Chainsaw Porting Theory

[tree monkey]

@TreeMonkey[/USER], does your 026 have a full circle crank?

no

 

[wigglesworth]

What actually determines max engine RPM in a non-limited coil engine?

Is it power vs. parasitic drag? Overall port flow?

I've heard it has a lot to do with actual engine design, now I'm wondering if that design is (at least in part) case volume to displace the ratio.

I believe, and have read, that it's the engines overall design that limits the max rpm in an engine. I don't get all hung up on "WOT" max rpm. All I care about is what rpm it holds in the wood, with a good, sharp, self feeding chain.

I see way too many "my saw four strokes at 18k rpm", and then they drop it in the cut and it struggles to hold 10k. It's quite comical actually....

 

[drf256]

I believe, and have read, that it's the engines overall design that limits the max rpm in an engine. I don't get all hung up on "WOT" max rpm. All I care about is what rpm it holds in the wood, with a good, sharp, self feeding chain.

I see way too many "my saw four strokes at 18k rpm", and then they drop it in the cut and it struggles to hold 10k. It's quite comical actually....

I'm trying to get the whole theory Wiggs.

I built an MS260 like that with a overly large intake. It would 4 stroke at 16,200 but didn't have the cutting speed of another that liked 13,800. I get it.

 

[riverrat2]

I would say less case compression plays a part of letting the saw run "freely"

I'm not sure as to why but a larger intake has more throttle response and rpm. Less intake is where the torque is at.

Once again, it's all about the balance and what each saw responds well to, or what the operator wants to do.

I tend to disagree, about the less case compression, allowing a 2smoke to run more freely and limiting RPM, not being disrespectful,
As that is not what I've seen in The 2.5 L Mercury V6 Drag boat motors I build, it is our goal to build plenty of bottom compression/velocity and we often spin them 11K + on the big end of the track and still have lotz (buttloads) of acceleration/torque out 0f the hole

Just sayin

 

[smokey7]

I'm having a hard time with all of this. As most of my experience is with a pipe. I am enjoying this alot.

 

[Barneyrb]

I believe, and have read, that it's the engines overall design that limits the max rpm in an engine. I don't get all hung up on "WOT" max rpm. All I care about is what rpm it holds in the wood, with a good, sharp, self feeding chain.

I see way too many "my saw four strokes at 18k rpm", and then they drop it in the cut and it struggles to hold 10k. It's quite comical actually....

That ain't no joke there, I can think of several that fit that description.

 

[Moparmyway]

That ain't no joke there, I can think of several that fit that description.

several ..................... shoot, there is just about an entire you tube channel dedicated to them

 

[drf256]

several ..................... shoot, there is just about an entire you tube channel dedicated to them

LMAO

 

[David Young]

I'm having a hard time with all of this. As most of my experience is with a pipe. I am enjoying this alot.

guess its time to set down the pipe smokey.

this is great stuff.

I think every part of the engine can effect working rpm. they all have to work together.

on a rc plane site they listed a formula that came from walbro. it had venturi, target rpm, displacement and a constant that you would select.

the constant for most chainsaw was .65 but for the saws with higher peak power rpms like the 242 and 346 the constant was more like .75 think how those saws run.

but it is one piece of the pie. You have to design through the whole motor to make things work.

 

[Mastermind]

David.......I'd like to see that formula if you can find it.

EDIT......

Found lots of stuff.

http://www.walbro.com/?s=service+manual

 

[jmssaws]

Interesting information there

 

[David Young]

from rc universe.com


Formula for calculating carb size
Here is a formula I located for determining the correct carb venturi size or at least to get in the ball park.


D = K x Square Root(C x n)

where:
D=carb diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000 (be realist, dreaming only will make you try carbs far bigger than the correct, with results below your needs)

Here are a couple of examples

For a 46cc running @8000 rpm
46*8=368 (8 for 8000 rpm)

Square root of 368=19.18

19.18*.65=12.47
or
19.18*.9=17.62

So that says a carb between 12.47mm and 19.18mm should work, After plugging in some values on known combinations that work a constant of .7 seems real close so

19.18*.7=13.42mm

Which is a value that is real close to what comes on the Poulan 46cc saw that seems to work well.

For a Ryobi 31cc running @ 8000

31*8=248

Square root of 248=15.75

15.75*.7=11.02

So 11.02mm should work on the Ryobi, Ill have to try that and see what happens.

One more for Bob (av8tor1977) and his 16cc Echo

16*8.5 = 136 (8.5 for 8500 rpm, that little piston should rev a little bit)

Square root of 136= 11.66

11.66*.7=8.16

So 8.16mm would be the size here.

I hope this helps. We will all need to do some serious testing to see how good all this is

 

[drf256]

Very cool David.

I need to do some calculations for my 026's.

I think an 044 carb runs well on it, but I'm quite sure that its more area than is needed for 49cc.

 

[David Young]

Thanks Al,
I thought it was cool too. most stock saws are close to the .65 constant.

here is the part I don't know, there is some sliding scale and what does that mean? we can put a bigger carb on a 346 which has a big carb for a 3 cube already, they seem to run ok and pick up some power. but like I said we need the rest of the "engine" tuned to the same rpm.

 

[jmssaws]

This carb talk is fantastic fellas

 

[huskihl]

This carb talk is fantastic fellas

Yep

 

[XP_Slinger]

Awesome info!

 

[lwhaples]

This thread moves right along,trying to take it all in.

 

[idiotwithasaw]

I've been lost since the first post. Lol

Sent from my HTC One using Tapatalk

 

[darkimpulse]

This is going to sound stupid, but when you guys are finished porting do you polish the surface afterwards? If so do you just give it a light polish or go as far to make it a silky smooth surface?