Compression value difference between dry and wet test

[SalvadorP]

What's the acceptable variation between compression values between dry and wet tests?

This is not actually concerning my volvo, but I couldn't find the answer online and I feel being a general question it is as valid for our cars as for any other.

I came back to Portugal and found out my mum had been driving my Polo IV low on engine oil, don't know for how long. The oil light came on and off and I checked the level and it was just bellow minimum. I have done a compression test with good test results (220psi), but since the engine was low on oil I was more interested in knowing if the piston rings were bad.
Wet test showed an increase of compression of up to 16% (260psi).

Is 15% increase an acceptable value?
Also, my intuition tells me if I'm puting oil on top of a piston, I'm efefctively reducing the size of the chamber and therefore increasing the compression. I'm wondering if that could justify the 15% increase or at least some of those percentile points!? But maybe I'm being dumb and this makes no sense.

Again, sorry for making a post that it's not volvo centered, but I wanna do a wet test on my 940 too, so this information will come handy.

[buick boy]

The reason a wet compression test is useful is to determine whether a compression loss is caused by worn/damaged piston & rings , or some other cause such as worn/damaged valve seats or head gasket etc . The oil on a wet test has the effect of sealing the piston and compensating for any wear in the bore or piston rings . All pistons leak to a certain degree when dry so don't be alarmed at a slight difference between wet & dry tests , 10-15% would be acceptable. If you still get a low reading with a wet test , further investigation will be needed.
Hope this helps

[Laird Scooby]

What he said ^^^^^

The compression figures seem extraordinarily high though - 220psi dry and 260psi wet - what's the CR on that engine?

[SalvadorP]

Originally Posted by Laird Scooby

What he said ^^^^^

Yes, I knew that. I didn't do the wet test because I thought the reading was low (on the contrary), but because I was already in there, and the oil was low, so I wanted to check the rings.

The compression figures seem extraordinarily high though - 220psi dry and 260psi wet - what's the CR on that engine?

As far as I can gather is 10,3 :1, but they have a 6v and a 12v version, mine is the latter, but I'm not positive that's the correct value. They made like a million versions of these cars.

EDIT: upon searching a bit more, seems to be 10.5 actually. I found a few posts online of people with similar values above 200psi.

[Othen]

Originally Posted by SalvadorP

As far as I can gather is 10,3 :1, but they have a 6v and a 12v version, mine is the latter, but I'm not positive that's the correct value. They made like a million versions of these cars.

EDIT: upon searching a bit more, seems to be 10.5 actually. I found a few posts online of people with similar values above 200psi.

It will not be over 200PSI: 10.5 x one bar is about 150PSI. That doesn't really matter though, it just means your gauge isn't calibrated to reflect the real world. Only the difference between dry and wet readings matter here - and 15% difference is fine (you were correct about the oil taking up some of the compressed volume of course).

[SalvadorP]

Originally Posted by Othen

It will not be over 200PSI: 10.5 x one bar is about 150PSI. That doesn't really matter though, it just means your gauge isn't calibrated to reflect the real world. Only the difference between dry and wet readings matter here - and 15% difference is fine (you were correct about the oil taking up some of the compressed volume of course).

It's a cheap tester from amazon. Thanks for clarifying 15% isn't too much.

[Laird Scooby]

Originally Posted by Othen

It will not be over 200PSI: 10.5 x one bar is about 150PSI. That doesn't really matter though, it just means your gauge isn't calibrated to reflect the real world. Only the difference between dry and wet readings matter here - and 15% difference is fine (you were correct about the oil taking up some of the compressed volume of course).

Originally Posted by SalvadorP

It's a cheap tester from amazon. Thanks for clarifying 15% isn't too much.

Thanks for doing the maths Alan, seems Geoff Bezos sent an overly-optimistic tester out!

[360beast]

Originally Posted by Laird Scooby

Thanks for doing the maths Alan, seems Geoff Bezos sent an overly-optimistic tester out!

Let me get this straight Dave, are you saying that Amazon (purveyors of only the finest quality products) have somehow managed to stock a chinesium product that doesn't work properly? This can't be possible as Amazon only sell the finest products known to man!

[SalvadorP]

Originally Posted by 360beast

Let me get this straight Dave, are you saying that Amazon (purveyors of only the finest quality products) have somehow managed to stock a chinesium product that doesn't work properly? This can't be possible as Amazon only sell the finest products known to man!

My biggest issue is that I bought it while I was in spain and now I'm in portugal so I cannot return it for being defective.

I'm planing on testing it with compressed air to see how off it is. Because it seems to give consistent readings at least. So if I know how off it is, it's at least usable.

[Othen]

Originally Posted by SalvadorP

My biggest issue is that I bought it while I was in spain and now I'm in portugal so I cannot return it for being defective.

I'm planing on testing it with compressed air to see how off it is. Because it seems to give consistent readings at least. So if I know how off it is, it's at least usable.

I think it will be perfectly usable, most of the time only relative rather than absolute measurements are required. As us surveyors say it is precise but not accurate.

If you could find a reference source to calibrate it against (it would need to be something that maintained a constant pressure when tested, something like a fairly large pressure vessel, but only at around 100 PSI - not like a diving air bottle (200 bar = 3,000PSI)) then you could just mark the scale accordingly.

You already know that the reading it gave (over 200 PSI if I recall) probably equates to 150 PSI), so I would tend to accept that as the calibration done and scale things from there. Good enough for government work.

I don't think this issue is much the fault of that nice Mr Bezos, most non-calibrated gauges are probably much the same. Unless an instrument is something that needs sending off for calibration every year and comes back certificated then that will probably be the case.