Chainsaw Repair
How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: aclarke on July 30, 2014, 04:36:33 pm
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Lots of people building "work saw" motors with 220+ psi compression by machining down the chamber and base. While this seems to be a common practice, at what point does the compression kill the power from the associated pumping losses?
On the flip side, many of the top 5 cube stock appearing race saws run 130-140 psi and make tremendous power on gasoline and alcohol.
Curious what everyone else's thoughts are to really high compressions and where it starts to become counter-productive?
Adam
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You cannot run that kind of compression for a full tank without over heating. So it is only good for cookie cutters
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I like around 170-180psi on my work saws and gtg saws in past. Ran just fine and held their own.
I have a stock porting echo 8000 right now with 195psi and no decomp.
Had a woods ported 166 with 190psi. Nothing to write home about IMO
I'll take good porting with 170-180psi, over super high 220psi plus and ? . But that is just me.
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Wayne told me he can pull his 5ci SA race saw over with 1 finger.
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I've used saws with 230 lbs of compression milling. Tuned richer than normal for sure. The last 046 cylinder had 205 and the husky 390 had 200. I could run all of them a whole tank with no overheating tuned to normalish rpms. I don't run work saws over about 13.7k.
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My Stihl 880 has 230 lbs and I have ran tanks of fuel through it without problems. Running it rich of course.
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I've machined a bunch of cylinders for folks (worksaw/Racesaw) over the years and generally believed that motors with can type mufflers did pretty well with higher compression. Recently, I've been fooling around with a husky 181 racesaw I built 3-4 years ago that ran well but didn't turn up as high as the port timing numbers might suggest. Re-cut the chamber to get about 150 psi static psi and it gained 1500 rpm. Makes me wonder.... lol. Adam
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It's hard to say without doing one cylinder on one saw and keep cutting the chamber and base and resetting the timing each time. If I had a lathe at home I would consider trying it.
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Yep, be a pain to test this!
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I thought Randy said something about testing early on when he got started that higher psi was his formula for work saws. Cant remember what all he said though or findings he had.
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That makes sense to a certain degree. Seems to me that it becomes counterproductive though after a certain point and it's always intrigued me how many racesaw motors have hardly any compression by design
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He found more torque with compression. I've got a couple jugs for my 046 I'm going to test when I get time. I had the chamber cut on one and the other is a D combustion chamber one. I may port both and see how the lower compression one does.
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Free spinning high rpm engines don't work well with too much compression......
Smaller engines like a 50cc work saw gain lots of torque with over 200psi.
Here's a graph of one of my MS261s compared to a stocker.....
(http://i1307.photobucket.com/albums/s582/Chad0723/Masterminds261cmvsstock261cm_zps7aecf5ff.jpg)
In my world race saws, and work saws are two completely different animals. In the past too many people tried porting a work saw like it was a race saw.....
I've tested bone stock Dolmars with over 200psi.......just saying.
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Makes sense, but we're talking an 3-4,000 rpm spread where a work saw performs well with say 200 plus psi and a racesaw does better with 140psi. Anyone know of any literature/studies on pumping losses at high compression/high rpm? would like to try and understand this more. Adam
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Adam, you might want to contact Joe (1manband)! I don't build race saws but nly work saws depending on size they run between 170-185 with 1194, 140-165 with gasket.
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I wonder if the fuel used in a race saw makes more dynamic compression than the regular gas in a worksaw? All this does seem really odd. In every instance more compression makes more power as long as the fuel doesn't break down under compression. It has to be because a single cylinder engine loses more to pumping losses.
Also something to consider is flywheel weight VS inertia. Do you guys run a heavier, or a lightened flywheel?
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I'm sure fuel combustion chamber design, etc plays a role. Just one of the many aspects of two stroke engines that keeps things interesting. Tim, had a long chat with Joe last week about port velocities, timing and CC design. Super knowledgeable and helpful!!
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Free spinning high rpm engines don't work well with too much compression......
Smaller engines like a 50cc work saw gain lots of torque with over 200psi.
Here's a graph of one of my MS261s compared to a stocker.....
(http://i1307.photobucket.com/albums/s582/Chad0723/Masterminds261cmvsstock261cm_zps7aecf5ff.jpg)
In my world race saws, and work saws are two completely different animals. In the past too many people tried porting a work saw like it was a race saw.....
I've tested bone stock Dolmars with over 200psi.......just saying.
backtracked some numbers to get some torque figures from that graph posted above. (more intrested in torque curves).
rpm/stock/ported: (lb-ft)
6K/2.107/2.472
6.5K/1.825/2.415
7K/2.238/2.507
7.5K/2.228/2.468
8K/2.340/2.500
8.5K/2.240/2.504
9K/2.241/2.534
9.5K/2.156/2.559
10K/1.769/2.557
10.5K/1.630/2.368
11K/1.451/2.304
11.5K/1.264/2.200
(did my best to decifer the 11K and 11.5K numbers on the graph)
stock sum avg TQ = 1.957 lb-ft
ported sum avg TQ = 2.449 lb-ft
20% avg increase.
still amazes me to see what an avg gain of about 1/4 lb-ft will do when its moved up the rpm band a few thou
worked out some bmep's to see a different view
stock = 3.97 bar (at peak)
ported = 4.34 bar (at peak)
+8.5%
compliment you on a nice increase that should give long reliable service.
regards
-joe
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I have no idea what you just said Joe.........but my 8th grade education gets tested a lot around you guys. lol
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.... torque is where its at, and always was, imo.
TQ= (HP x 5252)/ RPM
you moved the TQ peak up from the stock 7000 to 9000 range into the ported 7000 to 10500 range. the extra 1500 rpm or so, move up the ladder did the trick for the HP gains shown on the graph.
the bmep thing, just shows the stock peak TQ compared to the ported peak TQ using cylinder pressure. just find it easier for me to see the true gains this way.
HP graphs look nice, but imo, don't show the two most important measures of a motor. acceleration time and rpm recovery time (when a motor has come off peak TQ rpm).
i'm just a guy who does not know much of anything when it comes to saws, but have an incurable interest in how/why motors work. in my head, all my motors run on math. hahaha.
-joe
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Engine math is very cool......but sorta over my head.
All I've used is trail and error. Lots of videoed cuts to compare timing numbers and so forth. To get the gains I've found in most cases has taken more error than many would believe. I have some big crates with tested jugs that didn't impress. :(
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Well, don't let Joe fool you, he knows what he is doing and has an engineering background he uses to prove things he looks into. He IS the one that came up with the T/A program for chainsaws that I spoke to you about. You would have a ball talking to him! However, to Randy's point, Edison said 'invention is 1% inspiration and 99% perspiration'. I was once told that you could tell if a guy who ports 2-strokes was any good by the number of trashed jugs on his wall!
IMO everything an engine does or is capable of is based on math. There are some folks out there who have an inherent understanding of the math no matter if they they learned it in some sort of school or through trial and error. It is obvious to me that you are one of those folks. The fact that you were able to determine what the designers were after and improve on it proves it beyond doubt.
That is why I sent an extra jug with the Solo 665 in case one needs to end up in one of those crates!
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Potter?
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Subscribed. . ;D
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Well, don't let Joe fool you, he knows what he is doing and has an engineering background he uses to prove things he looks into. He IS the one that came up with the T/A program for chainsaws that I spoke to you about. You would have a ball talking to him! However, to Randy's point, Edison said 'invention is 1% inspiration and 99% perspiration'. I was once told that you could tell if a guy who ports 2-strokes was any good by the number of trashed jugs on his wall!
IMO everything an engine does or is capable of is based on math. There are some folks out there who have an inherent understanding of the math no matter if they they learned it in some sort of school or through trial and error. It is obvious to me that you are one of those folks. The fact that you were able to determine what the designers were after and improve on it proves it beyond doubt.
That is why I sent an extra jug with the Solo 665 in case one needs to end up in one of those crates!
Looks like I should learn to listen more. It might save me a few bucks in extra cylinders. :)
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I think you listen plenty good! So why do you think that the Solo 665s will be something special as quoted elsewhere?
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The design.....
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Cool, take your time, I'm in no hurry! It will be interesting to see if the timing numbers are close to the 7900/681 as you suspected. Would sure be nice if you could get the kind of gains you usually get like those that Joe commented on. If that is how it turns out, I am sure the rest of them will be headed your way.
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We would need a dyno test.......stop watches are so 1900 now. lol
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I suppose that we could do that if it will help you out.
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I think Chad and I have reached an agreement on his dyno. I'm hoping to have it in a dyno room someday......
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That would be slick! Are you gonna build one like Chad's? The room will be quite the challenge to build, what with all the remote operation and sound control stuff needed!
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I'm pretty sure I will be getting Chad's dyno. He wants to play with it a while longer......then we've talked about it moving to TN.
The room I build for a dyno room would be open on at least two sides........think porch with roof. :D
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Great. If you get his dyno, then ALL my saws need to go back to your place. Cause I want a Fancy Graph!! ;D
I'm looking forward to what your impression is on the 665 as well. I seem to have one of those that is yet to be messed with.
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I'm working on the last few saws that were backlogged. In just a couple more days I will be completely caught up for the first time in several years.
Then I hope to dig into the 665s.....
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At what point does the very high compression start degrading bearing life. If it is a work saw I think I would want to know that.
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Very good point Roger! ??? Unfortunately I have no idea, I suppose we would need to get the radial load specs on the bearings as a starting point. Mains, I believe would be the lesser problem, caged needles at the wrist pin would be my biggest worry or worse yet a twisted crank. But then again I have seen bent rods and the bearings suffered very little, messed up yes, but not catastrophic failures.
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I've run 32:1 since back in the 70's and never had a galled piston or bad rod or main bearing unless I bought the item that way.
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I use 32:1 with Belray H1R......and I've never seen a bearing failure. My 262XP is blowing 220psi, and has been used hard in 100 degree weather with nary a hiccup.
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That is certainly a good bit of info. I too run H1R @ 32:1. You think the 665s might be close to that 200 pound mark?
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On that size saw I shoot for 195 - 205.
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On that size saw I shoot for 195 - 205.
I have a feeling mine will be around 160-170...it'll be interesting to see the results.
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No one that I know of is in any big hurry so take your time. I think it makes more sense to come up with a good base set of measurements, and then develop a formula for porting the saw than to be in a big hurry to run the saw. I have a trashed 665 jug here and have been looking at it trying to determine in my feeble mind (since finger ports are beyond my experience), if finger porting would add to or diminish the torque the saw already produces or if it would ultimately be unnecessary. IIRC, Randy has done them on 262XP saws but I don't have a good understanding of what advantages they provide and in what situations they might have a positive effect.
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Hey Adam, what exhaust height would you be using?
On a 65cc engine I'll be looking for 102 - 103 atdc.