A question for the saw builders...

[Mattyo]

One thing I still haven't seen explained, and I have searched, is what happens if blowdown is adjusted to extremes... ie, 10 degrees of blowdown vs 40?

btw, old cat, that's a nice link...very interesting

 

[Mastermind]

And.......that's the hell of it.

Everything you find to study on two stroke performance assumes a piped engine. Since I've been doing this I've found more and more gains by veering away from the thinks that I was told when I first found this site. When I showed up it was all about raising the exhaust port and widening the hell out of the ports. A saw with a popup piston was rare. Cut squish bands were even rarer.

Wigglesworth studied ever jug he found.....mapping timing numbers to see why some jugs run better than others. The results of all that study led us to the port timing numbers we are using today.

Eric Copsey was the first guy that mentioned cutting squish......although I think Scott Kunz was doing it before Eric was.

I built a few saws with fair results without compression increases.......but coming from a drag race background I knew that higher compression equaled more torque across the entire RPM range. The first "specialty" tool I bought was a lathe......

I equate work saw VS race saw to full race engines VS street engines. Just because an engine makes uber HP and RPM does not mean it will be a good engine to have in a street car. The same is true for saws. If you build a saw engine with you thoughts only on RPM it will not make a good saw to use in real world working situations. Imagine this if you will......you have ten tons of tree over your head.........you stop to check the position of your bar tip.......and the saw doesn't have enough low speed grunt to get the chain moving again?

If you decide you're gonna build saw engines.......consider the customer.

 

[Mastermind]

One thing I still haven't seen explained, and I have searched, is what happens if blowdown is adjusted to extremes... ie, 10 degrees of blowdown vs 40?

btw, old cat, that's a nice link...very interesting

It depends on the engine.

One recipe does not fit all saws.

The 550XP and 562XP.........both of those saws I build with 10 degrees of blowdown. The Dolmar 7900? I use nearly 30.

 

[Mattyo]

ok ok ....lemme try to ask the question in another way. If the blowdown is low ...like 5-10 degrees... then does that increase the rpm of the saw? and perhaps lessen the torque ... or if you have higher blowdown, say 30-40, high torque but a tendency to not have as high an rpm?

for most guys porting saws we don't have 20 jugs of the same saw sitting around that we can mess with blowdown on .....

does that 10 degrees on the 550 also translate to the 346? etc? I assume the dolmar 7900 doesn't apply to nearly anything else because its designed differently anyway

 

[Mastermind]

I get what you are asking.......but by just concentrating on one aspect of the port timing you are missing the big picture.

The 550XP is a strato saw........that means the first few degrees of transfer flow are just fresh air.......by opening the transfers sooner......that fresh air is used to purge the combustion chamber of spent gasses, and the trapped gasses end up less diluted. That doesn't work well on "regular" engines.

There are five timing events that must be considered......and they must work in concert to achieve maximum gains.

Intake closing point.

Case compression time.

Transfer opening point.

Blow down time.

Exhaust opening point.

Think about all that.......and the engine design. Then try to figure out why the designer used the port timing number he did to begin with. Then decide what you can change that will improve on the design.

 

[jar944]

One thing I still haven't seen explained, and I have searched, is what happens if blowdown is adjusted to extremes... ie, 10 degrees of blowdown vs 40?

btw, old cat, that's a nice link...very interesting

Degrees alone does not tell the entire story either. One must also consider the port opening size (square mm is common) and shape. Time area and angle area are two "measurements" that take these into account.

For those unfamiliar with angle area:


"i understand time area . but ive never found a good answer for what angle area meens. is there a simple explanation for the definition of angle area ? i thought it had something to do with crank angle but im not sure."


"Frits Overmars
OK, here goes.
In the beginning there is a cylinder. With just one rectangular port, to keep things simple. That port has three linear dimensions: the distance from cylinder top plane to the top edge of the port, the distance from top plane to bottom edge of port, and the port width. That's all.

Now we put that cylinder on a crankcase containing a crankshaft with a certain stroke, a con rod with a certain length, and a piston.
Rotate the crank and the piston will descend until it starts to open the port. Keep rotating the crank, and after a certain number of degrees the piston will close the port again. Now we can say that in addition to its linear dimensions the port has also a timing.

Next: angle.area.
Let's assume that the port starts to open at 90° before Bottom Dead Center. We rotate the crankshaft 1° further and then the port will be open a little bit, say x mm. The open area is now x mm high times the port width. And this piece of open area will stay open all the way from 89° before BDC right until 89° after BDC on the next upstroke. That little piece of open port area has an angle.area of 178° * x * width.

We rotate the crankshaft 1° further still, and the port will open a little bit more, say another y mm. This additional open area will stay open from 88° before BTC till 88° after BDC. So this second little piece of open port area has an angle.area of 176° * y * width. And so on.
When we add all these pieces of open port area times the number of degrees that they are open, we find the total angle.area of the port.

Finally: time.area.
Now we look at a running engine. The angle.area of a port is a fixed value, but the time.area is not. How much time it takes to rotate the crankshaft 1 degree, depends on the rpm, as Graham explained above. Double the rpm, and that time will be halved, and so will be the time.area.
Emptying and charging the cylinder requires a certain time.area. If the rpm rises too high, the engine will run out of breath because the available time for emptying and charging the cylinder will become too short."

 

[old-cat]

ok ok ....lemme try to ask the question in another way. If the blowdown is low ...like 5-10 degrees... then does that increase the rpm of the saw? and perhaps lessen the torque ... or if you have higher blowdown, say 30-40, high torque but a tendency to not have as high an rpm?

for most guys porting saws we don't have 20 jugs of the same saw sitting around that we can mess with blowdown on .....

does that 10 degrees on the 550 also translate to the 346? etc? I assume the dolmar 7900 doesn't apply to nearly anything else because its designed differently anyway

It's all about volumetric efficiency. How full can you get the cylinder. Trial and error

 

[Mastermind]

Good thread guys.

 

[Fire8]

Husqvarna 51........

Nice saw to port.
I have a near mint 50 you got me thinking

 

[Mastermind]

Well Danny........all you have to do is put it in the mail. I owe you one my friend.

 

[Mattyo]

Good thread, but admittedly kinda discouraging... I really feel like I'm outa my league. I get it that the whole picture has to be taken into account, but there have to be some guiding principles along the way. The most interesting thing that I'm getting out this is that 2 stroke modding is really still in its infancy. What is impressive is how much we really don't know, and the more we know, the more we have that problem, AND the more that's found that flys in the face of guiding principles.... kinda discouraging when you just want to mod things a little bit

 

[old-cat]

Good thread, but admittedly kinda discouraging... I really feel like I'm outa my league. I get it that the whole picture has to be taken into account, but there have to be some guiding principles along the way. The most interesting thing that I'm getting out this is that 2 stroke modding is really still in its infancy. What is impressive is how much we really don't know, and the more we know, the more we have that problem, AND the more that's found that flys in the face of guiding principles.... kinda discouraging when you just want to mod things a little bit

Playing with air flow to get higher volumetric efficiency is Black Magik, trial and error. Raising compression to increase fuel efficiency is simply simple!

 

[Flattop Burns]

I want to understand how and why porting is the key to making power.

I wouldn't say porting is the key to making power, synergy is. Introducing a highly compressed and correctly proportioned air-fuel mixture to meet a hot spark at the right time in a state of balance with the exhaust and (with two-strokes,) transfer method and with the least resistance on the rotating assembly is the key to making power. Break down each component individually from there starting with fuel quality.

 

[Mastermind]

Good thread, but admittedly kinda discouraging... I really feel like I'm outa my league. I get it that the whole picture has to be taken into account, but there have to be some guiding principles along the way. The most interesting thing that I'm getting out this is that 2 stroke modding is really still in its infancy. What is impressive is how much we really don't know, and the more we know, the more we have that problem, AND the more that's found that flys in the face of guiding principles.... kinda discouraging when you just want to mod things a little bit

Do not be discouraged. Find a few saws on the cheap. The 026 is a great test bed, and the 350 Husky.....

I will tell you this.......90% of the time you can widen the intake to the max, and raise the transfers to 120 degrees opening point and get big gains in these engines. That is about as generic as a recipe can get.

 

[Mastermind]

I wouldn't say porting is the key to making power, synergy is. Introducing a highly compressed and correctly proportioned air-fuel mixture meet a hot spark at the right time in a state of balance with the exhaust and (with twostrokes,) transfer method and with the least resistance on the rotating assembly is the key to making power. Break down each component individually from there starting with fuel quality.

Great post.

 

[Mattyo]

I've done probably 20 saws at this point... down to crank rebuilds... 5 ported. But its not trial and error for me because once a jug goes down, I don't usually remove it unless something is wrong. I have the Bridgeport ... so I can machine a chassis if I need to. My 372xt project is almost finished so we'll see where that leads. I have my 385xp that I built for no particular good reason. I live on 1/2 acre plot and will never need the 385... so for me to mod it is fun, and not necessarily about chasing gains. I want SOME gain, something reasonable without having to go completely nuts with formulas and whatnot. I assume I'm not the only one on the forum that has this type of need/desire... so I've been reading here like crazy to find certain formula's / tendencies to see where average gains can be gotten without losing my mind entirely

 

[Mastermind]

The formulas for saw engines just ain't out there. Everything that is assumes a pipe.

 

[BigRed96]

Husqvarna 51........

Nice saw to port.

I'll do the machine work. Send me a PM.

Will do. Thanks!

 

[mathewsdxt75]

What a great tread thank you to all, who explain to some of us amateurs who are trying to learn. it is very much Appreciated

 

[Sty57]

I've done probably 20 saws at this point... down to crank rebuilds... 5 ported. But its not trial and error for me because once a jug goes down, I don't usually remove it unless something is wrong. I have the Bridgeport ... so I can machine a chassis if I need to. My 372xt project is almost finished so we'll see where that leads. I have my 385xp that I built for no particular good reason. I live on 1/2 acre plot and will never need the 385... so for me to mod it is fun, and not necessarily about chasing gains. I want SOME gain, something reasonable without having to go completely nuts with formulas and whatnot. I assume I'm not the only one on the forum that has this type of need/desire... so I've been reading here like crazy to find certain formula's / tendencies to see where average gains can be gotten without losing my mind entirely

I feel the same as you....
I haven't been on here for long and I don't post much, but I am on every day and read and search as much as I can.

It seems to me that once all the old Data was lost, and there was a big jump in technical data. So all the guys that have been doing it for a long time moved on and advanced.

I try to read every build thread out there, and to all of you that do them Thank You. What I would really like to see is a build up of a saw from the simplest mod to the more advanced.

I know this would take a lot of time and money for someone to do, but I think it would help a lot of people.

Thanks, Brian