Hi could you tell me please what the compression should be for a bmw mini coopers works i have tested all four cylinders and am getting 8 bar for all four the reason i have tested them is on cylinder 3 the spark plug is sooty there isnt any power lose there is a slight hesatation on ecellaration in 1st gear i have replaced all 4 plugs and leads do you know what the compression should be and can you tell me what else could be causing a sooty plug on cylinder 3 please thank you