Mastermind
Work Saw Specialist
Is the 288 stronger than the 2171?
Which ported 50cc saw
- Thread starter woodyman
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Is the 288 stronger than the 2171?
bryanr2
Addicted to ArboristSite
2171 might be a little fat- not quite tuned in right. I think the 288 is stronger than any saw I have.
Mastermind
Work Saw Specialist
I have no qualms running .016" on a 346 with 180-200 PSI turning 15,500 RPMs. I've done a fair amount of work with mine. It's been apart recently, and I see no signs of heat or detonation. I run Premium unleaded and 32:1 full synthetic oil.
Reminds me of a German gal I recently dated...
komatsuvarna
Arboristsite MVP
But why? Your not gaining much if any running is so tight for a .000005678 % gain. The gain doesn't outweigh the risk,,,, JUST MY OPINION OF COURSE
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But why? Your not gaining much if any running is so tight for a .000005678 % gain. The gain doesn't outweigh the risk,,,, JUST MY OPINION OF COURSE
I don't consider it overly tight for a 50cc saw, and do not consider it risky. If it were so risky, I wouldn't see 660s come from the factory with only .017" squish. Now that I have a lathe, I'll go closer to .020". But, I see no reason to go any looser than that. Why go to the trouble to build more compression, and then waste it with overly loose squish? Perhaps there's science as to why.
Disclaimer: This is only my opinion, and has not been voted on by the community as fact. Additionally, I reserve the right to change my opinion, should additional information come to light.
komatsuvarna
Arboristsite MVP
LOL. Just wonder was all
. I'd never run a 660 that tight for sure :msp_scared:.....but I'm not saying it wont work.
Tzed250
Addicted to ArboristSite
LOL!! Did your attorney advise you to lay the disclaimer in your posts?!? 
To little squish does have downsides. One being excessive squish velocity. This can cause scrubbing of the boundary layer on the piston, which can lead to excessive heat in the piston crown. Trapped end gas is also higher with a tighter squish. Goldilocks was on to something...
To little squish does have downsides. One being excessive squish velocity. This can cause scrubbing of the boundary layer on the piston, which can lead to excessive heat in the piston crown. Trapped end gas is also higher with a tighter squish. Goldilocks was on to something...
I think you can go tighter on a smaller saw for a variety of reasons-
The cylinder and piston don't have as much material in them overall and
thus expand less under heat and pressure; and there's less room in the
smaller jug for the fired charge to travel (or spread out.)
I haven't run a 60cc+ saw any tighter than .024" and I've had real good
luck with that. I have run several 45-50cc saws right at .020" and that's
worked fine.
Same thing goes for finger ports. I gave an 024S finger ports one time and
that was the strongest sub 50cc saw I've ever built. But I haven't had much
luck with finger ports in anything over 55cc.
The cylinder and piston don't have as much material in them overall and
thus expand less under heat and pressure; and there's less room in the
smaller jug for the fired charge to travel (or spread out.)
I haven't run a 60cc+ saw any tighter than .024" and I've had real good
luck with that. I have run several 45-50cc saws right at .020" and that's
worked fine.
Same thing goes for finger ports. I gave an 024S finger ports one time and
that was the strongest sub 50cc saw I've ever built. But I haven't had much
luck with finger ports in anything over 55cc.
komatsuvarna
Arboristsite MVP
LOL!! Did your attorney advise you to lay the disclaimer in your posts?!?
To little squish does have downsides. One being excessive squish velocity. This can cause scrubbing of the boundary layer on the piston, which can lead to excessive heat in the piston crown. Trapped end gas is also higher with a tighter squish. Goldilocks was on to something...
I've ran squish a little tight before..... After I seen the piston crown on the exhaust side I opened it back up. Sometimes I just have to learn for myself. I think I have a picture somewhere, But it looked like the exhaust side of the piston crown had been lightly sand blasted. I run a bit on the loose side now :msp_sneaky:.
Just built a 372 with .028 squish and 195psi.....
Mastermind
Work Saw Specialist
I love this post Brad.....I mean freaking love it.
I've been talking with some two stroke gurus that play with bikes......those guys have me trying several new ideas.
I totally agree that at .016 no damage will occur on a 50cc saw. I'm of the opinion (just mine at this point) that gains can be had by opening the squish.
I recently built a 461 with over .030 that was my best yet.
bryanr2
Addicted to ArboristSite
Here's some excellent reading I found. It doesn't tell you what the squish should be, but helps you understand the theory of how it works. It's not just about clearance, compression, and detonation.
The second paragraph adresses compression. It addresses my concerns with too much compression actually hurting performance.
Squish clearance is another important factor in head design. This is the clearance between the piston and the squish band of the head when the piston is at TDC. It has a strong influence on turbulence in the combustion chamber, which directly affects how fast the mixture burns. The old way of setting squish was to just make sure the piston didn't hit the head when running, but this is not the way it should be done. Like most engineering criteria, there is a correct range that needs to be targeted. Too much clearance, and the burning is slow and power is low; too little, and burning is too fast and puts extreme loads on the motor, similar to what detonation does. A correct value of maximum squish velocity, or MSV, will ensure good power output within safe limits. I use a computer program to calculate these values and ensure they are in the correct range. The factors that influence the MSV are RPMs, stroke, rod length, squish band width, squish clearance, and compression ratios. Only when all these factors are considered and adjusted can a head be considered well designed.
Compression is also important to consider when designing ports. Years ago, porting was targeted just at timing numbers for a desired RPM; if you wanted 10,000rpm, you needed 190 degrees of exhaust duration. Then a more accurate method became known, using port time-area values. These calculations suggested a target area range for ports to feed the motor at the desired RPMs, and was a big improvement. Further developments, and what is used now, uses the time-area values, but targets desired power output levels based on BMEP, or brake mean effective pressure. What this means, is that the motor needs more port if the compression is higher, even if the RPM's are not changed. So if you raise the compression on an engine and don't consider the port design, you may actually lose RPM capability. Conversely, if you have an engine with a lot of exhaust, it may respond well to a higher compression...as long as all the rest of the variables remain safe.
The second paragraph adresses compression. It addresses my concerns with too much compression actually hurting performance.
Squish clearance is another important factor in head design. This is the clearance between the piston and the squish band of the head when the piston is at TDC. It has a strong influence on turbulence in the combustion chamber, which directly affects how fast the mixture burns. The old way of setting squish was to just make sure the piston didn't hit the head when running, but this is not the way it should be done. Like most engineering criteria, there is a correct range that needs to be targeted. Too much clearance, and the burning is slow and power is low; too little, and burning is too fast and puts extreme loads on the motor, similar to what detonation does. A correct value of maximum squish velocity, or MSV, will ensure good power output within safe limits. I use a computer program to calculate these values and ensure they are in the correct range. The factors that influence the MSV are RPMs, stroke, rod length, squish band width, squish clearance, and compression ratios. Only when all these factors are considered and adjusted can a head be considered well designed.
Compression is also important to consider when designing ports. Years ago, porting was targeted just at timing numbers for a desired RPM; if you wanted 10,000rpm, you needed 190 degrees of exhaust duration. Then a more accurate method became known, using port time-area values. These calculations suggested a target area range for ports to feed the motor at the desired RPMs, and was a big improvement. Further developments, and what is used now, uses the time-area values, but targets desired power output levels based on BMEP, or brake mean effective pressure. What this means, is that the motor needs more port if the compression is higher, even if the RPM's are not changed. So if you raise the compression on an engine and don't consider the port design, you may actually lose RPM capability. Conversely, if you have an engine with a lot of exhaust, it may respond well to a higher compression...as long as all the rest of the variables remain safe.
Mastermind
Work Saw Specialist
Going by the quoted exhaust timing numbers you are looking at info related to a piped engine.....
Good point Randy. It's really hard to find good 2-stroke documentation where a pipe is not involved.
Mastermind
Work Saw Specialist
Exactly......and that makes what we are doing a bit tougher to figure out. I'm just going off of trial and error on most things. Some of the things I have found that work great were by missing my mark and thinking "let's see how this works".
I've heard it stated that minimum squish is .0125 x stroke. I believe this would be a mechanical limit, as it does not take any of the other factors into account. However, I've seen this rule significantly broken on a very good running saw. It was not a saw that I built. I've got my personal minimum limits, but they're basically just what I'm confortable with from experience, rather than science.
