Modifying Strato/X-Torq saws...

[Stihlman441]

Brad, all these various stratos are a bit different and take a slightly different approach to each of them. As I said 'on my saw' I couldn't make the strato ports any bigger. This is a Husky 450 that is based upon the 445 - the engineers had trimmed every bit of port work they could to open it up. In fact, the strato butterfly was a big 16mm, but the manifold was only 13.5mm. It became clear why it was only 13.5mm after I calculated the area of the strato ports in the cylinder, that's all it needed.

However, the piston did partially block the ports in the cylinder, so I reworked the piston so that the whole system could flow like it was designed to. I even trimmed back the ends of the pin boss so that the air could get past it.

Back to the fuel issue. It is real easy to get more fuel in a engine, just richen the mixture. The real trick is to get more air in the engine in order to burn the fuel - think super or turbo charging as a case on point.

The strato port is part of the intake cycle. If you can improve upon the flow of the strato system, you can increase the amount of air available for combustion. As I stated, there appears to be complete mixing of whatever is available in the cylinder by the time of combustion.

With the richen the mixture and get more fuel into the saw what about the MTronic and Auto tune saws ?,has anyone worked out how to adjust the electronics to acheave this ?.

 

[Mastermind]

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.

So you are saying you in effect raised the exhaust port two degrees and left the transfers as is.

Did you ever try raising the transfer ports?

 

[2stroked2smoke]

So you are saying you in effect raised the exhaust port two degrees and left the transfers as is.

Did you ever try raising the transfer ports?

Wouldn't that be going backwards and decreasing blowdown?

 

[Terry Syd]

Randy, my 450 was an odd ball saw to start working on. It is based on the 445 (45cc). When I checked the area of the transfer ports I saw that they were already 9% too small for the ports on the 50cc 450.

However, it got worse - I needed more transfer area, so I increased the width of the ports by another 12.5%. Add the 9% deficit to the 12.5% increase and I had to make the transfer tunnels another 21.5% larger - now that took some effort to get it all to flow correctly. I ended up grinding out the back of the transfer tunnel covers and building them up with fiberglass and GB weld.

I ended up with .0066 m/sec at 10,500rpm on the transfers, which works great.

Not a project for the faint of heart, but at 10.8lb, NK bar and 8 pin, it will stay with my mate's 365 in big Aussie hardwood. In fact, I have to get a re-match with him as I think that the last few changes may give me the edge.

Going back to the 562, if the revs are increased significantly by the additional blowdown, then there may be the need to widen the ports. I say widen rather than raising as the transfer tunnels may be easier to widen than to raise them. It will also prevent chasing more exhaust timing to keep the blowdown figures.

These Husky stratos have a lot of potential with the strato function, they can flat suck some air. I am running a 570 carb on my 450. The combined area of the strato function and 570 carb is equivalent to a 19mm carb. I expect that the 562 has a 12-12.5mm venturi in the carb. Since it is the same C1M carb as the 570, then someone will probably eventually slip on the larger 13.5mm 570 carb on the 562.

 

[parrisw]

So you are saying you in effect raised the exhaust port two degrees and left the transfers as is.

Did you ever try raising the transfer ports?

Ya, I'm kinda confused as well Randy??? I've never heard anyone say they just increased blowdown? To do that you need more spacing between the transfers and the ex, the only way to do that is raise the ex. Or lower the jug then raise the ex that amount, so ex would be unchanged, blowdown would increase, but intake would increase as well.

 

[Terry Syd]

You guys are focusing on timing. Remember it is time/area of the ports that really matters. That's why it helps to widen the exhaust port, you are increasing the area of the port during blowdown.

The 562 transfer tunnels look a lot easier to modify than the homeowner 450. If the 562 needs more transfer time/area I expect that you will find it easy to accomplish by making the transfer ports wider.

 

[Mastermind]

Wouldn't that be going backwards and decreasing blowdown?

If you raised the exhaust 4° and the transfers 2° you would net a 2° increase in blowdown. I just was wondering if raising the transfers would increase working rpm. There's more to the equation than blowdown.

Randy, my 450 was an odd ball saw to start working on. It is based on the 445 (45cc). When I checked the area of the transfer ports I saw that they were already 9% too small for the ports on the 50cc 450.

However, it got worse - I needed more transfer area, so I increased the width of the ports by another 12.5%. Add the 9% deficit to the 12.5% increase and I had to make the transfer tunnels another 21.5% larger - now that took some effort to get it all to flow correctly. I ended up grinding out the back of the transfer tunnel covers and building them up with fiberglass and GB weld.

I ended up with .0066 m/sec at 10,500rpm on the transfers, which works great.

Not a project for the faint of heart, but at 10.8lb, NK bar and 8 pin, it will stay with my mate's 365 in big Aussie hardwood. In fact, I have to get a re-match with him as I think that the last few changes may give me the edge.

Going back to the 562, if the revs are increased significantly by the additional blowdown, then there may be the need to widen the ports. I say widen rather than raising as the transfer tunnels may be easier to widen than to raise them. It will also prevent chasing more exhaust timing to keep the blowdown figures.

These Husky stratos have a lot of potential with the strato function, they can flat suck some air. I am running a 570 carb on my 450. The combined area of the strato function and 570 carb is equivalent to a 19mm carb. I expect that the 562 has a 12-12.5mm venturi in the carb. Since it is the same C1M carb as the 570, then someone will probably eventually slip on the larger 13.5mm 570 carb on the 562.

I remember that build very well Terry. I did a 450 shortly after and your build helped immensely.

Ya, I'm kinda confused as well Randy??? I've never heard anyone say they just increased blowdown? To do that you need more spacing between the transfers and the ex, the only way to do that is raise the ex. Or lower the jug then raise the ex that amount, so ex would be unchanged, blowdown would increase, but intake would increase as well.

You could raise just the exhaust and increase blowdown. I've been trying to maintain factory blowdown on most saws by raising both exhaust and transfers together. On some newer saws that doesn't work well though.

I've found that raising the transfers on some saws really helps them hold a higher rpm in the cut. I just wonder if the same would be true on these strato saws?

 

[Terry Syd]

Randy, I can tell you that when I finally got those transfers opened up on the 450 it held its rpm in the cut very well.

We still have not seen a picture of the side of the 562 jug with the transfer covers off. A picture of the inside of the transfer covers would also be informative. The ones on the 450 had some flow inserts that were relatively easy to modify in order to widen the ports - it was getting the rest of the tunnel to flow that port that was the real challenge, but the 562 looks easy.

 

[Mastermind]

Randy, I can tell you that when I finally got those transfers opened up on the 450 it held its rpm in the cut very well.

We still have not seen a picture of the side of the 562 jug with the transfer covers off. A picture of the inside of the transfer covers would also be informative. The ones on the 450 had some flow inserts that were relatively easy to modify in order to widen the ports - it was getting the rest of the tunnel to flow that port that was the real challenge, but the 562 looks easy.

I'll be starting a thread on this saw in a few days with lots of pictures.

I have several woods ports ahead of it, but I will get a stock video and pictures to study asap.

 

[w8ye]

What about these?

 

[Terry Syd]

If it needs more transfer area, then those ports will be a piece of cake to work on and widen. There's plenty of meat in the transfer covers to increase the area if need be.

Take a look at the second picture of the inside of the transfer cover. You can see all the soot from the blowback into the transfers. The covers on my 450 had the soot going halfway down into the crankcase. I expect with the crankstuffers on the 562 that the extra base compression may help to prevent some of the blowback, but in any case the added compression will help the transfer flow to make up for lost time.

The blowback into the transfers was one of the unusual things I found on the Husky stratos. I expect the long transfer tunnels are designed to contain the blowback and then use it to help purge the cylinder.

Once the transfer flow reverses, the blowback gases enter the cylinder first where they collide and mix with the exhaust gases that are in the cylinder. It doesn't matter if those gases get mixed together, it is just exhaust mixing with exhaust. Then as the flow gets established in the cylinder, the fresh air from the strato follows the train of gases up the back of the cylinder, followed by the rich mixture from the carb.

The result is less mixing of air/fuel mixture with the remaining exhaust gases in the cylinder.

Further, the relatively long transfer duration and late closing of the transfers allows a better trapping efficiency of the intake charge just before the exhaust closes.

The whole design is different from what we are used to, but when you think about it, it sure is clever.

 

[tree monkey]

all that black in the transfer tells me that there is not enough blow down

 

[parrisw]

all that black in the transfer tells me that there is not enough blow down

Does that mean its getting some reverse flow? Ex going down the transfers.

 

[parrisw]

Maybe I'm not getting this totally but blowdown to me is the time between the ex opening and the transfers opening. So how does giving the transfers more area increase this time? My thinking it cant. If you widen the area of the transfers, you're getting more flow, but is it really more time? I keep hearing time/area?

 

[tree monkey]

Does that mean its getting some reverse flow? Ex going down the transfers.

yes

 

[Terry Syd]

Yep, that's what the black soot is, blowback. On a conventional two-stroke you may only find that around the transfer ports, but the Husky stratos it gets way down the tunnels.

However, here's the point to remember - how much blowdown is enough? The answer is easy - "enough".

You can find that out by taking a bit off the front of the front of the piston to increase the blowdown. You keep going until you go past the 'sweet spot'. Now you know how much is 'enough'.

 

[parrisw]

yes

Thanks. Tis what I thought.

 

[Terry Syd]

Will, Google 'time/area two-stroke' and you should find lots of references to time/area for porting. If you can pull up a copy of Jennings 'Two-stroke Tuners Handbook' he has a good discussion on it.

It is an old 'rule of thumb' of tuners to go for area before going for timing.

 

[tree monkey]

Maybe I'm not getting this totally but blowdown to me is the time between the ex opening and the transfers opening. So how does giving the transfers more area increase this time? My thinking it cant. If you widen the area of the transfers, you're getting more flow, but is it really more time? I keep hearing time/area?

you need to do some reading on port time area and port mean time, jennings is good for this. it's hard to exlplain.
macking the ex port wider gives the port more time area so the ex gas gots out faster. so you don't need as mutch blow down.
does that help?

 

[tree monkey]

terry beat me to it