Author Topic: How much compression is too much?  (Read 1275 times)

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Offline 660magnum

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Re: How much compression is too much?
« Reply #40 on: December 28, 2011, 08:58:06 pm »
As many saws as i have, that may take a while unless I use it exclusively
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Offline EHP

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Re: How much compression is too much?
« Reply #41 on: December 28, 2011, 09:13:58 pm »
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 ,

Offline EHP

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Re: How much compression is too much?
« Reply #42 on: December 28, 2011, 09:21:17 pm »
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 

Offline Mastermind

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Re: How much compression is too much?
« Reply #43 on: December 30, 2011, 11:22:46 am »
Good reading here. I was wondering where you fellers ended up.  ;D
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Offline EHP

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Re: How much compression is too much?
« Reply #44 on: December 30, 2011, 09:36:31 pm »
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

Offline Al Smith

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Re: How much compression is too much?
« Reply #45 on: December 30, 2011, 10:14:41 pm »
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 . ;)

Offline EHP

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Re: How much compression is too much?
« Reply #46 on: December 30, 2011, 10:41:48 pm »
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

Offline rms61moparman

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Re: How much compression is too much?
« Reply #47 on: December 31, 2011, 01:17:46 am »
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

Offline Al Smith

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Re: How much compression is too much?
« Reply #48 on: December 31, 2011, 06:25:59 am »
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 .

Offline EHP

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Re: How much compression is too much?
« Reply #49 on: December 31, 2011, 08:23:46 am »
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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