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[Webby the Bear]

You know you love my questions 

Humour me...

Joe Blogs owns a 2.5 V6 Omega. The expected compression if he were to do a test would be around 175 to 220psi.

Joe gets 205 on each cylinder.

Joe is happy because all his cylinders are within spec and more importantly they're all the same with no drops across the cylinders.

QUESTION

Why such a huge gap between acceptable compression readings? This suggests to me that the dogs 'dangle berries' as left from factory shoulda been 220 and joe has lost 15 psi somewhere?

I know some of you will say that the figures don't mean anything and it's the comparison between the cylinders that's the key. But I can't believe that omega owner A has 220 on all cylinders and omega owner B has 175 on all cylinders will perform the same from a speed aspect.

And don't beat me up too much for my very unrealistic scenario. .. Just humour me 

[Andy B]

you're thinking too much! 

[Webby the Bear]

I think so Andy. But I was bored and this has taken my mind off turbos for two minutes 

[4x4]

[Andy B]

 

[Webby the Bear]

  fekkers 

[Entwood]

Simple, single word answer ....

Wear.

(a more elaborate answer is available on request ...   )

http://en.wikipedia.org/wiki/Wear

[Kevin Wood]

Also, the compression figure you achieve from a healthy engine depends on many factors.

Temperature
How fast the starter is cranking the engine
Exact cam timing
How wet the bores are
Vagaries of the compression tester you are using to measure it

Etc.

If things have worn or failed, then it's very unlikely to have affected all cylinders identically, hence the spread of readings between cylinders gives you more information than the absolute pressure.

[Webby the Bear]

Thanks guys for yuor input

So, i am right in that all cars SHOULD have come from factory with that top figure of the compression range (220 in this case) on all cylinders?

Thus the wear has cost Joe Bloggs 15psi on all cylinders that WILL equate to power loss! Even though technically on a 14 year old car he's actually doing really well. 

[05omegav6]

Me thinks Joe should have bought a 3.0/3.2

[Webby the Bear]

Me thinks Joe should have bought a 3.0/3.2

I did say humour me

Am i right or wrong

[05omegav6]

In principle yes, but a well maintained engine that is lightly used will be less worn than one which has been neglected and ragged

Also, manufacturing tolerances will be aiming for the ideal of 220 psi, but might allow for 10% either way...
So hypothetically, Joes engine might have only left the factory with 198 psi, or 240 psi 

[Webby the Bear]

In principle yes, but a well maintained engine that is lightly used will be less worn than one which has been neglected and ragged

Also, manufacturing tolerances will be aiming for the ideal of 220 psi, but might allow for 10% either way...
So hypothetically, Joes engine might have only left the factory with 198 psi, or 240 psi 

Spot on! Thanks Al.

[Webby the Bear]

...so just re-reading what you put Al......

Basically without knowing what the compression from factory was when it rolled off the line it's impossible to say whether the figure we get today has dropped.

Although, is it fair to say that after 14 years, even with it being driven sedately, it's likely that there is some wear and thus some affect on compression?

[Andy H]

The actual pressure achieved depends a lot on cranking speed and ambient temperature.

Do the test on a hot summer day with a perfect battery and you should see the higher figure, do it in November with a cold tired battery and an unworn engine may struggle to achieve the lower figure.

A new engine might be quite tight and only turn over slowly (so giving low figures).

A big part of the pressure rise is caused by the heating of the air as it is compressed - slow cranking or a cold cylinder allows the air to cool before it has achieved the peak pressure.

[Webby the Bear]

Thanks Andy,

i suppose thats why they say to do it on a hot engine anyway.

i assume then that the compression during engine running will be far greater then as thats obviously a lot faster than cranking speed.

but for testing purposes im just really trying to find out if joe's engine has actually worn even though he's got ''acceptable'' compression figures if ya gets me

is it reasonable to assume that, whether the engines been flogged or driven sedately that its lost power due to age and wear (as nige mentioned)

[05omegav6]

Was going to add that a gently driven, low mileage engine could be more worn if it is only used for infrequent short journeys rather than for regular long runs...

There are two many variables to get consistent, accurate readings on the same engine, let alone for comparing two engines. Hence Kevins point about looking for consistentcy across all the cylinders in an engine

So if one cylinder is way off, up or down, then this is more useful than specific pressures

[Webby the Bear]

Thanks guys

Out of interest.... how would you wear your engine more if driven slow and not up to temp etc? i understand the concept of blockinmg breathers etc.

is it cos the engine will be pushing more all the time with the heavy cold oil?

[Kevin Wood]

Yes, there is no "correct" figure, which is why you are given quite a wide acceptable range.

You also have to consider that, to make the compression figure worse, you need substantial blowby of the piston rings or valves that don't seal at all (or a failed head gasket).

Wear isn't going to make a difference until the engine is showing other signs of being worn out (i.e. excessive smoking and oil consumption, poor running, etc.).

On most modern cars, that doesn't happen due to wear alone, as the body has rotted away by the time there is a problem with engine wear.

[Webby the Bear]

Ok thanks kev,

So if ive absorbed the knowledge and got this correct....

As long as youre performing the compression test on a warm engine and youre within the manufacturers spec. for all cylinders and no cylinder is down on the other then this is definitively not a compression problem causing the power loss

[4x4]

Correct

[Webby the Bear]

Correct

We got there in the end

Didnt take too long