Compression test VS compression ratio

[downhillwolf]

Hi, just thinking about that and i would like to make a study around that if there are some connection and make a graphs with figures... Maybe something comes out and will help someone.
So please tell me your numbers of hot compression test pressure, theoretical or measured compression ratio and what cam are you using...

 

So i would like from you to share this info if you have measured it or you know exactly what numbers you have on the same 

I dont want to hear i think i have...
For my car:

1991ccm
8,37:1 CR

hot 165psi compression test pressure

stock cam

 

[williamggruff]

2.23L

10:1 CR

197 psi hot compression test

Schrick 292 cam

[PaulTWinterton]

Stock US 73tii

175psi hot compression test

[downhillwolf]

Stock US 73tii

175psi hot compression test

US tii had 9,3:1 right?


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[PaulTWinterton]

19 minutes ago, downhillwolf said:

US tii had 9,3:1 right?

 

9,0:1 I believe.  Some would say that the Canadian imported tii`s are 9,5:1.  Can`t say for sure.

[schwarz75]

Hi, just thinking about that and i would like to make a study around that if there are some connection and make a graphs with figures... Maybe something comes out and will help someone.
So please tell me your numbers of hot compression test pressure, theoretical or measured compression ratio and what cam are you using...

 

So i would like from you to share this info if you have measured it or you know exactly what numbers you have on the same 

I dont want to hear i think i have...
For my car:

1991ccm
8,37:1 CR

hot 165psi compression test pressure

stock cam

 

2152 cc
11.8:1 effective compression (12:1 forged pistons)
Schrick 316 degree / 11.1mm lift
Two Weber 45 DCOE
Low mileage Stage III Korman engine
Perfect across all cylinders: 158-159



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[downhillwolf]

Bump up!
Come on guys, we need your help.

[burndog]

I am still not sure what this experiment / results is supposed to provide? Total compression PSI = what? Plus, with as many variables as described above it seems the resulting number is meaningless except to itself. Please expound...

[avoirdupois]

1 hour ago, burndog said:

I am still not sure what this experiment / results is supposed to provide? Total compression PSI = what? Plus, with as many variables as described above it seems the resulting number is meaningless except to itself. Please expound...

 

Wondering the same thing.

 

This recent thread seems apropos:   compression test results

[burndog]

Exactly!


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[downhillwolf]

Want to make an excel file with as much measurments and make some guidelines that could help someone determine cr without getting engine apart or even determine how healthy it is... based on the values that can be measured and seen without engine apart...


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[vacca rabite]

When folks are measuring compression tests and compression ratio, not using an apples to apples measure is going to make a graph somewhat pointless.

 

Someone trusting a stock CR ratio set of pistons and quoting that CR is going to vary quite a lot from the guy measuring swept volume, head volume, etc.  Likewise the guy who has never taken the head off his engine will have no idea what may have been changed over the past 40 years.

 

Likewise on a compression test, engine being hot isn't really enough (though it is a good start.)  Compression tests should only be considered as accurate if the valves are known to be properly adjusted and the engine is hot and the throttle is wide open.  Otherwise, again, its not apples to apples.

 

You may want to consider leak down numbers as well.  Its the always forgotten but critical variable when you are trying to determine norms for a healthy engine, and doing a leakdown test will help determine where your issues are.

 

Zach

[downhillwolf]

2 hours ago, vacca rabite said:

When folks are measuring compression tests and compression ratio, not using an apples to apples measure is going to make a graph somewhat pointless.

 

Someone trusting a stock CR ratio set of pistons and quoting that CR is going to vary quite a lot from the guy measuring swept volume, head volume, etc.  Likewise the guy who has never taken the head off his engine will have no idea what may have been changed over the past 40 years.

 

Likewise on a compression test, engine being hot isn't really enough (though it is a good start.)  Compression tests should only be considered as accurate if the valves are known to be properly adjusted and the engine is hot and the throttle is wide open.  Otherwise, again, its not apples to apples.

 

You may want to consider leak down numbers as well.  Its the always forgotten but critical variable when you are trying to determine norms for a healthy engine, and doing a leakdown test will help determine where your issues are.

 

Zach

Cannot argue about what you are saying, because I agree.
There is error everywhere, most of compression testers give some % of error, even if you measure combustion chamber and piston there is amount of error. I belive noone here has Pharmacy equipment to be 0,001ml accurate.
But this might help someone that does not know what CR does he have in his car. Car is running well, leakdown test proved as valves did seal ok. He did make valve adjustments and know that he has #2 camshaft... If he want to know if engine is good for another Xkkm, he must do compression test to see if all 4 cylinders have same amount of pressure. If they have he can determine what CR does he have...
Considering that everyone that has done pressure test has valves that hold and have little to none leakdown.
Maybe if we got it far enough there will be a excel file in the Articles for all newbies that did not open the engine to get some guidelines what engine they have.

[vacca rabite]

5 hours ago, downhillwolf said:

Cannot argue about what you are saying, because I agree.
There is error everywhere, most of compression testers give some % of error, even if you measure combustion chamber and piston there is amount of error. I belive noone here has Pharmacy equipment to be 0,001ml accurate.
But this might help someone that does not know what CR does he have in his car. Car is running well, leakdown test proved as valves did seal ok. He did make valve adjustments and know that he has #2 camshaft... If he want to know if engine is good for another Xkkm, he must do compression test to see if all 4 cylinders have same amount of pressure. If they have he can determine what CR does he have...
Considering that everyone that has done pressure test has valves that hold and have little to none leakdown.
Maybe if we got it far enough there will be a excel file in the Articles for all newbies that did not open the engine to get some guidelines what engine they have.

Yeah, you are right.

And with enough data points you can normalize the error out. 

I was perhaps a bit dramatic to say it would be pointless without built in error correction. 

What I should have said was that without some controls there may not be a big enough sample size data group in order to tell where the optimal range is.

 

Zach 

[avoirdupois]

20 hours ago, downhillwolf said:

Want to make an excel file with as much measurments and make some guidelines that could help someone determine cr without getting engine apart or even determine how healthy it is... based on the values that can be measured and seen without engine apart...


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Compression test results offer a useful diagnostic tool regarding engine health, but it is not the most reliable or accurate means of determining engine compression ratios.  If you looked at the other linked thread regarding compression tests, one device provided some of the very information you are attempting to compile, although it used the word “approx[imate].”

 

Setting aside unknown variances between testing equipment, and different camshaft profiles, a relatively high reading, e.g., 190 psi,  might suggest a compression ratio of ~ 10.5:1.  However, in one such instance, some of that higher pressure was due to excessive carbon buildup on combustion chambers and piston crowns (probably resulting from extreme high mileage and leaded fuels).  Conversely, readings at the other end of the spectrum might be artificially low due to a slow cranking speed possibly caused by a weak battery or starter, or cold temperature.  Unless all measurements are made using the same test bed and similar conditions, "approximate" remains the operative phrase.

 

One more time.