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[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