Club 80-90 Forums

Howdy, tomorrow when the sun is at it's highest (some chance), i'm going to do a compression test on the van. Can anyone tell me the ball park figures that I ought to wish/hope/dream of? The engine starts on the button in a fraction of a second which absolutely delights me, so, i'm hoping for all 4 pistons to be relatively similar in figures, fingers crossed. Cheers.

What injun you will be asked!

Most of the petrol engines are quoted at 10 - 13 bar (145 - 188.5 PSI) DJ's a little higher at 11 - 14 bar CU's lower as their compression ratio is just 7.4 : 1.
1 bar (atmosphere at sea level) is approx 14.5psi so the above seems rather generous TBH.

Worked out this way.......the common 1.9 DG petrol has a compression ratio of 8.6: 1 so 8.6 X 14.5 = 124.7 PSI and that of course would be no cylinder leakage AT ALL!

I would have said 110 - 125 PSI but the important thing is that they are all similar (within 15% of eachother)

Martin

doh, was in such a hurry that i forgot the bleedin obvious. 1.9 DG.

can't wait till the moro, will be having a go once my brew is made. will report back later - please don't laugh at my results as i'm a sensitive type!!!!

Don't forget to hold the throttle wide open to get true readings.

CRAP. Just come in my results and guess what? Forgot to floor the throttle!!!! As promised (no sniggering at the back):
Test order = 3 4 2 1
No.1 = 140 psi,
No.2 = 120 psi,
No.3 = 138 psi,
No.4 = 153 psi.

This was after a 35 minute drive, the gauge was left on for 300 seconds to see if any pressure drop was observed - it was noticable on any of the cylinders. Obviously, by the time I tested No. 1 , the engine had cooled, not sure if this has any significant effect on the figures obtained.


Thoughts please.

what2d wrote: This was after a 35 minute drive, the gauge was left on for 300 seconds to see if any pressure drop was observed - it was noticable on any of the cylinders. Obviously, by the time I tested No. 1 , the engine had cooled, not sure if this has any significant effect on the figures obtained.


Thoughts please.

Thoughts are: like I said...You can't really get 153 PSI not with a compression ratio of 8.7: 1
Results are not too bad....but you really should have done them all within a few minutes for accuracy.
No need to leave the gauge connected as I'm sure you'll find a one way valve in the compression gauge so the reading won't go down anyway.

Martin

California Dreamin wrote:

what2d wrote: This was after a 35 minute drive, the gauge was left on for 300 seconds to see if any pressure drop was observed - it was noticable on any of the cylinders. Obviously, by the time I tested No. 1 , the engine had cooled, not sure if this has any significant effect on the figures obtained.


Thoughts please.

Thoughts are: like I said...You can't really get 153 PSI not with a compression ratio of 8.7: 1
Results are not too bad....but you really should have done them all within a few minutes for accuracy.
No need to leave the gauge connected as I'm sure you'll find a one way valve in the compression gauge so the reading won't go down anyway.

Martin

Hope you don't mind me jumping in - trying to understand compression and all that.

My Haynes says a DG should get 10 - 13 bars of pressure (145psi to 188.5psi), with a wear limit of 8 bar (116psi).

So, as I understand it, he got between 153psi (10.5bar) and 120psi (8.3bar). But I don't understand why the engine shouldn't be able to get 153psi?

Thanks

Feel free to jump on in james, the water is warm. A valid point that you have made.


Yet another DOH moment, the gauge does indeed have a non-return valve which explains why i was obtaining zero pressure loss after 300 seconds!!!!!! Will do the test tomorrow when the engine is stone cold and also when it is warm, with a warming of the engine for 5 minutes to bring the temp up for each cylinder test. Back to the drwaing board. It can't be all bad due to the fact it's an ace starter. Cheers.

jamesandtheopenroad wrote:

California Dreamin wrote:

what2d wrote: This was after a 35 minute drive, the gauge was left on for 300 seconds to see if any pressure drop was observed - it was noticable on any of the cylinders. Obviously, by the time I tested No. 1 , the engine had cooled, not sure if this has any significant effect on the figures obtained.


Thoughts please.

Thoughts are: like I said...You can't really get 153 PSI not with a compression ratio of 8.6: 1
Results are not too bad....but you really should have done them all within a few minutes for accuracy.
No need to leave the gauge connected as I'm sure you'll find a one way valve in the compression gauge so the reading won't go down anyway.

Martin

Hope you don't mind me jumping in - trying to understand compression and all that.

My Haynes says a DG should get 10 - 13 bars of pressure (145psi to 188.5psi), with a wear limit of 8 bar (116psi).

So, as I understand it, he got between 153psi (10.5bar) and 120psi (8.3bar). But I don't understand why the engine shouldn't be able to get 153psi?

Thanks

As you sit there reading this thread you are being subjected to atmospheric pressure...although that goes up & down a little, it is accepted that at sea level the mean pressure value is approximately 14.50 PSI (ie: 1 bar equaling 14.50 PSI)

OK...a DG has a compression ratio of 8.6 : 1 what this means is the charge of air/fuel in the cylinder of this normally aspirated engine is subjected to being squeezed into a space 8.6 times smaller than it's own volume.
As the 'charge' in the cylinder is at atmospheric pressure (14.5 PSI) that means the piston compressors the charge volume by 8.6 times...8.6 X 14.5 = around 125 PSI....that would be 100% volumetric efficiency (the most you could get)

Could be I've missed something...anyone else care to add corrections..

Martin

You're right, something is missing here, especially as all my figures (nearly) were above the calculated figure of 124 psi. HELP.

No mate....you have misunderstood me...your figures are pretty typical and there isn't anything wrong.
Compression gauges just aren't super accurate and neither is the method of getting the readings, they are only a guide, there are just so many variables.
Thats why I said the most important thing is to get even figures or at least between 10 & 15% of eachother.

Martin

Got this from Wikipedia:

"The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.[1]Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio."

With that in mind, aren't the compression ratio and compression pressure two different things?

Compression ratio being the change in volume in the cylinder between top/bottom of the stroke and the compression pressure being the pressure created in the cylinder as the piston moves from the bottom to the top of the stroke? Then any differences in the compression pressures of each cylinder is due to any leaks where pressure, and thus performance, is then lost? So a higher compression pressure gives better power and any marked difference between the compression pressure the four valves, gives a noticeable lumpy drive as the cam isn't being driven equally?

Again, just trying to understand and apologies if I'm confusing things.

jamesandtheopenroad wrote:With that in mind, aren't the compression ratio and compression pressure two different things?

Not unless Boyle's Law just got repealed.

P1 x V1 = P2 x V2
Pressure 1 x Volume 1 = Pressure 2 x Volume 2

P1 = 1 bar (atmospheric pressure)
V1 = 8.6 thingies
V2 = 1 thingy (the actual volumes don't matter, it's only their ratio that does matter)

1 x 8.6 = P2 x 1

therefore P2 = 8.6 bar

So a higher compression pressure gives better power

It certainly contributes to it, yes - the compressed mixture gives a bigger explosion, pushing the piston down the bore harder. Too high, and the mixture will ignite before the spark - pre-ignition, detonation or pinking. Too much of that, and the compression will reduce itself as a hole appears in the piston...

and any marked difference between the compression pressure the four valves, gives a noticeable lumpy drive as the cam isn't being driven equally?

ITYM "crank". As a worst case - the HT lead removed - then on a four-pot, with one cylinder failing to fire completely, you'll notice a loss of performance, as you're losing a quarter of your power AND compressing "dead" air to sap a bit more power - but even idle speed gives 7.5 "failure to fires" every second (to 22.5 fires). At 3,000rpm, it's 25 "misses" to 75 "fires". Certainly at higher revs, you'll not notice any lumpiness - the job of the flywheel is to damp that sort of thing out. At idle, you're more likely to notice it, but it's possibly more likely that you're noticing the exhaust note being a bit "odd".

Certainly, 20% or so drop in compression won't be felt or heard at all.