Modifying Strato/X-Torq saws...

[Terry Syd]

"I'll be building a Husqvarna 562 in a few days." - Hot damn, now were going to see some interesting saw building. I hope you take heaps of pictures as you go.

I've been of different minds about staggering the strato and intake timing for different applications. There may be a small increase in fuel economy, emissions and low down lugging torque by having the strato timing open a few degrees before the intake.

The increase in low down lugging torque would be a result of having the 'back flow' through the intake at low revs begin in the strato port before the intake port. In other words, the there would be less tendancy for the mixture to become excessively rich at the low revs.

Having said that, I have still been able to lug the strato down quite well with the timing matched. It appears that the 'back flow' is split between both ports, so there is only half the back flow going up through the carb compared to a conventional engine. Basically, if you can build an engine with matched timing that will lug down lower in the rev range than you will ever practically use in the field, then there is no advantage to staggering the port timings.

The most obvious advantage of identical timings is being able to ensure that you are maximising the crankcase compression. The crankcase timing won't begin until both intake and strato timing are finished. If the strato timing finishes later, then that is when the base compression begins - and the contribution of the intake port has been 'short-changed' by finishing early.

I'm still in the 'matched timing' camp when it comes to the stratos.

For all out power, it would appear that compromising the strato port so that it flows fuel mixture would give an advantage. The fresh fuel mixture would tend to cool the upper ports and combustion chamber better than straight air and thus allow more charge density.

The compromised strato port may become a defining technique between a 'woods port' and a 'race port'.

 

[2stroked2smoke]

This is good info. I took the cylinder off my 562 tonight and am trying to wrap my mind around this configuration and what I'm gonna do. Here's some pics.

 

[mweba]

The cylinder quality is superb is it not

 

[2stroked2smoke]

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

 

[parrisw]

The cylinder quality is superb is it not

Ya it looks real nice!

 

[Terry Syd]

Here's some other pictures you need with the jug on the saw.

Take one of the transfer covers off and take a picture of the strato cutaway just peeking over the bottom of the transfer port. You want to check that the opening is completely flat (no curvature from the cutaway). This will give you the fastest opening of the strato port.

Take another picture from the back of the jug (no manifold) with the piston at TDC. You want to check that the piston does not block any of the flow in the strato ports.

Check the timing of the strato port with the transfer cover off, then compare it to the intake timing. You'll probably find there is about 10 degrees difference.

 

[Terry Syd]

"looks like the strato ports open way before the intake." - nope.

This has been a common misconception with guys starting out to mod stratos, they use old habits and look at the back of the jug for the timing figures. The strato port opens at the transfer port.

The openings at the back of the jug must be open BEFORE the transfer port opens so that the transfer port openings have an open conduit to work with. If the back ports opened at the same time, it would just create more restriction, so the back ports are opened first to get ready for the transfer port opening.

 

[mweba]

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

Yes. When studying my 555 cylinder, I was trying to figure a way to reduce the advance on the strato ports. In turn loading less clean air in to the transfers. Lost interest and never sorted it out......

 

[tree monkey]

can you give us the stock timing numbers ? thanks

 

[mweba]

"looks like the strato ports open way before the intake." - nope.

This has been a common misconception with guys starting out to mod stratos, they use old habits and look at the back of the jug for the timing figures. The strato port opens at the transfer port.

The openings at the back of the jug must be open BEFORE the transfer port opens so that the transfer port openings have an open conduit to work.

I don't remember this being the case but could have completely over looked it on this configuration. Interesting

 

[mweba]

can you give us the stock timing numbers ? thanks

I believe they are earlier in this thread, Scott. Will check

 

[blsnelling]

I don't find them so different to port than a non-strato saw.

 

[Chris-PA]

Cool pictures, neat parts. The engineer in me really likes these strato designs. Just puking raw fuel out the exhaust port is gross, this is a much more elegant solution. There is more complexity, but that complexity is mostly in the design - it manifests in the shape of the ports and the piston, plus and extra butterfly. The finished design is not that mechanically complicated with lots of fussy moving parts, and the basic function remains the same. It's just filling the end of the transfer runner with air before charging the cylinder, so that air is the first thing pumped in. There's no reason they couldn't have made this stuff a long time ago.

 

[2stroked2smoke]

"looks like the strato ports open way before the intake." - nope.

This has been a common misconception with guys starting out to mod stratos, they use old habits and look at the back of the jug for the timing figures. The strato port opens at the transfer port.

The openings at the back of the jug must be open BEFORE the transfer port opens so that the transfer port openings have an open conduit to work with. If the back ports opened at the same time, it would just create more restriction, so the back ports are opened first to get ready for the transfer port opening.

OK, I see what your saying now. Looks like the strato is opening into the transfer port about 4 degrees after the intake opens. This is a pic of the piston with some tape on it to mark how much I would have to remove to make the timing on both ports open at the same time.

 

[Mastermind]

OK, I see what your saying now. Looks like the strato is opening into the transfer port about 4 degrees after the intake opens. This is a pic of the piston with some tape on it to mark how much I would have to remove to make the timing on both ports open at the same time.

You said the strato ports open after the intake ports. Interesting. I would have thought the stratos would have open slightly ahead....

This is a great discussion. I really expect to see more and more of these so we MUST learn everything we can about them and share it freely.

 

[2stroked2smoke]

Well I went ahead and changed the strato port timing to match the intake timing. I also took the exhaust port width from 58% of cylinder width to 65% of cylinder width. Started it up and it runs pretty good. I'm gonna go cut some firewood tommorrow. I'll let ya know how it runs.

 

[Terry Syd]

Yeah, I was also expecting the strato timing to be more advanced/longer than the intake timing.

I notice that the cutaway on the piston has been smoothed from the factory. It is already shaped for good flow, whereas the first stratos had pin bosses sticking out and disrupting the flow.

EDIT: I'm wondering if you matched the intake of the STRATO port rather than the intake of the intake port where the CARBURETOR is located.

 

[2stroked2smoke]

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?

 

[mweba]

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?

That is the way I understood his explanation as well. This is why I was confused about the port opening after the intake. The stratos I've dealt with, the strato port opens a couple degrees before the intake, causing negative pressure to draw fresh into the upper trans. If the timing is matched,with the intake, less fresh is drawn into the uppers for the incoming charge.

 

[mweba]

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.