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Discussion Starter · #1 ·
Does anyone know if there is a mathematical way of working out what the compression figure should be on a healthy engine?

I'm getting just less than 13bar on a 2 litre 175bhp engine - at least the handbook says it's 175bhp, though it's a 1989 FPT no cat & water cooled turbo.

Thanks in advance
 

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Figure seems about right (around 180 psi in old money). However it's not really the value of the figure that's critical, that will vary with ambient temperature etc. What you should look for is any significant difference between cylinders and the number of engine turns it takes to get there.

For example:

If only one cylinder is down it's likely to be a burnt valve.

If two adjacent cylinders are down to a similar value, it's likely to be the gasket between them.

Also if you think the piston rings are worn, put a small amount of oil in through the plug hole and re-test. If the rings / bores are worn you will get a significant increase in pressure.

I'm sure there are other things you can do or check but that's the limit of my experience. I stand to be corrected by the experts.
 

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Discussion Starter · #3 ·
Thanks for that. I've tried the oil trick and it made no significant difference.

The compressions are approx 13--12.5--8--13 and it's losing a little water so I guess it is just the gasket.

Many thanks
 

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Originally posted by Mark_A:
[qb]Sorry, may be a stupid question.. but how does one test compression?[/qb][/b]
Not a silly question if you don't know! to test compression you remove the spark plugs and then you screw a compression tester (a gauge similar to a tyre pressure gauge) into the plug hole and then turn the engine over for a few seconds and the compression reading is stored on the gauge
 

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I'm sure Saabman meant to say "then turn the engine over for a few seconds WITH THE THROTTLE FULLY OPEN and the compression reading is stored on the gauge".

There is also a static leakage test which can be done if you have the kit. This involves screwing an air line and gauge into the plug hole. Pressurise the cylinder to 100 psi (7 bar) or so with the piston at bottom dead centre, shut off the air supply, and plot pressure against time as the air leaks past the rings and valves.

However, DaveS has probably already found his problem...........
 
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