Effect of altitude on Compression?

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highdesert

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Jun 17, 2009
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My 850 reads 125 right, 120 left throttle full open with warmed up motor, five good kicks.

I am at 6000 feet altitude.

Anyone know what this might read at sea level?

warning: I do not make my own clutch cables.
 
highdesert said:
My 850 reads 125 right, 120 left throttle full open with warmed up motor, five good kicks.

I am at 6000 feet altitude.

Anyone know what this might read at sea level?

warning: I do not make my own clutch cables.
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Isn't 125psi enough?
 
I don't know if 125 is considered enough. I have read of other Commando owners reporting 150 psi.
I know 125 is not great, but I suspect my altitude is giving a little lower reading than if I was at sea level.
 
I thought the rule of thumb is that if the two cylinders were close (5-10%) then that was a good sign.
 
swooshdave said:
I thought the rule of thumb is that if the two cylinders were close (5-10%) then that was a good sign.
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yes this is correct - esp. since the difference between the readings on two different compression testers can be quite different
 
Air density decreases with Altitude therefore it would be reasonable to assume that a bike may run lower compression at 6000 feet (or 2000 metres roughly) as the air you are compressing is rarer or less dense. I just looked at a chart which appears to say that at 6000 feet you would have air that is only 80% as dense as Sea level.

My 850 Commando's vary but around 130Psi seems right and even 125 wouldn't worry me, my 750 on the other hand (Lumpier pistons and higher comp ratio) read 155 when I checked it a couple of months ago. At as near as damn to sea level.

Simple answer, it seems fine, if it aint broke don't fix it!
 
Hello Highdesert,
Air pressure at sea level is said to average 14.7 lbs./square inch, at 6000 feet its only slightly less. Although, I don't think it makes any difference because its not that less air is entering the cylinder its just that its under less pressure. Once the intake valve closes and piston begins the compression stroke its compressing the same amount of air, and the gauge is probably just compressing a spring which doesn't care if its at sea level or ten miles. If I am overlooking something and I am wrong I hope someone will point out my error
GB
 
Hmm, at sea level, 10-1 compression, 14.7 x 10 = 147psi. Maybe some heat as it enters the tester & could show a bit more.
120/125psi at 6000ft for a standard 850 seems high to me at 8.5-1 compression. Must be in good nick.
 
geo46er said:
Hello Highdesert,
Air pressure at sea level is said to average 14.7 lbs./square inch, at 6000 feet its only slightly less. Although, I don't think it makes any difference because its not that less air is entering the cylinder its just that its under less pressure. Once the intake valve closes and piston begins the compression stroke its compressing the same amount of air, and the gauge is probably just compressing a spring which doesn't care if its at sea level or ten miles. If I am overlooking something and I am wrong I hope someone will point out my error
GB
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Be glad to:

Sea level is 14.696
6000 ft is 11.78

I think that's a little more than slightly less.

There is less air entering the chamber so there is less to compress, thus less compression. At least that's my take on it.

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
 
i think the real question should be why did you check it in the first place? was it smoking? rough idle ? noisy top end?

if it was running fine, great- who cares what the compression test said, if not then you need to address it is - altitude - sea level or mt Everest - what ever is a bunch of crap if it was running fine
 
It seemed to me when I took my BSA B33 to Yosemite from Monterey in '63, the bike seemed to run better the higher in altitude I went, but then it was probably because I was getting farther away from the tourists.

Dave
69S project
 
DogT said:
It seemed to me when I took my BSA B33 to Yosemite from Monterey in '63, the bike seemed to run better the higher in altitude I went, but then it was probably because I was getting farther away from the tourists.

Dave
69S project
Click to expand...

Actually the bike was probably running too lean at sea level.
 
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