Chainsaw Repair
How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: Old Iron Logging on December 26, 2011, 12:21:03 pm
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How many pulls are needed for a true compression #?
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I will pull until the needle on the Gage stops.
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I have to go recheck the 5000 then.
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The old 5000 is holding 240lbs. Not bad for an old relic.
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:o :o Is it tweeked? I have a stock 5000 that gets 190lbs and my woods ported with pop up is at 210lbs.
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It has a 246 piston, if that is what you mean.
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Yeah. Thats what i was getting at. ;D
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Is there a point where is too high.
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Are you racing or working?
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Just playing at racing. Work saws are strictly MM only for me.
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I don't know .They say 180 on a work saw .I have a d-handle on an 038 mag that tells you anything ,no problems .
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In my opinion yes, I have pushed compression over 220psi with a 2 piece head and it was the wrong way to go. I think it hurts flow and remember you have to dead start the damn thing if you're gonna race it. And forget about doping the fuel......
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Art what did you do besides the 246 piston install on the 5000 to get that kind of compression? I have a stock one torn down I might put a 246 P in just to see how it is stock.
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I put the 246 piston in and milled the cyl base to get .019 squish. Then raised the exhaust. We race over the block so start is not an issue. Going to try to get the 152 Efco into the same range.
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You must have pretty low exhaust timing to get that kind of compression? I read 240psi in the other thread?
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how much compression really depends alot on how high your port timing is , follow me here , if you got a motor with 200 pounds of compression with a ext. port opening of we will say 85 degrees , it should live or be close to living in race forum on small wood, put that same compression on a motor with 92 degrees of opening on the ext. and she will get hot in just a couple seconds , If the port timing is quite high then 175 pounds is a safe number if running high test pump gas , This is on a low port motor to start with, your transfers also have quite abit to do with getting hot fast
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I have a 50 with a 490 top end with a 2 piece head running 220psi.
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holy crap, is that 50 a gas motor? pipe?
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Its a alky-nitro stock appearing(http://i1139.photobucket.com/albums/n553/snoopy013/Picture015.jpg)
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This 046 shows 140psi ??? ............
http://www.youtube.com/watch?v=VVUOgA4m76I
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I am thinking i am going to take some of that compression out to about 180-190
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I know I really like to see threads like this, I'm not much of an engine builder, I need all the help I can get!!
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TF that is understandable if you were porting the 046 for running on pipe.
That Dave Neiger built pump gas piped 084 didnt have any compression to speak of with it's stock cylinder head. De-comp was not needed.
It sure was a runner though on the PIPE, but on muffler it was no more then stock saw. Had nice big finger ports in it too.
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Like I said , it really depends on what motor your talking about cause some motors have higher porting to start with and it really depends on what fuel your running your saw on, the 2 transfer port motors run different than the 4 transfer ports motor to , crankcase pressure is different
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This was with a crap practice square chain. The hand filed chain I bought for this saw flew through the cuts compared to this video.
http://www.youtube.com/watch?v=HDpqWxQm2qQ
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That looks strong! have you ever had a compression guage on it? But, Like you said it's got a pipe......
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The cyl came off of a 50 Husky. I will have to make a degree wheel and get some #'s.
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I'm pretty sure I did way back when Aric, but cant remember. Put it this way it had the least psi of any saw I have ever pulled over stock to modified.
I'm going to combine the 2 threads guys.
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It seems i have a lot to learn yet. :-\ I always thought you wanted comp up around 220-240 for two piece heads and 200 for hot woods port saws.
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You and me both, it seems the more I learn the less I know :-\
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also remember the gauge itself makes a difference , I know on my gauge 200 psi will sound good but unless you are running real good fuel it will fall flat on its face in big wood . On say a 281 woodsport saw I can make 200 psi like nothing but in about the 3rd or 4 cut in 20 inch plus wood I can feel it start to slow down so I keep those saws at 175 psi and no trouble , BEMP is what you have to look at but also remember the smaller the bore the higher you can run your BEMP but also just because the computer says it will work with a 19.9 to 1 compression it lies to sometimes ;D, on gas if you are getting real close to the edge once you make a run in say 3 cuts in 8 by 8 , shut the motor off and open up the throttle and listen, you should be able to hear the gas boil in the crankcase and in most cases if you can hear this , you have went to far with either compression, ignition timing to advanced or if running a pipe the pipe is drawing to hard on the transfer ports
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Well if you get upwards of 16 to 1 ,that's diesel compression .Might not work out so well .
I'm not exactly certain where the edge is and am only guessing it to be about 12 to 1 .At that you'd probabley have to run Cam II or av gas .
What I'd be concerned about is the darned thing getting hot and auto ignition like a diesel at about 50 degress ATC and blowing the cylinder into a million pieces .Geeze that would be about like holding a live grenade . :o
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See Al this is where I have a big problem with compression ratios on 2 strokes , everyone talks about 14 to 1 or 16 to 1 but they are counting the whole cc's of the engine from BDC to TBC, to me this is pure BS cause you got ports in that area so that volume is totally wrong , reason , if you have 2 motors the same type but 1 motor has the ext. port timing at 100 degrees ATDC and the other has its 80 degrees ATDC , both engines would have the same compression ratio by using this method but the amount of compression each will make it totally different , To me the proper way is to only count the number of cc's above the ext. port to get your compression ratio
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I see your point Ed but PSI is still PSI no matter how you look at it .--Then you get a pipe back stuffing a charge and the whole thing goes haywire . ;)
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But, you can make lots of psi by stuffing a small space..... but have no flow. What would you rather have, a 7cc chamber at 240psi, or a 11cc chamber at 170psi? I don't know the anwser, there will be more air/fuel trapped in the bigger head, but more pressure in the small one.....
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Ed is refering to trapped volume in the static state . Then comes along a little thing called dynamic which deals with pumping efficiencies running at speed .
You can reach a state where it's possible to actually trap more volume that the cylinder holds .Now how you actually measure it or figure it into an equation I have no idea .
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Al that is why I go with PSI instead of Compression ratio , yes you still have to think cause you can still build a motor where it fights itself all the way instead of working like you think it should plus when you throw a pipe into this mess it changes everything again and what fuel you are using makes all the difference in the world
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What are signs off too much compression when running? Ed mentioned heat, is there anything else? The Partner in question can make multiple cuts with no heat issues.
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It usually starts with little pin holes burned in the piston top.
Sometimes the pin holes are around the plug area and other times they are around the squish more towards the exhaust.
You can often hear the noise. The saw will not sound right.
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660MAG, let me know when you get 10-15 tanks through the 026. I would like to check the PSI then.
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As many saws as i have, that may take a while unless I use it exclusively
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heat most times if your just racing will quit making power or just lay down in the cut , you can burn a piston but most times you never get that far before you know something is wrong and if only cutting something like 8 by 8 its over before you get to far , you can hear detonation in a motor quite easy, it sounds like something rattling in your carb or pinging in that area ,
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the very first saw I ever built was a 084 , I thought compression was the answer to all questions , now this is a long time ago when the very first 084's came out . I built a motor with 300 plus pounds of compression and tried to run it on pump gas and the strange thing was it would make 4 or 5 cuts before getting hot , problem was first sure it was faster than stock but not as fast as it ended up being by a long shot second non of our races around home were cold start and I thank God for that cause that thing was a bear to start , third I could not keep any thing tight on it , it would loosen the heads bolts all the time , So I learned from that first saw that compression is not the all answer I thought it was but also there is more than 1 way to skin a cat
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Good reading here. I was wondering where you fellers ended up. ;D
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Trout as far as a race saw I will take the 170 psi over the 240 psi every day of the week, month, year , I can run the saw on alcohol/nitro at 170 but thats about its limit before its start going backwards , you loose rpms and build to much heat on a good pipe saw , My goal when building a new racesaw is 165 to 175 , even the smaller bore saws like 272's donot like that much compression , 150 to 160 is good for them cause of all the rpms they turn
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Yeah but are you talking piped or muffler ? Piped you get that back stuffing ya know which will jack the traped comp ratio .That's how you guys can get away with those high exhaust ports . ;)
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Al if its a racesaw then its on alcohol /nitro mix so even on muffler I will take 170 psi over anything higher than that
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Can one of you guys tell me how volumetric efficency figures in here?
I know that a 3400 Poulan with 135# will run VERY well. As will a 372 with 160. Why do some saws need so much more compression to run well?
Mike
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Well again it gets confusing .130 PSI is static pressure meaning that's what registers when you rope it over .
Once it's running the rings seat outwards under pressure and that figure climbs .I have no way to measure it but have read and been told it could climb another 30 or so PSI.
Now another puzzle is that increase in pressure due fully to ring expansion or the fact that under speed it's trapping more volume than under static conditions ? I personally don't know for sure .
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crankcase volume plays a big part of this plus the number and size plus the degree of your transfer /ext ports
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I would be curious if somehow an engine could be spun up like an air compesser exactly how much it would raise over static .Of course with no source of ignition it would leave out the factor of a rapid expansion of the combustion to tighten the rings .
I know for example that a 2100 Huskey thick ring version like I have is one of the toughest SOB's to roll over ever made .The highly sought after thin ring piston pulls a lot easier and evidently then relies on ring expansion because that's the one they want if they are making a cookie cutter .
I can't imagine trying to drop start the version I have unless you weight about 300 pounds and have gorrilla arms .
Like Kevins big Sachs or those couple of 084s' you just shove down on the handle and pull the rope ,they fired right up .That damned Husky you have to have a foot in the handle and a good grip on the d-handle else it will try and suck you backwards through the recoil .
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I have both thick ring 2100 and thin ring , the only reason everyone wants the thin ring is the ring and piston does not break as easy as the thick ring but still the thin ring piston will break the ring land , on my computer program it has just about everything to do with this kind of stuff , it will even tell you if you punch in your porting numbers if your numbers make sense , if your ext or transfer ports are to high it will tell you so
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Wonder what ole Aric aka Troutfisher is up to now days. Good with the file and other chainsaw stuff.
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In the late 60s/70s one of the things we sometimes used a dykes ring on the race bikes, just wondering if that type of ring has ever been used on a saw? If so did it make any difference?
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The ERA of the Dykes has passed. Too much friction (during cylinder firing?) and doesn't last very long?
I remember 50 years ago that it was going to be the ring of the future.
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660 you are correct about the wear! On the bikes less than 250cc we were replacing the rings every other race. 250 and greater we were doing a ring job every race. However at that time the big Husqvarnas were using an aluminum jug with a spun cast iron liner rather than the chrome plated bore on the Japanese types. So wear on the cylinder liner was not so much of a problem.
You knew when the ring needed to be replaced because usually the bike could barely pull itself unless WFO!
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lots of good information.
think it was ehp(?) mentioned trapped compression ratio. donno if its a common practice to cc the chamber. could be worthwhile if someone knows what would be a good number to shoot for. i sure dont.
regards
-joe