[mjh93sa]

I used to be a user of BP Ultimate diesel, but switched to Shell V-Power diesel shortly before the change to the E91 due to better local availability.

Both have a higher cetane rating, which is (sort of) the equivalent to petrol's octane rating. But whereas octane rating is the precentage of octane in heptane that will create the same level of pre-ignition, cetane is a measure of the precentage of cetane in napthalene that has the same ignition delay. Practically a higher cetane rating will ignite more quickly, allowing more time for combustion to occur and so allow more power to be gained from a given amount of fuel.

As far as V-Power and Ultimate go the car seems to run that little bit quieter and smoother, with a slight improvement in mpg and a bit more on the butt dyno. However, I wouldn't like to say if it offsets the price hike or not. With both I found the primary benefit to be a distinct reduction in the amount of soot coming out of the back end in the days of the non particulate filter equipped E46.

In the case of Shell V-Power diesel it is actually made in the reverse manner to that traditionally employed in refining. Rather than using fractional distillation to extract it from crude oil they start with natural gas and reform it back into a liquid. The practical upshot of this that you get less impurities and so a cleaner burning product. Obviously incomplete combustion will still cause soot, but it should mean a cleaner burning engine.