Altitude and Compression Testing

[litefoot]

I found this table to adjust for altitude when performing compression tests:

Altitude Factor
1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860

It looks like a logarithmic curve starting at roughly 3% loss in pressure every 1000'. So at my elevation (5500'), I'd be looking at about a 15% loss. That explains why my as-new 361 only reads 148 psi. It's actually 170.

 

[Lakeside53]

good post... I was just looking for a simple table just like that!

 

[chowdozer]

I found this table to adjust for altitude when performing compression tests:

Altitude Factor
1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860

It looks like a logarithmic curve starting at roughly 3% loss in pressure every 1000'. So at my elevation (5500'), I'd be looking at about a 15% loss. That explains why my as-new 361 only reads 148 psi. It's actually 170.

You're really getting into this compression testing thing. It's good to get an understanding of what's going on.

 

[litefoot]

You're really getting into this compression testing thing. It's good to get an understanding of what's going on.

Sorry if I'm getting a little anal...I just want to understand.

 

[chowdozer]

Sorry if I'm getting a little anal...I just want to understand.

no reason to be sorry. learn everything you can, you only live once.

 

[breymeyerfam]

just went out and checked comp on my 361: 180lbs. im at pretty much sea level. and did the tread where this got brought up get deleted??

 

[Patrick62]

I was thinking too much

I always thought it was 3 1/2%, never the less it is somewhere in there!

So that means I have only a 22% power loss (I am at 8000 ft)...:jawdrop:

You don't miss anything you never had. I have never run a
saw at lower elevation.... I did lay rubber with a 4 banger
toyota once tho.... at sea level

-Pat

 

[mtngun]

All my saws test low at my 4500 foot altitude.

33cc Homelite = 120 psi = 137 psi corrected

Oly 980 = 140 psi = 160 psi corrected

Stihl 066 = 130 psi = 148 psi corrected

Now I don't feel inferior when I hear you guys talk about 160 - 170 psi compression.

 

[Thumper650]

An old thread brought back

This is a interesting thread so I thought I would bring it back instead of starting a new one.
I was a little bit concerned when the best compression test any of my chainsaws was 145#.
After some research I found a couple of charts that actual say the loss is greater then those posted in this thread.
From what I gather here at 3600 feet my loss is closer to .875
So my tests of 145# are really at sea level 166#.
Even with the chart above the compression would be 160# so no worries.

Here is the link that I found.

http://www.challengers101.com/Pressure.html

And the calculator within the link.

http://www.digitaldutch.com/atmoscalc/

 

[KRP]

Let's not forget that altitude is not the only variable. The individual test gauge is probably the greatest variable. Compression readings of different cylinders are best with the same gauge. This is why manufactures may give an acceptable range in repair manuals, but don't list this spec for the consumer. "My 50cc saw is better than your 50cc saw because it has higher compression." Use the gauge for comparing within your stock of saws, not with some guy using his own gauge, under different conditions, altitude, temperature, etc.

 

[Steve K]

Wow this is great info. I never could get the compression I wanted and didn't know how to figure out a percentage loss due to elevation. At 4200' I'm at a 15% loss. So a saw with 135psi compression is really about 155psi. I feel much smarter!

 

[WetGunPowder]

3% rule

We have always used the same 3% rule for tuning 2 cycle snowmobiles for elevation- 100HP sled at 10000ft has 70HP. I would think chainsaws would be the same

 

[maico490]

In the same vein do you guys at altitude get away with doing less work by saying "But that would have been 30% more wood at sea level"
The highest I've ever been was 10000ft in Switzerland. It was a day's work to smoke 5 cigarettes to my mind.

 

[litefoot]

In the same vein do you guys at altitude get away with doing less work by saying "But that would have been 30% more wood at sea level"
The highest I've ever been was 10000ft in Switzerland. It was a day's work to smoke 5 cigarettes to my mind.

LOL! Hey, I never thought of that! You just need to be skeptical of anyone in the Western US who says he's cutting wood above 12,000'.

 

[BobL]

Don't forget atmospheric pressure varies significantly over time at the same altitude.
If you measured compression on a day when it was 1030 mbar and got 150 psi and a couple of week of so later when it was 990 mbar thats a difference of 4%, and assuming nothing else had changed you would then read only 144 psi.

 

[farmer steve]

bump

 

[Sawyer man88]

So I'm at about 1400 ft above sea level and my ms460 is 155psi so really it's about 165 to 170psi