Compression ratios

[edisto]

Kind a like calculating windage and 500 yard trajectory when you still have not got a gun shooting to hit a pie plate a 100 yards.

Like I said, I have a tendency to get too far ahead of myself.

Shooting a plate at 100 yards doesn't seem very difficult or interesting...but probably is a helpful step.

 

[timberwolf]

Shooting a plate at 100 yards doesn't seem very difficult or interesting...but probably is a helpful step.

Exactly, you're talking 500 yard details with 100 yard shooting. You have cleaned up some ports now get at it and build a few good ported worksaws, never mind

REALLY, REALLY, REALLY

fast that comes later, no one builds a real good saw the first time.

Don't worry about going too far, you really have not gone anywhere at all yet, just lots of talk about the amazing jouney you're setting off on. Take each saw a little further, try to only change one thing at a time or two complmentry things at the most so you can learn what actually makes a difference and what does not.

 

[edisto]

Don't worry about going too far, you really have not gone anywhere at all yet, just lots of talk about the amazing jouney you're setting off on. Take each saw a little further, try to only change one thing at a time or two complmentry things at the most so you can learn what actually makes a difference and what does not.

Well, I only have one shot at it (I won't be adding any saws any time soon), so I want to get it right. That means I want to have a plan going in. Prior preparation prevents piss-poor performance...or lowers the probablility anyway.

Can't afford the P&C until I get the refund check, so I've been having a lot of fun thinking about it reading modding threads here and planning, and discussing.

I appreciate all of your help and advice, and I'll keep my enthusiasm and ideas to myself until I have some real work done.

 

[timberwolf]

You want to learn to port saws or just get a good running saw?

If you wan't to learn to port, tuition is going to set you back more than one P&C. They say it takes 10 years of continnual practice to get really good at anything, I figgure I might be half way there with porting. Problem is my attention span is no where near 10 years long.

Otherwise, learning asside, if you want a really good runner with low risk of making a dog you are best to send the saw to Treeslingr or someone who has already worked out the experimental stuff.

Cost of an OEM P&C is as much or more than a good port job.

 

[edisto]

You want to learn to port saws or just get a good running saw?

No...it might sound weird, but I wouldn't get any joy out of running a saw someone else modded. It's the journey not the desination? Some cheesy crap like that.

I know I'll never get good at porting, because I don't have the (life)time to devote to learning. I also don't harbor any expectation that I'll get it even close to right the first time, no matter how much I read. I just want to get as little wrong as possible.

The whole exercise is just a learning experience, and to the extent that I can, I'll do it in stages so I can learn as much as I can...but for the next little while, I won't be able to afford too many mistakes either.

My 61 is in great shape...no reason to mess with it at all. I'm going to put a 272 P&C on it, so I'll still have the original P&C to fall back on, but if I screw up the 272 P&C, I won't be getting another for a while.

I think I'll actually start with my 141, cause there's not much to lose there, and not much to gain either, so I'm less likely to get overambitious.

 

[timberwolf]

Cool, I hear you on the joy of doing it yourself.

But just seems you have many mutually exclusive quallifiers going.

Cheep but high quality
Fast but reliable
DIY right first time but low risk
Want to learn but can't afford mistakes
Want saw done full out with known results but don't want to send it to anyone...

Hey, fire me a line once you got the parts together, if I can help with some ideas.

 

[056kid]

In a combustion engine, compression is KING






















unless you are forcing the charge...



I do know that when my 250r is running on over 220lbs of comp, it has MORE top end ass than when i change the dome for about 194...

 

[edisto]

Cool, I hear you on the joy of doing it yourself.

But just seems you have many mutually exclusive quallifiers going.

Cheep but high quality
Fast but reliable
DIY right first time but low risk
Want to learn but can't afford mistakes
Want saw done full out with known results but don't want to send it to anyone...

Hey, fire me a line once you got the parts together, if I can help with some ideas.

Is that unreasonable?

I would say:

Best quality I can afford with room for improvement.
Noticeable improvement without requiring monthly replacement of parts.
Don't completely trash cylinder first time.
Learn from the mistakes of as many others as possible before making your own.
Plan your mods so you can learn from unexpected results.

Just because I want it all doesn't mean I'm unrealistic to think I can have it all.

Thanks for the offer, and for the help you've already given me.

 

[parrisw]

I'm worried about me going too far, not the reliability of what the guys here do. It took me two months to convince myself that there was no way to fit a useful expansion chamber to my saw and still get some work done with it.

I have a tendency to go wayyyy overboard.

There is no way you can get too much compression, unless you mill the cylinder and make a popup.

 

[edisto]

I have a tendency to go wayyyy overboard.

There is no way you can get too much compression, unless you mill the cylinder and make a popup.

So...once I finish porting the engine and modifying the exhaust, keeping the compression ratio the same (unless the squish is too high), I'll do some timed cuts (and monitor temperatures), and then perhaps try a pop-up to increase compression, and see what happens to the cut time.

Told you! I get carried away.

 

[timberwolf]

Even little pop ups like this won't put you over the line on most saws.

 

[timberwolf]

This one bumps compression up close to the limit of pump gas though.

 

[edisto]

This one bumps compression up close to the limit of pump gas though.

Ceramic too.

See...just when I start to think sensibly, you toss out some pretty pictures of your work and I start thinking again.

IF I do want higher compression, it would be better to plan to do so before the porting, because milling the cylinder will change the timing...or am I missing something again?

 

[timberwolf]

Nope, bang on. Thats where it gets hard to get it all right first time. Port timing also affects compression as it controls trapped volume and dropping the jug to gain compression affects port timing... circular reference.

Seriously though, just build a good saw, keep it simple, then make the next one better. Wild ideas, radical redesigns and significant departures from the original factory thinking usually result in poor running saws. Most times factory designs are pretty good, just build off what they got going. Look for the bottle necks and fix them one at a time.

 

[parrisw]

Even little pop ups like this won't put you over the line on most saws.

Are those what I think they are??? Hmm, I'm smiling!!

 

[edisto]

Seriously though, just build a good saw, keep it simple, then make the next one better. Wild ideas, radical redesigns and significant departures from the original factory thinking usually result in poor running saws. Most times factory designs are pretty good, just build off what they got going. Look for the bottle necks and fix them one at a time.

Roger that! I'll try to be reasonable...but it isn't easy. I can recognize sound advice, I just have a little trouble following it.

OK...I've been convinced that increasing compression won't reach a point where you will get power loss in a saw unless you do something drastic.

I am, however, still interested in the why. I'm not being argumentative, I just am interested in the theory just for theory's sake. I know it is annoying, but if it annoys you, just ignore me and I'll stop. Maybe...

What caught my attention in Bell's and Jennings' stuff was not reliability or safety issues, but actual power loss, or at least diminishing returns. The returns got less and less at higher rpms and greater scavenging efficiency.

A lot of this, it seems, could be a function of using a tuned pipe, because it is more efficient at higher rpms. TWolf said that you have to really try to make a saw knock...does it get easier with a pipe? Do you have to start worrying about detonation with an expansion chamber? (Don't worry...I'm not going there with a saw, just in my head)

The relationships between piston speed and flame propagation rates is key here to understanding some of the reasons a saw will beneficially tolerate higher compression ratios in respect to higher output.

Certain things like flame propagation rate are fixed, while distances and surface area to volume relationships vary greatly from one size engine to another.

So, for a constant air-fuel mixture, propagation rate is the same for any engine...but pressure has an influence too, does it not?

From what I've read, propagation rate will be affected by the "quality" of the mixture as well. Atomization of the fuel improves at higher rpm (compensated for to some extent by a richer mixture at low rpm) and, in some cases, scavenging efficiency improves with increasing rpm (up to a point), so do propagation rates to increase (up to a point) as rpm goes up?

Is the relationship between propagation rate and piston speed a separate issue from the relationship (if any) between propagation rate and rpm? If so, that would go a long way to explaining the difference between bikes and saws. With a relatively shorter stroke, the piston covers less distance for the same rpm, so piston speed would be slower...if propagation rate and piston speed were positively correlated, then the lower rate for the "squarer" engine would make it "safer".

Lastly...does the relatively short stroke of a saw engine compensate for some of the mechanical and cooling losses that occur at high compression in motorcycle engines?

Like I said, if all (or any) of my questions are annoying, just ignore them (and me). Discussing things like this is tremendous fun for me, but I can definitely see how it could be burdensome to someone who felt obligated to supply answers.

if you want to know "why" in the most precise mechanical way or at the molecular level then you will likely want to expand your library beyond Blair and Jennings lol!

Or, you could make some suggestions (as long as it isn't that >$200 book!), and get a temporary reprieve!

Thanks for all the help thus far.

 

[Meadow Beaver]

Maybe you guys should shoot some pie plates and see if you can hit them at 10 yards first.

 

[timberwolf]

Can you point to the pie plate?

 

[Crofter]

OK...I've been convinced that increasing compression won't reach a point where you will get power loss in a saw unless you do something drastic.

Caution applies to older and larger or low rpm engines

I am, however, still interested in the why. I'm not being argumentative, I just am interested in the theory just for theory's sake. I know it is annoying, but if it annoys you, just ignore me and I'll stop. Maybe...

What caught my attention in Bell's and Jennings' stuff was not reliability or safety issues, but actual power loss, or at least diminishing returns. The returns got less and less at higher rpms and greater scavenging efficiency.

As piston speed increases to nearer maximum flame propagation rates the return from increasing compression will reach diminishing returns, and at greater scavenging rates the optimum static compression ratios can be lower to achieve optimum dynamic compression. certainly going beyond the point of diminishing returns will cause just that; diminishing returns!

A lot of this, it seems, could be a function of using a tuned pipe, because it is more efficient at higher rpms. TWolf said that you have to really try to make a saw knock...does it get easier with a pipe? Do you have to start worrying about detonation with an expansion chamber? (Don't worry...I'm not going there with a saw, just in my head)


Yes, it definitely would get more probable on pipe!



So, for a constant air-fuel mixture, propagation rate is the same for any engine...but pressure has an influence too, does it not?

From what I've read, propagation rate will be affected by the "quality" of the mixture as well. Atomization of the fuel improves at higher rpm (compensated for to some extent by a richer mixture at low rpm) and, in some cases, scavenging efficiency improves with increasing rpm (up to a point), so do propagation rates to increase (up to a point) as rpm goes up?

Progagation rates do no go up in response to rpm but some of the causes affecting it could be in step or out of step with it. Do a search on factors affecting flame propagation rates in general. You could spend a weekend contemplating the implications

Is the relationship between propagation rate and piston speed a separate issue from the relationship (if any) between propagation rate and rpm? If so, that would go a long way to explaining the difference between bikes and saws. With a relatively shorter stroke, the piston covers less distance for the same rpm, so piston speed would be slower. Whoops, look at expansion ratio of combustion chamber volume per degree of crank rotation in both short and long stroke motors. Accelleration is greater in short stroke! Also if you shorten the stroke, to maintain the same displacement you have to increase bore.if propagation rate and piston speed were positively correlated,They are not then the lower rate for the "squarer" engine would make it "safer".

Lastly...does the relatively short stroke of a saw engine compensate for some of the mechanical and cooling losses that occur at high compression in motorcycle engines? Read TZ250's reply; basically that bikes are already tuned to have all the compression they can tolerate. Conditions that take the charge compression to just short of detonation conditions usually get the most BTUs out of it. Pumping losses are probably quite linear but combustion efficiency is not and there is a sharp jog in the curve at the onset of detonation.

Like I said, if all (or any) of my questions are annoying, just ignore them (and me). Discussing things like this is tremendous fun for me, but I can definitely see how it could be burdensome to someone who felt obligated to supply answers.





Or, you could make some suggestions (as long as it isn't that >$200 book!), and get a temporary reprieve!

Thanks for all the help thus far.

Well I dont know how much help it is. Some people would certainly not be interested in doing the amount of reading required to get around all the implications and seeming contradictions. Combustion engineering and design theory of heat engines would be some good searches. A whole lot of engine modding search hits, whether they be on bikes, sleds or saws will get you information that is pretty weak on explanations and lots simply BS.

 

[edisto]

Well I dont know how much help it is. Some people would certainly not be interested in doing the amount of reading required to get around all the implications and seeming contradictions. Combustion engineering and design theory of heat engines would be some good searches. A whole lot of engine modding search hits, whether they be on bikes, sleds or saws will get you information that is pretty weak on explanations and lots simply BS.

Thanks a bunch Frank! I'm just interested because I find it interesting...I'll see if I can find some more reading with which I can confuse myself further.

I'm sure I'll be back with more contradictions.