C2 3.4l Compression Test ....

I've just had my C2 compression tested .. and thankfully it showed only slight variation between cylinders, nothing significant. Just had me thinking ... what readings should you acheive from a 'good' engine is there a guide for permissable variation between cylinders ? Many thanks, Peter.

Hi Peter. I'm not sure about a compression test, but can give you an idea of what the figures should be for a cylinder leakage test, as this is what most indies do to gauge the state of the engine. I am not exactly sure what the leagake percentages relate to (I need to find out more), but they usually reckon a healthy engine will give up to 5% leakage past rings and valves on each cylinder. Once you get to 10%, then things are getting worn. Using this method, they can tell if the leakage is from blow-by of the piston rings, or from leakage across the valve seats. I once very nearly bought a 3.2 Carrera - until I had the leakage test done. They varied between 4% and 20%. Hope this helps.

Thanks Richard. I'd assumed that if my compression results looked OK, then general cylinder/bore condition must be reasonable - is that not necessarily the case? As one of my spark plugs is darker than the others (noticed at recent service) I was originally concerned it was oil entering the combustion chamber, but following the compression results, felt it was more likely to be a fuel mixture issue/injector. /Peter

If the compression tests were OK I am sure you have nothing to worry about. The advantage of the cylinder leakage test is that they can determine wheter the leakage is past the rings or past the valve seats. I think you are correct. Different colour plugs could also be a fuelling issue, possibly a worn injector giving an odd spray pattern and poorer combustion on that cylinder.