Compression readings & compression ratio

[cliff.b]

Read this on another site and wondered if it is accurate information

 

"To calculate the compression ratio, try this formula: compression gauge reading divided by 14.7 (atmospheric pressure) i.e 140/ 14.7 = 9.5; 110 / 14.7 = 7.5"

Makes sense I suppose and just wondered as I tested mine today and all cylinders where reading very close to 150psi, which this calculation suggests means a CR of about 10.2:1

 

I know nothing about the history of the engine so it could be, I suppose, or maybe the gauge isn't accurate 🤔

 

What's a normal test pressure for a 1500 engine, measured warm?

[Pete Lewis]

that calculation is the general observation but doesnt take into account drag from ait filters, carb pistons  etc  so reality will be a bit lower 

always do the test with  good battery, all plugs out and throttles held open 

the 150 you have sounds very good , the one thing is they all should be within 5% of each cylinder 

its when you get 3 at 140 and one at 90 you need to get involved 

Pete

[cliff.b]

I thought it sounded a bit "simplistic" lol

I tested as you described and the gauge appeared to read 150/148/148/150

Only checked out of interest as I had never done it before 😊

[johny]

1 hour ago, cliff.b said:

"To calculate the compression ratio, try this formula: compression gauge reading divided by 14.7 (atmospheric pressure) i.e 140/ 14.7 = 9.5; 110 / 14.7 = 7.5"

Think this is very rough calc as you could have a very high CR but a low reading because the valves dont seal or the bore/piston rings are worn!

[cliff.b]

Aha, I suspected there may be a little more to it 🙄

Further reading uncovered a more detailed calculation.

"Gauges very, as does valve overlap. The actual formula for comparing pressures and CR is P = Po X CR^g where P is the achieved pressure, Po is the ambient pressure, CR is the Compression Ratio, and g is gamma, or the specific heat of the gas, for air 1.4 may be used. The ^ means the CR is raised to the gamma power. In practice this doesn't work anyway because of the aforementioned valve timing and the following relationships.

Since CR is derived from volumes, not pressure, CR is V1 / V2. Pressure Ratio PR is P2 / P1. Assuming an adiabatic compression, and air being a perfect gas, we can define the change in volume and pressure as follows:

P1V1^g = P2V2^g implies P2 / P1 = (V1 / V2)^g.

Because, in reality, the specific heat will change with temperature and significant deviations from adiabatic behavior will occur, there is no good way to relate CR to the compression readings"

 

So there you have it 🤔

[JohnD]

Compression gauges are cheap, uncalibrated tools. Plus the  technique of taking a reading isn't straightforward - engine warm or cold? throttle open.  Wide, wired open? how many engine turns?  And the atmospheric pressure, which is only 14.7psi on average  can vary from 13-16psi.     So their readings on different days, on different cars, by different people cannot be compared, and applying a "formula" as if it gives an absolute answer is nonsense.

The only and correct use of a compression gauge is as you have done, comparing the readings across the block.  That is a perfectly valid comparison.

JOhn

 

[cliff.b]

4 hours ago, JohnD said:

Compression gauges are cheap, uncalibrated tools. Plus the  technique of taking a reading isn't straightforward - engine warm or cold? throttle open.  Wide, wired open? how many engine turns?  And the atmospheric pressure, which is only 14.7psi on average  can vary from 13-16psi.     So their readings on different days, on different cars, by different people cannot be compared, and applying a "formula" as if it gives an absolute answer is nonsense.

The only and correct use of a compression gauge is as you have done, comparing the readings across the block.  That is a perfectly valid comparison.

JOhn

 

Yes, that is my understanding. Was just curious when someone appeared to be suggesting otherwise but clearly it is far more complicated and of no real importance anyway. 

 

[johny]

well compression ratio can get important if, like me, you hope to skim the head. Then I will have to work out how much to take off to get a little boost in ratio👍

[cliff.b]

4 hours ago, johny said:

well compression ratio can get important if, like me, you hope to skim the head. Then I will have to work out how much to take off to get a little boost in ratio👍

Yes, of course. I just meant it's not important that you can't directly relate a compression test reading to the CR. 

[JohnD]

Johny,

May I humbly suggest you read my article, " How to raise the compression ratio safely and effectively" at  https://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/#comment-99739 ?

John 

[SpitFire6]

So bar = CR. 
 

[JohnD]

1 bar = atmospheric pressure (approx) or 100 kPa.

[SpitFire6]

6 minutes ago, JohnD said:

1 bar = atmospheric pressure (approx) or 100 kPa.

1 Bar = 100kPa. 

[JohnD]

There's an echo in here.

[johny]

Seems to be a very empty bar😂

[SpitFire6]

Mixing a&g?