850 Compression numbers

[dttrusko]

Correct me if I am wrong but I believe elevation and barometric pressure should have nothing to do with the compression results on the gauge. You are measuring gauge pressure and not absolute pressure here. If there is less air pressure pushing into the cylinder at higher elevation, there is also less external pressure exerted on the Bourdon tube of the compression tester. They cancel each other out and it is a simple multiplier of volumes of air.

You're measuring the pressure of the air compressed in the combustion chamber. Since air is less dense at elevation (lower mass for a given volume), the molecules of air aren't compressed as much, which equates to lower reading on your compression gauge. Same general idea as why NA engines make less power at elevation.

Gauge pressure is just zero-ing your reference pressure in this case since it isn't a closed system. If it were a closed system like hydraulics or water pressure, you would be right.

 

[Teth-Air]

Yes but equal and opposite pressure acts on the outside of the Bourdon tube of the pressure gauge. At low elevations the denser molecules are both entering the cylinder and are pressing on the exterior of the Bourdon tube. They cancel and the only reading you see on the gauge is the same at all altitudes.

 

[BBP]

Yes but equal and opposite pressure acts on the outside of the Bourdon tube of the pressure gauge. At low elevations the denser molecules are both entering the cylinder and are pressing on the exterior of the Bourdon tube. They cancel and the only reading you see on the gauge is the same at all altitudes.

That's not correct. I've tested it same day, same snap-on guage. lost 10 psi from 1000' - 4500' of elevation.

 

[Teth-Air]

That's not correct. I've tested it same day, same snap-on guage. lost 10 psi from 1000' - 4500' of elevation.

Not arguing that you have seen that but it could be other factors, If you think about it a bit different you may agree with me. If the gauge reads in absolute pressure it would start higher at a lower altitude and multiply as the piston hit TDC. The piston sucks in one cylinder volume and then reduces its volume. It would always compress down to the same volume and the gauge pressure compression tester always reads zero to start whether at sea level or 10,000 ft. It can be looked at a pressure multiplier or a volume divider. Why do you think the pressure reads zero when look at your gauge at sea level or if you are at 10,000 ft when there is clearly more air pressure pushing into the gauge at sea level? It is because the pressure external and internal to the gauge is canceling out. This happens when you actually do the test on you motor too, the altitude difference is cancelled out too. Now it is true that your motor does get less air at altitude but you will not see it on the compression tester.

 

[BBP]

Altitude Correction Factor Table
for Compression Testing
Expected compression tests are specified at sea level. Altitude has an effect on readings obtained and a correction factor must be applied to expected readings taken at other than sea level.

For example: Specified compression = 150psi, Altitude = 2000ft, (150 x .943 = 141.5psi expected@2000ft)

Altitude (ft)​	Correction Factor​	​	Altitude (ft)​	Correction Factor​
500​	.987​	​	3500​	.907​
1000​	.971​	​	4000​	.888​
1500​	.960​	​	4500​	.880​
2000​	.943​	​	5000​	.862​
2500​	.933​	​	5500​	.853​
3000​	.915​	​	6000​	.836​

 

[Letsride4]

You said correct if you are wrong. You are correct compression reading on a gauge is gauge pressure, absolute pressure less atmospheric pressure. When you add compression ratio into the equation all things are NOT equal.

Theoretical compression reading on a gauge is: Ambient (Absolute) pressure X compression ratio = absolute pressure in cylinder when compressed less absolute pressure = pressure on gauge reading

10 to 1 compression ratio - different absolute pressures

Sea Level - 14.696 absolute pressure
14.696 X 10 = 146.96 - 14.696 = Theoretical reading on your gauge 132.264 PSIG


10,000 ft elevation - common riding elevation in Colorado 10.1 absolute pressure
10.1 X 10 = 101.00 - 10.1 = Theoretical reading on your gauge 90.9 PSIG
That why we are about 30% down on power riding in Colorado when compared to sea level.

So theoretical difference on the gauge reading with 10 to 1 compression ratio between sea level and 10,000 ft is 41.361 PSI difference.

I don't think you can actually expect to see theoretical reading on your compression gauge. When testing compression you probably don't get 100% filling of the cylinder, you don't get 100% seal on piston rings to cylinder wall, if you have a compression tester with a hose between where you thread it into the head and where the gauge is, the volume in the hose changes the compression ratio. If that hose has some flex when pressurized that has an effect on the reading. There is always a tolerance factor on the accuracy of any gauge when new, and the gauges on automotive compression testers are not what I would consider high quality gauges.

 

[kiliki]

One thing I see missing among all the smart kids math is you have to have the motor up to operating temp before the test.
you don't ride a cold motor down the trail.

 

[TRS]

One more, decompression holes.

 

[Teth-Air]

You said correct if you are wrong. You are correct compression reading on a gauge is gauge pressure, absolute pressure less atmospheric pressure. When you add compression ratio into the equation all things are NOT equal.

Theoretical compression reading on a gauge is: Ambient (Absolute) pressure X compression ratio = absolute pressure in cylinder when compressed less absolute pressure = pressure on gauge reading

10 to 1 compression ratio - different absolute pressures

Sea Level - 14.696 absolute pressure
14.696 X 10 = 146.96 - 14.696 = Theoretical reading on your gauge 132.264 PSIG


10,000 ft elevation - common riding elevation in Colorado 10.1 absolute pressure
10.1 X 10 = 101.00 - 10.1 = Theoretical reading on your gauge 90.9 PSIG
That why we are about 30% down on power riding in Colorado when compared to sea level.

So theoretical difference on the gauge reading with 10 to 1 compression ratio between sea level and 10,000 ft is 41.361 PSI difference.

I don't think you can actually expect to see theoretical reading on your compression gauge. When testing compression you probably don't get 100% filling of the cylinder, you don't get 100% seal on piston rings to cylinder wall, if you have a compression tester with a hose between where you thread it into the head and where the gauge is, the volume in the hose changes the compression ratio. If that hose has some flex when pressurized that has an effect on the reading. There is always a tolerance factor on the accuracy of any gauge when new, and the gauges on automotive compression testers are not what I would consider high quality gauges.

Thanks for your answer, I agree with you here. I simplified my explanation to get my point across that gauge pressure really is not ideal to compare compression ratios at different altitudes. I do not believe you can just throw in a reliable multiplication factor to come up with reliable numbers. The design of the gauge simply does not account for barometric pressure. I do realize that if you were on the moon you would not get the same reading as on Earth.