. That is a cover after I had already raised the exhaust port two more degrees. You can see where I wiped off the exhaust soot at the bottom of the cover. The exhaust soot actually went all the way down and into the case.

The other cover is being modified in that picture. I took a casting of the cross-section of the transfer port opening and then of the tunnel. The tunnel was about 10% smaller than the transfer port opening at BDC. I expect this is because the 450 is based upon the 445 and uses the same port tunnels.

I carved out the back of the covers and used fiberglass and GB Weld to build it back up. I also had to do extensive opening at the base of the tunnels to get the tunnel to flow. The tunnels now start out bigger at the base and continue to taper down to the size of the transfer port opening. This mod was good for increasing the torque over the whole range of the engine.

I was surprised that after raising the exhaust port two degrees and putting the scallops on the piston crown my cranking pressure was 143 psi. Recalibrated from my 2,000 ft altitude the cranking pressure is an honest 150 psi at sea level.

The saw is cutting a lot faster than a stocker and retains enough torque that you have to be heavy handed to bog it. The powerband is broad and very forgiving.

 

[Terry Syd]

Intake and strato

Here is a shot of the bottom of the transfer port where I added some GB Weld to shorten up the strato timing. The stock jug looks like it was modded after the jug was cast. That area was filled in and shortened the strato timing two degrees.

The shot of the piston shows the amount I took off the skirt in order to match the intake timing to the new strato timing.

Since the strato timing on this model of Husky is quite long, the strato timing dictated the amount of intake timing I went for. By matching the two timing figures I have maximised the amount of time for base compression. I would have preferred to have a bit shorter timing for both the intake and strato (since I have the bigger carb), but the saw is revving very well and perhaps needs the timing it ended up with.

 

[Terry Syd]

Base compression

When I ported out the bottom of the transfers, I opened up the web again that creates the reservoir at the bottom of the transfers. Since I opened up the volume of the transfer tunnels, there is probably less need for the reservoir. It would be very difficult to get the bottom of the transfer tunnels to feed well with the web fully in place.

Even though the 450 uses the smaller 445 case (which bumps base compression), you can see by the amount of blowback in the transfer tunnels that some more base compression would probably help prevent some of the blowback down the tunnels. When I finally get around to pulling the crank out I will stuff the center of the crank pin and the area below the transfer reservoir in order to increase the base compression. This will probably lift the power at the top of the rev range a bit.

You can see the first picture where I filled in my previous port of the reservoir web. That has all been taken back out and the port is now even larger. If I had to do it all over again, I would start the hole a bit lower on the web and take off the bottom of the web. I could get the same flow, but would not have reduced the volume of the case as much. As it is, the crankshaft counterweights are just begining to impede the flow into the transfers at the end of the transfer cycle. By dropping the hole/transfer access down lower I could avoid that problem and not have to take as much out of the case.

The second shot shows the stock access port to the reservoir. With the web opened up the access port in the case is not needed. I can go back into the cases and fill the area with some screws into the case to help hold the GB Weld. The reduced case volume should bump up the base compression a little more.