Modifying Strato/X-Torq saws...

[2stroked2smoke]

This is the first strato saw that I've ported. It is a little confusing but I'm starting to understand how it works. Have you timed your ports to see when they open MWEBA? I'm just curious if your findings match mine. My intake was opening 3-4 degrees before the strato port.

 

[mweba]

This is the first strato saw that I've ported. It is a little confusing but I'm starting to understand how it works. Have you timed your ports to see when they open MWEBA? I'm just curious if your findings match mine. My intake was opening 3-4 degrees before the strato port.

Not in the following pics it isn't. Is the side piston port not open to the upper transfers at this point? I have not degreed mine or the 555 I owned. Never had the time or interest in porting one.

top notch. looks like the strato ports open way before the intake. See what you think.

 

[2stroked2smoke]

The only way I could figure to do this would be to lift the piston skirt or lower the intake....loosing crank case pressure. Not sure less pressure is bad but it is more expensive to correct later.

The crankcase on my 562 is the tightest I've ever seen. The clearances between the crankshaft and crankcase were very tight. It even had a plastic stuffer between the crank and connecting rod. Just enough room for the rod to cear and that's it. This has to build some good crankcase pressures.

 

[Mastermind]

My mind is at work on the 562 as we speak. BUT I have three saws ahead of it and two of those are 7900s and I have a lot of learning on those beasts as well to do. They are pretty straight forward though.......lots of blowdown and gobs of compression.


These strato saws are coming clearer in terms of port work though. I seems that by altering the timing of the strato ports one could change the characteristics of the engine, and that is just one variable.

I'm thinking that by increasing intake and strato timing plus evening the two that could serve to fill the case. Then raising the exhaust port and transfer inlets together should raise the working rpm just like any other saw.

Has anyone done much on compression increase in a strato?

 

[mweba]

My mind is at work on the 562 as we speak. BUT I have three saws ahead of it and two of those are 7900s and I have a lot of learning on those beasts as well to do. They are pretty straight forward though.......lots of blowdown and gobs of compression.


These strato saws are coming clearer in terms of port work though. I seems that by altering the timing of the strato ports one could change the characteristics of the engine, and that is just one variable.

I'm thinking that by increasing intake and strato timing plus evening the two that could serve to fill the case. Then raising the exhaust port and transfer inlets together should raise the working rpm just like any other saw.

Has anyone done much on compression increase in a strato?

All in all they are the same as any 2 stroke. Just searching for the most efficient charge per compression. The strongest 372XT I built was at 187psi.

 

[2stroked2smoke]

Not in the following pics it isn't. Is the side piston port not open to the upper transfers at this point? I have not degreed mine or the 555 I owned. Never had the time or interest in porting one.

The strato ports on the intake side of the jug open way before the intake does, but the strato ports on the piston don't open up into the transfers until about 3-4 degrees after the intake port starts to open. The strato ports on the piston is what I changed to make the timing match. I just raised the port onthe piston until it opened into the transferports at the same time as the intake opened.

 

[mweba]

The strato ports on the intake side of the jug open way before the intake does, but the strato ports on the piston don't open up into the transfers until about 3-4 degrees after the intake port starts to open. The strato ports on the piston is what I changed to make the timing match. I just raised the port onthe piston until it opened into the transferports at the same time as the intake opened.

Got ya. This is different than the 372xt. Basically, you are adding more fresh air duration to your stroke cycle. Some kind of a running change must have take place? Easier to recast a new piston than a cylinder?

 

[Terry Syd]

I matched the strato port on the piston opening up to the transferport with the bottom of the piston opening up the intake port. That's the way I was understanding you. Did I misunderstand you?

Nope, that is what I was referring to. It looks like this 562 is the first strato we've worked on where the strato timing was less than the intake timing.

 

[Terry Syd]

The only way I could figure to do this would be to lift the piston skirt or lower the intake....loosing crank case pressure. Not sure less pressure is bad but it is more expensive to correct later.

When you match the two timings, you don't loose any base compression. The base compression can only start when the last port closes, which ever port it is. By matching them you retain the same compression, but increase the potential flow into the crankcase.

I'm a fan of cutting the piston as they can easily be replaced.

 

[Terry Syd]

These strato saws are coming clearer in terms of port work though. I seems that by altering the timing of the strato ports one could change the characteristics of the engine, and that is just one variable.

I'm thinking that by increasing intake and strato timing plus evening the two that could serve to fill the case. Then raising the exhaust port and transfer inlets together should raise the working rpm just like any other saw.

Has anyone done much on compression increase in a strato?

You got it. Once you have the ports matched, then if you need more 'intake' timing, then both intake and strato ports are moved together.

These Husky stratos are famous for having very low blowdown figures - around 12 degrees. As I recall this 562 was measured at 11 degrees. It doesn't take much of an increase to create a significant difference to the blowdown. If you raised the blowdown just 2 degrees to 13 degrees, that is a 18% increase in the time for blowdown. Add in the extra area by widening the port and you have a big increase in blowdown.

I took the cautious route and trimmed a couple of degrees off the front of the piston (about .5mm off the edge - again pistons are cheaper than jugs). The change to the engine speed was impressive. I went back in and cut another 2 degrees on the piston for a total of 16 degrees of blowdown, but the engine didn't need it. I then went back and cut the jug for 14 degrees and put in a fresh piston.

 

[2stroked2smoke]

Went out with the 562 today, and downed about 30 elms. Cut up three of them into firewood. I'm impressed with my saw with the new cylinder mods. It actually felt smoother. It started out cutting a little hyper but the autotune brought it back down after a little cutting. This is one thing that I don't understand about porting an autotune. The autotune works at adjusting the carb based on engine rpm. So if you do cylinder mods to raise the working rpm, won't the autotune adjust the carb to bring it back down. I can see getting more torque but don't see you being able to raise the working rpm.

 

[tree monkey]

Went out with the 562 today, and downed about 30 elms. Cut up three of them into firewood. I'm impressed with my saw with the new cylinder mods. It actually felt smoother. It started out cutting a little hyper but the autotune brought it back down after a little cutting. This is one thing that I don't understand about porting an autotune. The autotune works at adjusting the carb based on engine rpm. So if you do cylinder mods to raise the working rpm, won't the autotune adjust the carb to bring it back down. I can see getting more torque but don't see you being able to raise the working rpm.

i was thinking the same thing

 

[parrisw]

There has to be something else the fuel curve is based upon besides just RPM. It just wont work well with 1 input, even just two inputs is very crude compared to Automotive standards.

 

[Terry Syd]

When you increased the duration of the strato port, you increased the amount of air going to the cylinder. In other words, the saw started out initially a bit lean, or as you said 'hyper'. The autotune eventually worked out the change and set the mixture.

The autotune works to set the mixture, it doesn't work on the rpm. It does this by changing the mixture and recognizing any change in rpm. If it leans it out and the rpm goes up, then it needs to change the baseline mixture to run a bit leaner. If it leans it out and the rpm goes down, then it needs to change the baseline mixture to run a bit richer. The autotune is looking for an optimum fuel mixture at a given load.

EDIT: If you want to see an increase in cutting speed, try adding just two degrees of blowdown. You can nip the front of the piston to get that extra blowdown. The amount of loss in compression from the removed material is minimal and you probably wouldn't notice it - especially if you are increasing the ability of the engine to charge the cylinder.

If you don't like the change, then it is simple to put a stock piston back in.

 

[tree monkey]

When you increased the duration of the strato port, you increased the amount of air going to the cylinder. In other words, the saw started out initially a bit lean, or as you said 'hyper'. The autotune eventually worked out the change and set the mixture.

The autotune works to set the mixture, it doesn't work on the rpm. It does this by changing the mixture and recognizing any change in rpm. If it leans it out and the rpm goes up, then it needs to change the baseline mixture to run a bit leaner. If it leans it out and the rpm goes down, then it needs to change the baseline mixture to run a bit richer. The autotune is looking for an optimum fuel mixture at a given load.

are you saying that if you can feed it enough air/ fuel it will turn, lets say, 20,000 rpm? or is it rev limited? will it be way rich?

 

[Terry Syd]

Naw, all engines have their limits. For example, you can put a limit on a saw's working rpm by limiting the blowdown - the saw will eventually hit a point where there is so much blowback down the transfers that it can't rev any higher.

After going through Blair's work it appears that most, if not all, new saws are being set up with a shorter exhaust duration in order to decrease emissions. In the case of these Husky stratos the stock blowdown is very low compared to other saws. I found that a two degree increase on my Husky strato (from 12 degrees of blowdown to 14) really woke it up.

I don't know if the 562 is limited, but I would expect that it is. An increase in cutting speed will probably still be well within that limited coil.

The autotune appears to be rather simple. It will change the mixture and then note whether the rpm went up or down. It does that whether you are dogging in at 7,000 rpm or cutting at 10,000 - it will probably continue to do that function right up until it hits the limiter.

 

[tree monkey]

Naw, all engines have their limits. For example, you can put a limit on a saw's working rpm by limiting the blowdown - the saw will eventually hit a point where there is so much blowback down the transfers that it can't rev any higher.

After going through Blair's work it appears that most, if not all, new saws are being set up with a shorter exhaust duration in order to decrease emissions. In the case of these Husky stratos the stock blowdown is very low compared to other saws. I found that a two degree increase on my Husky strato (from 12 degrees of blowdown to 14) really woke it up.

I don't know if the 562 is limited, but I would expect that it is. An increase in cutting speed will probably still be well within that limited coil.

The autotune appears to be rather simple. It will change the mixture and then note whether the rpm went up or down. It does that whether you are dogging in at 7,000 rpm or cutting at 10,000 - it will probably continue to do that function right up until it hits the limiter.

i guess what i'm asking is will the auto tune work at 20,000?

 

[2stroked2smoke]

When you increased the duration of the strato port, you increased the amount of air going to the cylinder. In other words, the saw started out initially a bit lean, or as you said 'hyper'. The autotune eventually worked out the change and set the mixture.

The autotune works to set the mixture, it doesn't work on the rpm. It does this by changing the mixture and recognizing any change in rpm. If it leans it out and the rpm goes up, then it needs to change the baseline mixture to run a bit leaner. If it leans it out and the rpm goes down, then it needs to change the baseline mixture to run a bit richer. The autotune is looking for an optimum fuel mixture at a given load.

EDIT: If you want to see an increase in cutting speed, try adding just two degrees of blowdown. You can nip the front of the piston to get that extra blowdown. The amount of loss in compression from the removed material is minimal and you probably wouldn't notice it - especially if you are increasing the ability of the engine to charge the cylinder.

If you don't like the change, then it is simple to put a stock piston back in.

That makes sense. So you think if I increase my blowdown by 2 or 3 degrees that it will wake it up. I'll give it a try.

 

[mdavlee]

The 555 has the same exhaust and transfer numbers as the 562. I want to say it was 12° of blowdown. The 576 was 102° exhaust, 13° of blowdown and 74° intake. Just raising the exhaust 2° seemed to make a huge difference. I lowered the intake 4° and left the strato ports alone. I don't remember the timing number on them off the top of my head. It seems to cut with a lot more rpms in the cut and sounds pretty much the same out of the cut.

 

[Terry Syd]

I'm old school. I prefer to cut the piston and see where the 'sweet spot' is. You can try 13 degrees, then 15 degrees, etc. At some point you are going backward. Now you know where the sweet spot is and you can cut the jug to that duration.

I expect with just two degrees you will be surprised. If it is just two degrees, then for most people it won't be worth cutting the jug, just nip the crown at the exhaust port and run it. The factory cylinder liner isn't compromised and you can still go back to stock if you want it.