Static and Mathematic Compression Ratio Q.

[nabilhpe]

Hello Guy;

I would like to ask you what is the difference between Static and Mathematical Compression Ratio.

I thought that when a car has a CR or about 8 or 8.5:1 the compression test results shows about 155psi on the gauge in each cylinder, last week end we tested a VW VR6 12v AFP engine code, and we found that the test results were about 185psi / cylinder, and that car has a 10.5:1 CR, then yesterday we did the test on a VW AGU engine 1.8T, and the results were about 205psi / cylinder. How come? I thought that car has a 9.5:1 CR and as such the compression vlaues should be lowe than those of the VR6 car.

I checked online today and it seems that 195psi - 205psi seems to be a logical number with the 1.8T engine.

So, what does really affect the values of the compression test, and is there a way to tell the CR of an engine by using the compression test kit?

Thank you so much
Nabil

[JonReynolds]

Compression test tells you the condition of the sealing of the cylinder, i.e. rings and valves. You could have a high CR of 15:1 with bad rings and get very low readings.

[Guy Croft]

Let me explain it this way:

1. Take two identical engines - the one with higher CR will give higher cranking compression on the gauge.
2. Take two engines which are identical but one has different cams. The engine with longer duration inlet cam will give lower cranking compression because more air is lost (forced back out of the inlet valve) on the compression stroke after bottom dead centre.

As Jon says, ring and valve sealing condition have an effect on the cranking compression, state of bore lubricant, inlet/throttle loss too, also cranking speed and engine temperature. A turbocharger will generate a loss too over a normnally aspirated one, a supercharged one will not. Carbon on the valves can show no pressure at all but a quick blast for a mile or so will bring the cranking compressions back up. Comparisons engine-to-engine are thus pretty meaningless.

An engine that has never been run will have fractionally lower readings than one that has been run under load for 1/2hr or so, after that the reading will not really go any higher although it does take longer than that for the rings and bores to fully bed-in and for the oil consumption to stabilise.

Compression tests are a handy tool for comparing cylinder-to-cylinder condition (less than 10% variance should be the aim) and overall engine state, no more than that. Wetting the bores with oil thru the plug holes is a great way of determining whether the rings or valves are at fault if the readings dry are low. How long it takes to achieve the highest reading on cranking is a good determination of valve sealing condition too. A good engine will fly up to nearly max on the first stroke and reach peak within 3 or 4 strokes.

GC

[sumplug]

I assume compression tests are/were done on a hot engine to get a more accurate reading.?

Andy.

[nabilhpe]

Thank you for the replies,so basically from what GC said, compresison test has nothing to do with the actual CR of an engine, and an 8:1 CR engine might have higher presure values than a 10.5:1 CR engine with a set of higher profile cams.

But I am still wondering how come the 1.8T would give +200psi pressure / cylinder, I am not getting that, its a 1.8L 20v turbo engine.

Sumplug: yes both engines were hot when tetsed.

Thank you
Nabil