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compression testing

H

HFJ

Well-known Member
a friend sent me a link to an article which states that the speed at which an engine turns over affects compression readings, and a weak battery can result in artificially low numbers. wanting a second opinion, i searched for other articles on the topic and found many that said the same thing.

this made sense to me at first - but then i thought about 6 volts versus 12. obviously, my N turns over a lot faster with 12 volts than it did with 6. would readings be artificially inflated in this situation?
 
Compression is only One of the indicators of Engine Condition..... But it can be a useful tool for the fella looking for a "reason" to overhaul his toy...

While compression IS necessary to function properly, it ain't the whole story.
 
philosophical musing about the relative merits of diagnostic procedures notwithstanding, if u think i'm looking for a reason to do it again, you're wrong. u also seem to think i know a lot less about this than i do.

i rebuilt my engine 2 years ago, and i did so without testing the compression. why? because, strangely, i didn't need to know the numbers to know it needed it. i'm not looking to find a reason to do it again, as u so quaintly put it. i am trying to understand something.
 

to put it another way...

if these people are right, why does no one ask the voltage of the tractor in question when discussing compression? are they right, but the difference is trivial? is there some other explanation?
 
Minimum acceptable compression readings (90psi) were provided by FoMoCo and assumes original configuration as built with 6 volt battery.

Low compression will make difficult starting that is why some can be pull started but won't crank with a battery.

Most often compression readings are to indicate the status of rings, head gasket or valves.

If trying to wring a little more life out a worn engine, converting from 6-12 volts was a quick fix for starting problems during growing or harvesting season.
 
(quoted from post at 06:59:36 04/28/18) Minimum acceptable compression readings (90psi) were provided by FoMoCo and assumes original configuration as built with 6 volt battery.

Low compression will make difficult starting that is why some can be pull started but won't crank with a battery.

Most often compression readings are to indicate the status of rings, head gasket or valves.

If trying to wring a little more life out a worn engine, converting from 6-12 volts was a quick fix for starting problems during growing or harvesting season.
Click to expand...

i understand all of that, but my question(s) remain unanswered.
 

let's say an engine turned over by a fully charged 6 volt battery shows 100 psi. if that same engine is then turned over by a fully charged 12 volt battery, will the results be significantly > 100 psi?

how about a 6 volt battery that's fully charged compared to the same battery that's been run down and turns the engine over slower. will there be a significant difference between compression readings between the two scenarios? if so, i find it odd that when people report low compression numbers, people don't ask "is your battery fully charged?"
 
(quoted from post at 11:09:36 04/28/18)
let's say an engine turned over by a fully charged 6 volt battery shows 100 psi. if that same engine is then turned over by a fully charged 12 volt battery, will the results be significantly > 100 psi?

how about a 6 volt battery that's fully charged compared to the same battery that's been run down and turns the engine over slower. will there be a significant difference between compression readings between the two scenarios? if so, i find it odd that when people report low compression numbers, people don't ask "is your battery fully charged?"
Click to expand...

Take your tractor and do compression test with 6 volts and them test the same engine with a 12 volt battery and see if you get any difference. do the test the same way on both, three complete revolutions of the crankshaft. let us know what you find.
I would do it but I do not have compression tester.

A test that is better is one that makes no difference of battery voltage, and that is a leak down test.
 
(quoted from post at 15:43:37 04/28/18)
(quoted from post at 11:09:36 04/28/18)
let's say an engine turned over by a fully charged 6 volt battery shows 100 psi. if that same engine is then turned over by a fully charged 12 volt battery, will the results be significantly > 100 psi?

how about a 6 volt battery that's fully charged compared to the same battery that's been run down and turns the engine over slower. will there be a significant difference between compression readings between the two scenarios? if so, i find it odd that when people report low compression numbers, people don't ask "is your battery fully charged?"
Click to expand...

Take your tractor and do compression test with 6 volts and them test the same engine with a 12 volt battery and see if you get any difference. do the test the same way on both, three complete revolutions of the crankshaft. let us know what you find.
I would do it but I do not have compression tester.

A test that is better is one that makes no difference of battery voltage, and that is a leak down test.
Click to expand...

i would love to. unfortunately, i don't think i have a 6 volt battery anymore. i'll look around, but i'm not optimistic.
 
There are many things that change the compression reading of an engine. Most don't open the butterfly on the carb, you can only compress what you can pull in. Most do it on a cold engine, a warm engine should read higher. Many do it on an engine that has been sitting for a long period of time without running, the rinngs and cylinder are dry which will definitely lower compression readings. Most have 1 compression gauge, can be variation between gauges. But most reasons for checking compression relate to identifying a bad cylinder in diagnosis. In this case you have 4 on the same engine at the same temp cranking the same speed using the same gauge.
 
Hfj,

If I was looking for the most accurate representation of compression to assess ring and valve conditions, I'd just as soon go with a slower cranking speed.

If the 12 volt spin boosts compression, then that faster turning over would just be overriding the leaking compression I would think.

T
 
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