Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Squish

The squish on my saw is 41 thousands at the edge of the piston.

If I was setting up a racer I would plasma coat the top of the piston to bring the squish down to 20 thousands. This would also reduce both the exhaust and transfer timing a few degrees. Since the transfer timing is already a bit long, this would help the blowdown problem. I could raise the exhaust port back up to stock timing and have the blowdown I want.

I could also have more material sprayed in center of the piston to create a dome and bump the compression ratio.

The transfer ports are very easy to make larger. If you take a look at the picture of the transfer tunnel cover, you will see that the insert into the transfer port can be trimmed back another 1.5mm. In other words, you can widen the transfer port another 1.5mm to get back the flow you lost when you dropped the transfer timing.

The Time/Area of the existing transfer port is enough and perhaps just a tad too large for the RPM that the saw is presently cutting. So, it may not be necessary to widen the transfer port, however, a racer may wish to raise the exhaust port some more and the extra transfer area may be necessary. In fact, a second set of transfer ports can be cut into the jug wall from the existing tunnel. All it would take would be a drill and some needle files to do the work. The potential transfer area is there, it just needs the tunnels enlarged to feed them.

All in all, I am very happy with the way the saw performs. It took a bit of work to figure out how to mod the new Husky strato, but anybody who says these stratos can't be modded to perform as well as the conventional two-stroke just hasn't played around with them yet.

If I was racing a Husky strato, that strato port would get a second carburetor. Perhaps in the future we will see someone step up to the log with a tricked out Husky strato so we can see what some serious mods would do.

 

[bitzer]

Hey Terry,

Its been a while since I've been in here. Good pics. I agree on increasing the case volume by raising the bottom of the transfers. I believe that it helped a lot on my 455 build and it is where I am headed next on the 460 top end. I have also been seriously contemplating finger ports starting at the base of the cylinder and ending at the bottom of the upper transfers. I am thinking about small windows in the piston on the intake side through the strato cut outs. I mean small. I don't want to upset the weight balance of the piston. The finger ports would be small too. Maybe an 1/8" wide. I know symmetry and equal volumes are huge here and I have no experience with finger ports, but increasing the case volume and fingers may make real monsters out of these saws. There would be added mixing with the strato air intake down the transfers and through the piston windows. I would test it on the 455 first as the 460 is nearly pristine right now. It may be a while before I get to it, but its an interesting thought at least.

Bob

 

[Terry Syd]

Exploring the Husky strato

Bob, good to hear you're still tinkering with the stratos. I'm waiting right now to go out to a mate's place to try my most recent mod - increased transfer area. It has been more than 30 years since I used Gordon Jenning's formulas and I stuffed up the calculations. The ports were too small.

I discovered that the stock transfer tunnels were too small for the area of the transfer ports. I opened them up to get sufficient flow to the ports and picked up some power across the powerband.

This led me to take the leap to enlarging the transfer port windows up the full amount possible by removing metal from transfer cover insterts (the 1.5mm I referred to). I did the new calculations before hand and the time/area was deficient for the rpm I was running. Timberwolf (Brian) had indicated in another thread that a good running saw had a transfer time/area that was just a bit below the minimum area suggested by Godon Jennings in his book on Two-Stroke Tuning.

The increase in area I made was 12.5% over stock and brought the time/area up to about .0007 at 10,000 rpm, which was still well below the Jennings time/area figure of .0008. I figured it should work, and it looks like it has.

I tried cutting some of the smaller wood I had in my back yard and the cutting speed was about the same. The limited blowdown on the saw does keep the revs down. However, I noticed that the saw wanted to keep the speed under load.

I'm off this afternoon to try and get the saw into some bigger wood to see how much better it holds its revs. I'll report back in a few hours.

Good luck on the finger porting. I've went through that phase of tuning years ago. I didn't get much of anything for the amount of work I did. You may wish to do some calculations with Jennings formulas and then see how much you need. It may be that a slight tweaking of the existing transfer ports would save you a lot of work. On the other hand, every tuner has to try finger ports at some time or another, just because they can LOL

 

[Terry Syd]

Back from cutting

Yep, the extra transfer area worked like a charm. I was using the 18" bar on the saw, buried in a freshly fallen Aussie gum tree, and the saw held its revs much better than before.

Max torque did come in a bit higher in the powerband, but the powerband was sufficently broad that the saw wasn't 'peaky'.

I'm running out of potential mods for this saw. I might try 'stuffing' the crankcase and see if bumping up the base compression a bit will extend the powerband a little higher in the rev range.

 

[bitzer]

Terry,

I raised the bottom of the transfers on my 460 yesterday. I brought them up about 5/8" from where they were. I actually ran an angle grinder on them for the ruff, quick removal and then cleaned it up with the dremel. From tear down to rebuild it was less than 1 hour. It made a huge difference. My average time was around 17.5 and I even saw some 16s on the stopwatch. Same cherry log and saw chain from the last test. I haven't touched either one since. Thats a 18-20% increase over stock and 7-9% increase over the initial work. If I can find the time again there is at least another 3/8" to go until I reach the cylinder. I may be skipping the fingers for now or all together. Thanks for the tip on that! I have yet to strap the stock muffler to it and test it to see if I can get some more torque. I haven't lost any I just think my modded muffler may be too wide open.

Bob

 

[Terry Syd]

Stuffing

Those lower transfers must have really been restricting the flow in order to get that kind of improvement. It may be that Husky has restricted the transfer flow on the stratos in order to help pass some EPA regs. The 450 was also limited by the transfer tunnels, perhaps opening up the transfers will become a common mod on these stratos.

My next mod is 'stuffing' the crankcase. I'm going to get a set of sealed bearings for the crankshaft and also stuff the center of the crankpin with cork and epoxy.

I took out a fair bit of metal in order to open up the transfer flow. That, along with increasing the area of the transfer port window 12.5% has dropped my port velocity. The increase in crankcase compression should help with the blowdown problem and probably extend the powerband out a bit on the top-end.

 

[weimedog]

My next mod is 'stuffing' the crankcase. I'm going to get a set of sealed bearings for the crankshaft
.

How would those sealed bearings get lubed? How would the oil get to the inner bearing?

 

[Terry Syd]

Packed

The're packed with grease. If you do a search on this forum of 'sealed bearings' you can find a few threads that discuss using sealed bearings on the crank. However, not one of the threads mentions using them to help stuff the crankcase. I got that hint from some other forums. I even found some posts of model aircraft engines using the sealed bearings in their engines.

Stuffing the crankcase is not just about making the volume smaller. Many times it's WHERE you stuff it that is important. Having the volume located away from the transfer tunnels doesn't help the flow as much as having the volume located right at the entrance of the tunnels. The mixture down in the main bearings has too long a convoluted journey to do much good.

On a piped engine a larger crankcase volume can be an advantage by using the sonic waves of the pipe to help fill the larger volume - larger volume, more fuel/air mixture. However, a can muffler engine simply uses the pumping efficiency of the crankcase (perhaps there may be some resonance packing off the intake, but I doubt it looking at the intake/strato/aircleaner ducting on my saw).

These Husky stratos are coming out with very low blowdown figures. As a result there is considerable blowback into the transfer ports. I'm hoping by stuffing the crankcase a bit, I can reduce some of that blowback and also bump up the velocity through my larger transfer ports.

I doubt whether I could ever get a high compression ratio of say 1.5-1.7, I would need a full circle crank to get something like that. Since I have carved out a lot of metal to open up the transfers, I may only be able to get it back to somewhere around the stock ratio.

 

[weimedog]

The're packed with grease. If you do a search on this forum of 'sealed bearings' you can find a few threads that discuss using sealed bearings on the crank. .

Most sealed bearings come packed with grease. Not certain of the quality. Just I have never heard of using sealed bearings in a continuous High RPM application, especially where heat is a factor and last but not least, pressure fluctuations can help the now more fluid grease escape over time. I'm only familiar with using sealed bearings on low RPM applications such as wheel bearings (Motorcycles come to mind). Guess you learn something every day!

I guess if a race saw was built that way, the low run time might not expose those bearings to issues and they can be serviced between events. But for a work saw..I don't know. I would be curious to see how this concept works over a normal saw life span.

 

[Terry Syd]

Shields

I did some more research on sealed bearings and I think I have a better idea about what kind I can use. It appears the 'shield' bearings would be the best type for the crankcase application.

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=1_1_4a

While going through the SKF catalog I was surprised to find a sealed bearing that had 50% less friction than a standard non-sealed bearing. I had assumed that a sealed bearing would always have more friction than a non-sealed bearing. Even within the 'sealed' catagory there are several types of seals, some have seals that are tighter than others and aren't low-friction.

Yeah, I always swapped out the standard wheel bearings on my dirt bikes for sealed bearings. The deep water crossings would ruin a standard bearing rather quickly.

 

[ratman36]

modding

you guys have done some interesting work!!!like to know if you have any ideas on modding a ms 211?good info keep it coming!

 

[Terry Syd]

You might want to try a search of the forum on mods for the ms 211. Somebody must have done some work on one.

I believe that the ms 211 is a strato, so what we worked through here on the Husky stratos can also be used on other stratos. The biggest thing I worked through and came to realise was that a matching of the intake and strato timing gave me the best results.

Usually on a strato the intake timing is shorter than the strato timing. That allows the engine to start a little easier and the longer strato timing gives more air to the cylinder for better emissions. However, the longer strato timing will decrease base compression without any increase in fuel/air mixture. By matching the strato and intake timing you can maximise base compression and the time/area of the intake.

You may not have to increase the intake and strato timing over stock (strato timing) as the combination of the two ports allows a high rate of flow when they are both open. However, if you did decide to increase the intake timing, I'd suggest you also increase the strato timing to keep them the same.

The old mod of widening the intake is virtually impossible on the stratos. Not to worry, the combination of the two ports allows a good size 'gulp' when they open together at WOT. If you can find a larger carb for the intake you can maximise what area the intake port provides. Since the carb will open before the butterfly on the strato, there is plenty of velocity in the carb to give good metering of the fuel at and off of idle.

All the rest of the mods are similar for all other saw engines.

 

[weimedog]

And what is your opinion on using the Strato ports as yet another way to flow fuel air mixture such as Brad Snelling did on the Husqvarna 372XT? Where does the Strato charge enter both from a physical location perspective in the combustion chamber along with its possible impact of the size of burnable charge and also the timing where its introduced relative to the combustion cycle?

 

[Terry Syd]

Brad was chasing some flow restriction in the intake system. When he did hog out everything, the result is a matching of the intake and strato timing. However, the saw was no longer a strato when he finished.

There are lots of different configurations of stratos and each may require a different technique to get the most flow out of the system. What I was trying to communicate was the matching of the intake and strato timing as an important part of that porting process.

I initially thought that a strato kept the two charges (air and air/fuel) quite seperate (just like the advertising visual aides showed), however I found that a lot of mixing did occur. There will be less fuel pushed out the exhaust with the initial air charge entering the cylinder, but by the time of ignition the two charges have mixed together. Thus, maximising the flow of the strato porting is as important as maximising the flow of the intake. Both of the ports contribute to the bulk flow in the engine.

That observation is opposite of my original ideas about 'biasing' the charges. I now feel that maximising both both the intake and strato ports is the better approach. Having said that, each strato configuration is different and may require a different approach.

For example, when I made my transfer ports wider, it also made my strato port timing wider. In other words, my engine then got a bit more of a gulp of air with the change in the width of the transfer port. I checked to make sure that the ports would flow that bigger opening and they did, however on some engines that may not be the case and a bit of additional porting in the piston and strato tunnels may be necessary.

The wider strato opening at the transfer port may be a concept that can be used to increase the time/area of the total intake cycle without increasing the length of the intake cycle. If a person cuts the rear transfer port back at a different angle, they are also potentially changing the width of the strato port. They should check that the cutaway on the piston can take advantage of the wider port. It may be that the cuttaway also needs to be extended and/or the tunnels widened.

 

[weimedog]

Brad was chasing some flow restriction in the intake system. When he did hog out everything, the result is a matching of the intake and strato timing. However, the saw was no longer a strato when he finished.

There are lots of different configurations of stratos and each may require a different technique to get the most flow out of the system. What I was trying to communicate was the matching of the intake and strato timing as an important part of that porting process.

I initially thought that a strato kept the two charges (air and air/fuel) quite seperate (just like the advertising visual aides showed), however I found that a lot of mixing did occur. There will be less fuel pushed out the exhaust with the initial air charge entering the cylinder, but by the time of ignition the two charges have mixed together. Thus, maximising the flow of the strato porting is as important as maximising the flow of the intake. Both of the ports contribute to the bulk flow in the engine.

That observation is opposite of my original ideas about 'biasing' the charges. I now feel that maximising both both the intake and strato ports is the better approach. Having said that, each strato configuration is different and may require a different approach.

For example, when I made my transfer ports wider, it also made my strato port timing wider. In other words, my engine then got a bit more of a gulp of air with the change in the width of the transfer port. I checked to make sure that the ports would flow that bigger opening and they did, however on some engines that may not be the case and a bit of additional porting in the piston and strato tunnels may be necessary.

The wider strato opening at the transfer port may be a concept that can be used to increase the time/area of the total intake cycle without increasing the length of the intake cycle. If a person cuts the rear transfer port back at a different angle, they are also potentially changing the width of the strato port. They should check that the cutaway on the piston can take advantage of the wider port. It may be that the cuttaway also needs to be extended and/or the tunnels widened.

So now I am confused as what you are implying is that the strato saw's mix the air charge with the fuel/air charge in the combustion chamber thoroughly and prior to combustion. So does that imply they run a really rich mixture, greater than the magic 17:1 ratio thru the carb, and then blend in a straight air charge to bring it back to a more combustible mixture? And that is more efficient than the standard carburetor mixture device? Is the magic in the fact they can run leaner because the additional air flow is also a cooling agent? Just confused on the concept now. Why not just use a better carb setup use the added port capacity to flow fuel/air to begin with?

I guess my misconception is the strato's added a little air in the fill/exhaust cycle at a specific location & time to promote a more thorough burn of what would eventually go out the exhaust...timed & directed not to interfere with the more optimal mixture at the plug during initial combustion. But I guess in retrospect..how could that possibly be??? So now I just have no idea how that all works.

 

[Terry Syd]

There's some good animations of how a strato (stratified charge) on this thread -

http://www.arboristsite.com/showthread.php?t=150358&highlight=strato+works

 

[drkptt]

So now I am confused as what you are implying is that the strato saw's mix the air charge with the fuel/air charge in the combustion chamber thoroughly and prior to combustion. So does that imply they run a really rich mixture, greater than the magic 17:1 ratio thru the carb, and then blend in a straight air charge to bring it back to a more combustible mixture? And that is more efficient than the standard carburetor mixture device? Is the magic in the fact they can run leaner because the additional air flow is also a cooling agent? Just confused on the concept now. Why not just use a better carb setup use the added port capacity to flow fuel/air to begin with?

I guess my misconception is the strato's added a little air in the fill/exhaust cycle at a specific location & time to promote a more thorough burn of what would eventually go out the exhaust...timed & directed not to interfere with the more optimal mixture at the plug during initial combustion. But I guess in retrospect..how could that possibly be??? So now I just have no idea how that all works.

Terry's post described it correctly--the initial air pushes the exhaust out; scavenging the cylinder with air and not air/fuel mix. There's no 'promotion of a more thorough burn.'

Magic 17:1 ratio? Did you mean stoichiometric for gasoline at 14.7:1?

Yes, it is correct that the carburetor supplies a richer mixture that is diluted with some of the strato air charge. The mix trapped in the cylinder should blend thoroughly and be of a uniform, homogeneous mixture when burned. This is described as stratified charge, homogeneous combustion. A standard carbureted 2-stroke has both homogeneous charge and combustion.

There are actually two air/fuel ratios to consider. Trapped (or burn-zone) AFR is the composition of the mixture in the cylinder after the exhaust port closes. Overall AFR is the ratio of all the air and all the fuel running through the engine. These numbers are identical for a carbureted 2-stroke because the mixtures are homogeneous throughout, and are around 12.5:1 for best power and durability. Trapped AFR for a strato engine should also be around 12.5, but overall AFR will be in the order of 14:1 to 18:1 because some extra air runs through the engine but is never compressed in the cylinder and never takes part in combustion.

People describe a strato engine as running very lean. Looking at the overall AFR gives that appearance, but as far as the burn zone is concerned it's running as rich as a standard engine.

 

[weimedog]

Terry's post described it correctly--the initial air pushes the exhaust out; scavenging the cylinder with air and not air/fuel mix. There's no 'promotion of a more thorough burn.'

Magic 17:1 ratio? Did you mean stoichiometric for gasoline at 14.7:1?

Yes, it is correct that the carburetor supplies a richer mixture that is diluted with some of the strato air charge. The mix trapped in the cylinder should blend thoroughly and be of a uniform, homogeneous mixture when burned. This is described as stratified charge, homogeneous combustion. A standard carbureted 2-stroke has both homogeneous charge and combustion.

There are actually two air/fuel ratios to consider. Trapped (or burn-zone) AFR is the composition of the mixture in the cylinder after the exhaust port closes. Overall AFR is the ratio of all the air and all the fuel running through the engine. These numbers are identical for a carbureted 2-stroke because the mixtures are homogeneous throughout, and are around 12.5:1 for best power and durability. Trapped AFR for a strato engine should also be around 12.5, but overall AFR will be in the order of 14:1 to 18:1 because some extra air runs through the engine but is never compressed in the cylinder and never takes part in combustion.

People describe a strato engine as running very lean. Looking at the overall AFR gives that appearance, but as far as the burn zone is concerned it's running as rich as a standard engine.

So it actually pushes the exhaust so that strato charge is introduced somewhere between the exhaust and the incoming charge? Almost a air barrier..with some bleeding out the exhaust and some bleeding back into the fresh charge?

 

[drkptt]

So it actually pushes the exhaust so that strato charge is introduced somewhere between the exhaust and the incoming charge? Almost a air barrier..with some bleeding out the exhaust and some bleeding back into the fresh charge?

Yes. I have seen the term air fence used in technical papers to describe it.

 

[weimedog]

Yes. I have seen the term air fence used in technical papers to describe it.

That make more sense again...thanks!