Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Neil, I live on the Comboyne Plateau, to the West of Port Macquarie.

A piped 576! Wow, I'd like to run that beast through some wood. What carburetor do you have on that engine? I actually have the same carb (C1M, 13.5mm venturi) from a stock 576 on my 50cc 450.

A bigger carb is the way to go with these Husky stratos. The strato system is split with the first half of the throttle opening working just the carb - so you can run a big carb because you still have good intake velocity for the low speed circuit. The stock time/area of the intake/strato system can accomodate a lot of flow, but the bottle neck is in the carb and strato butterfly.

I kept the strato and carb flow seperate on the 450 as mine is a work saw, but a racer could do all sorts of wild things. I would probably lean towards two carbs with the strato port carb running a richer mixture. The richer mixture would help flush heat off the piston, the upper transfers and the combustion chamber (then get blown out the exhaust), that would allow for a denser charge in the cylinder and might also allow for some extra compression on an engine. However, if a pipe was packing that rich mixture back in I could forsee some problems. Just got to try it and see.

 

[ausneil 1]

terry,
here is my 576 when i got her, i will find a line up of my saws which i compete with

 

[Terry Syd]

OK Neil, where did you get that pipe? That thing looks like it was put together from exploded forms inside a mould. Is that a bike pipe you adapted to your saw?

Whatever, it sure fits the saw nicely. I'd probably go with an internal stinger to shorten up the package, but then maybe you want all that noise going on behind you!

 

[ausneil 1]

terry,
thats the way i bought the saw, the pipe and specs were sold to me on certain conditions, (privacy). i have since tuned the pipe a bit and it only runs in large events like aust titles ect. now i know where you are you may get a look sometime.
Heres a ruff pic of my gear ready to pack for 2010 australian titles in wallumbilla QLD, not much detail but it shows what we need to be competative in all events.
the line up is...
576 pipe
394 pipe
3120 pipe
2 x 3120 for vis standard speed and open post
394 vis standard speed
575 vis standard speed / disc
395 vis standard spare

 

[Terry Syd]

Wow, what a line up. You are definitely into the racing scene - and I note that the two other piped saws do have an internal stinger, sounds like I was telling you how to suck eggs.

Edit: I looked at your avatar and I think I may have seen you at the Kendall showgrounds last year.

I tried sending Rick a reply to a PM he sent, but I think I had a brain fart and lost the message. Essentially, I was refering to reversing the strato function on a racer. In other words, the lean mixture comes in the intake and the rich mixture comes in the strato port.

Running two big carbs would allow the carb on the intake port to have a proper metering low speed circuit and a LEAN high speed circuit. As the second carb came in on the strato port it would have a rich low and high speed circuit.

By running two big carbs, you could maximise the potential of the increased time/area of the intake/strato system. It may be possible that you could get away with the stock 'intake' duration and still get good filling of the crankcase. In which case, you maximise base compression, or to state it another way, retain the swept volume of the piston. A lot of people don't consider that when they are increasing the intake duration that they are also DECREASING the swept volume of the piston (or decreasing the size of the gulp the piston can make).

The conventional two-strokes have been tweaked to their limits, I think we are just beginning to see what can be done with a strato.

 

[ausneil 1]

thanks for your comments, and your right regarding these new engines, its a new concept to most and in time i intend to learn all involved with these motors, i best get to work.
later neil

 

[Terry Syd]

One last comment on that pipe for the 576. The Husky stratos have very low blowdown figures. I figured a good running pipe would have a very short mid-section (space from the divergent cone to the reverse cone) on the pipe. That pipe on your saw has a fairly long mid-section, completely opposite of what I thought would work. I should take a look at the formulas again, it has been a while since I built a pipe.

 

[ausneil 1]

One last comment on that pipe for the 576. The Husky stratos have very low blowdown figures. I figured a good running pipe would have a very short mid-section (space from the divergent cone to the reverse cone) on the pipe. That pipe on your saw has a fairly long mid-section, completely opposite of what I thought would work. I should take a look at the formulas again, it has been a while since I built a pipe.

the geometry of this pipe is unique and i donnot fully understand how and why, but it performes better than i originaly hoped.
I am extremely happy with this saw and i donnot beleive in unnessescary developement, only time i look for more gains is when i cannot make finals.

terry stay with this thread, it is very imformative, and well explained.

BTW.. i was not at kendal last year, as i raced at gressford that weekend but i have raced at kendal many, many times.
later neil

 

[SDB777]

Just wondering if any of you guys in this topic have video of your saws in action?


Very interesting...







Scott B

 

[Terry Syd]

Sorry Scott, if you way back to the early posts you will see that I finally had to purchase a new cheap camera just so there would be a few photos for the thread. If I lash out and get a new 560XP to play with, I'll get another new camera to go with it (I'll claim it as a 'present' to the wife to offset my purchase).

TDI Rick sent me a link to Gordon Blair's book on designing two-strokes. I had heard of the book, but never seen any pages from it - it is the best ever on designing/modding a two-stroke. Unfortunately, it is very, no, let me say, VERY math oriented. It is a book for engineers.

However, if you do not let the math get in your way, you can read a section and understand the bones of what he is saying. Once you grasp the concepts of what he is saying, you can work with the concepts and leave the math for the engineers. I like his analysis on things like scavenging efficiency, trapping efficiency, charging efficiency, short circuiting and leakage.

One of the mods on this forum has been to sweep back (and sometimes up) the rear of the transfers. Nobody has ever explained why they do this (IMO) stupid mod, but if they were to read sections 3.35/36 they would quickly understand more about about the actual transfer flow direction. Paragraph 3.35.51 has a good paragraph on what makes a good basic running engine. Lots of titbits, here's the link -

Design and simulation of two-stroke engines Gordon P. Blair.pdf - 4shared.com - document sharing - download

 

[ausneil 1]

One last comment on that pipe for the 576. The Husky stratos have very low blowdown figures. I figured a good running pipe would have a very short mid-section (space from the divergent cone to the reverse cone) on the pipe. That pipe on your saw has a fairly long mid-section, completely opposite of what I thought would work. I should take a look at the formulas again, it has been a while since I built a pipe.

hey Terry, just a thought.....
Should you decide to play with some numbers and design a different pipe, i would be happy to make it to your specs and try it on my 576. It would be an interesting comparasin to the one i use now, if its better than my one, great and if not you can have the specs from mine.
just a thought.

 

[ausneil 1]

SDB777.... if you visit my utube channel, ausneil 1, you will see some of our racing here in australia.

later neil

Just wondering if any of you guys in this topic have video of your saws in action?


Very interesting...







Scott B

 

[Terry Syd]

Neil, I am definitely not a pipe builder. I've only built one back in the early 1970's and it was butt ugly. It worked, but I never built another after the factories started coming out with decent pipes on the bikes.

Perhaps I was 'overthinking' the strato pipe. It just seemed to me that with that short blowdown that the transfers/crankcase would be quickly pressurised with the blowback and once the cylinder pressure dropped and the transfers started to flow they would flow fast with the extra crankcase pressure. Then after BDC the transfer flow could use the low pressure from the pipe to keep pulling on the long transfer timing. Then a quick return wave to pack the cylinder after the transfers close.

However, looking at your pipe, a quick scavenging pulse to get the transfers flowing after being hit with all that blowback seems just as logical. - Anyway, what works is just that, what works.

Here's something I picked up over the years that might give you an advantage. I built a few bikes with modified expansion chambers where the exhaust outlet was on the side of the pipe. By taking the exhaust off the mid section, the amplitude of the exhaust is significantly reduced (noise), it also shortens up the pipe. Since doing those couple of bikes, I have seen the same mod being done to Aero engines and moped bikes in Europe.

The first bike had a baffled muffler attached to the underside of the pipe in the mid section. The baffled muffler prevented any resonance in the pipe outlet. However, the second bike had a stinger attached to the mid section leading to a muffler.

I soon found out that the length of that stinger was now a critical dimension. The pipe now had a 'hit' in it that wasn't there before. I discoved that by changing the length of the stinger, I could shift that 'hit' up and down the powerband to just about anywhere in the powerband. Since this was an enduro bike, the last thing I needed was a hit in the power. I started playing around with the shape of the pipe at the end of the stinger.

I heated the end of the stinger and flared it out in a cone. That helped to spread the hit, but wasn't the right shape. I spent a few weeks re-working the cone until it was more of a bell shape. Essentially, a large radius bend initially and then getting progressive smaller radius as it approached the end. That worked to completely spread out the hit so that I could use it on the enduro bike.

Here's the thing - I have no idea what the wave dynamics are that made up this phenomena. All I know is that I could use the phenomena to fine tune the power characteristics of the pipe.

So here's what I'm thinking. The next time you build a pipe, you might try the external stinger. You could put a second tight fitting pipe over the stinger to slide back and forth and thus change the length of the stinger. On the powerband of the chainsaw you could move that hit to anywhere you wanted it. You might even make up some other pipe extensions with different endings to change the nature of the hit. Essentially, you could adjust your 'sweetspot' in the powerband for the kind of wood/race on the day.

One caveat is the edge of the internal pipe in the composite stinger will create its own wave. That edge has to be ground back in a long bevel, even a 45 degree bevel on that edge will still create a small pressure wave.

 

[ausneil 1]

i understand what your saying Terry, i have a way of using different legnth stingers and testing performance gains / losses.
Your idea should work also and i could give it a go on a 090 pipe i have, its is lent to another mate at present but i wanted to experament with it further. the 090 for me is a fun saw but gets raced in exclusive 090 races in QLD.

 

[Terry Syd]

I've found a few nuggets in Blair's work and I thought I would pass them on. In a previous post I mentioned how I opened up the width of the transfer ports using Gordon Jennings formula and the advice of Timberwolf. Timberwolf stated that he found transfer time areas that worked on a saw to be a little less than Jennings lower range of .00008.

I opened up my transfers and set them at .00007 at 10,000 rpm. That worked quite well.

Blair's work looked at over 1,800 different two-strokes, some of those engines were chainsaws. In fact, he uses a chainsaw (loopsaw) in one of his developement exercises. His computer calculations indicated that the transfers should have .000066 at 9,600 rpm (note he uses meters in his calculations where Jennings uses centimeters, so the figure in his book is .0066).

The measured time area on the saw was actually .000067. The time area of the transfers being fine, he went on to the other ports.

I recalculated my area using the .000066 figure and I hit that time area at 10,500 rpm. Perhaps that helps to explain why my saw holds it revs in the cut.

Since the time area of the transfers is so important, it would be a good exercise to measure the transfer time area before a lot of development work is done. If the transfers can't flow at those higher revs, then raising the exhaust port is only going to make a peaky engine that will fall on its face when stuck in the wood.

 

[Terry Syd]

For decades I have believed that it was the piston crown temperature that heats the charge in the crankcase and drops the power of a two-stroke. That drop in power can be up to 20% between a flash cold dyno reading and a hot reading.

I don't know what 'expert' I read that information from back in the 60s/70s, but it stuck in my mind. However, I still questioned the concept. For example, a water-cooled two-stroke doesn't loose as much power as an air-cooled two-stroke, but the BMEP on the piston is higher. That means the piston crown gets even hotter than the air-cooled engine. Of course, there is conduction of the heat through the ring and piston skirt into the cooler cylinder, but the center of the piston crown is still getting hotter than the air-cooled piston. Hmmm

Blair's work clarified what is really happening in the two-stroke with actual measurments of the temps. Here's a quote, "The peak temperature of the crankcase air rises to 120°C and, as shown in Fig. 5.21, and can be heated within the cylinder up to 600°C during the early stages of scavenging. Not surprisingly, the majority of fuel vaporization occurs within the cylinder. Pg391-392"

Bingo! It is the remaining exhaust gases and cylinder temperature that raises the temperature of the incoming charge. The water-cooled two-stroke has a cooler cylinder, therefore it doesn't radiate as much heat into the incoming charge.

So how can we use that information? Remember the idea of the strato with two carbs? What if the strato carb was running extra rich, that would help wash the heat out of the cylinder. Since as Blair says "the majority of fuel vaporization occurs within the cylinder", that extra fuel vaporizing in the cylinder would be a sort of 'liquid cooling' to the cylinder.

That cooling blast through the strato port would flow across the piston skirt first, then down into the tops of the transfer tunnels, then across the top of the hot piston and then into the cylinder - liquid cooling where you need it.

Much of the initial blast of rich mixture would then get pushed out the exhaust port just like the fresh air of the standard strato does, but who cares, it has done its job - to cool the cylinder.

 

[Terry Syd]

I was surprised to see the heat and pressure curves of the ignition process. It is almost universal that there is a 10 degree delay between when the spark occurs and when the combustion gets going. In some engines that can be up to a 14 degree delay.

Peak temperatures and pressure occur between 5 and 10 degrees ATDC, but wait, there's more! That peak in temperature coincides with the engine reaching the burning of 50% of the available fuel air mixture.

Here's Blair, "Nevertheless, the common factor that prevails for these mass fraction burned curves (and the comment is equally applicable to Fig. 4.5) is that the position of 50% mass fraction burned is almost
universally phased between 5° and 10° atdc. In other words, optimization of ignition timing means that, taking into account the ignition delay, the burn process is phased to provide an optimized pressure curve on the piston crown and that is given by having 50% of the fuel burned by about 7.5° atdc. The 50% value for the mass fraction burned, B, usually coincideswith the peak heat release rate, QRQ .- pg 310"

"The optimized burn profile rarely has this point before 5° atdc nor later than 10° atdc. It can also be observed, from Fig. 4.5 and Figs. 4.7(a), (b), (d), and (e) that the ignition delay for engines, with scavenging efficiencies at 0.75 and above, are quite commonly between 10° and 14°, pg 313"

So what does this mean? Well, it means that all the talk about air/fuel mixture trapped under the squish band and not burning is bumkin - it all gets burned. Granted, there is a whole science related to squish velocities in high compression engines, but that is irrelevant to the backyard modder of a chainsaw.

In fact, Jennings has this to say, "You may be interested to know, too, that in many cases a non-squish combustion chamber, with its complete utilization of the mixture to offset the power-limiting effects of a necessarily-lower compression ratio, has proven to be best in absolute terms of power and economy".

I also no longer have the reference, but I read a paper a long time ago on flame propagation with flame speeds across the chamber and it concurred with Blair's work. Basically, the piston has already moved well away from the squish band by the time the flame front reaches the squish area and the burn continues under the squish band as if it wasn't there.

So what does this mean? Well, when we drop the jugs we aren't really 'tightening up the squish', we are actually just increasing the compression ratio.

It also means, as I found out years ago, that if you cut the side of the piston crown to increase the port timing, that any mixture in that area under the squish band will not be lost, it will get burned.

 

[tdi-rick]

Terry, I don't think I've heard anyone suggest that the squish charge doesn't get burned, more that by the time it does start to burn it doesn't contribute anything significant to the pressure curve as it's happened too late.

 

[Terry Syd]

I've seen guys write that the charge trapped under the squish doesn't get burned, on this forum even. As Blair states, the 50% burn rate by about 7.5 degrees ATDC pretty much indicates that there is another 50% left to burn after that point.

EDIT: It is also informative to look at the pressure charts relating to the burning of the charge. The peak pressure is around 5-10 degrees after TDC and then drops off about as fast as the pressure rises. When we take into consideration the rod/crank angle, then the resulting burn and thus pressure on the crank has finished and started dropping well before the maximum 90 degree leverage on the crank.

 

[Terry Syd]

Here's another tidbit from Blair, "This is also helpful, for the inlet port maximum area is a
very useful guide to carburetor size, i.e., its flow diameter. If one matches ports in this manner,
the carburetor flow area, for an engine with the inlet port controlled by the piston skirt,
can be set to between 85% and 95% of the inlet port flow area, as an approximate guide." Pg 434

In other words, the common approach is that we are seeing people port massive increases of the widths of the intake port without a coresponding increase in the size of the carburetor. The extra widths do give a slight increase in the time area (the 'gulp') of the port, but the maximum flow rate is being set by the stock carb.

In a previous post I posted a link to a formula for approximating the size of the carb, that may be a good guide to setting the carb size for a modified saw.

On the strato, the bottleneck is the size of the carb and the strato butterfly. The combined areas of the intake and the transfer ports is more than enough to accomodate the larger flow from a bigger carb and strato butterfly.

I'm a big fan of rotary valves as they normally open slightly before the transfers are closed by the piston. The result is that the rotary valve engine can 'inhale' through the intake as soon as the transfers close. Unlike the rotary, in the piston port engine, the piston moves upward from the closing of the transfers creating a depression (say vacumn) in the crankcase until the intake port opens at say 80 degrees before TDC - it is like a skin diver coming up for a gulp of air.

The rotary valve engine can fill the crankcase a lot easier than a piston port engine. Sometimes the rotary valve will close around 65 degrees ATDC because it doesn't need any more timing. However, the piston port engine is still trying to make up for lost time and may need the extra time of 80 degrees ATDC.

Since we are dealing with piston port engines, the faster we can fill the crankcase when the port opens the better - we won't need as much intake timing. The strato engine provides lots of time area for the intake cycle, however it will be the carb and strato butterfly that will limit the use of that time area.