Dark smoke from exhaust

[Asian Panda]

I recently had the car on the Dyno and it seemed to have hit some kind of limiter at 5000rpm but when driving hard on the road the car is runs ok with no issues, I also noticed dark grey smoke coming from the exhaust. There was a C7 on just before me and his had little smoke.
I am aware that some smoke, is normal but I wasn't thinking a lot was???

I seem to getting different results from my searches regarding spark plugs, people saying OEM is fine for stage 2 or no you have to change to upgraded plugs. So I have a set of NGK R-7438-8 (4905) waiting to be fitted.
Car is fitted with milltek turbo back system and Celtic tuning stage 2 map.

So question is; could it be that the own plugs aren't suitable for a stage 2 and could this possibly be the cause for the dark smoke?

Any advice be great?

[HDPete]

I had similar under full acceleration. Turned out to be the faulty fuel pump seals. Replaced both c.£650 inc labour and full oil change.

Take off the oil cap and see if it smells of petrol. My oil was so diluted it stank of fuel.

[Asian Panda]

How much were the pumps?

I know that some smoke is to be expected but I was also told that when on a dyno the car doesn't get as much air unlike when you're on the road and that the smoke is nothing to be too concerned about. I've watched loads of YouTube videos of stage 2 cars accelerating and there is little to no smoke at all.

Will check the oil cap when it decides to stop raining or I'll leave it to Thursday when I get the plugs changed.

[Tadass910]

How many miles on your car?

[Asian Panda]

How many miles on your car?

A little over 41000.