Over 3 years it constantly returns 54.7 running about on BP but drops to 48-49 on supermarket everytime and the responsivenes goes

A 10% drop in economy is very significant. I'm sure BP would be delighted to hear your results. So delighted that they'll take a 1.6TDCI and ask an independant organisation to put it on a dyno and compare the BSFC and BTE on Ultimate-Chuck-Norris-Optimum fuel to supermarket swill-and publish the results. In fact you'd have thought they would have done that already...

The formulation of diesel, like petrol, varies throughout the year. The calorific value will also vary. Cetane number is poorly inderstood by the masses. Higher is not necessarily better. It will tend to give easier starting and quieter combustion, but more diffusion controlled combustion and greater soot formation. Much of the noise of a diesel isn't combustion 'knock' but simply the high pressure fuel pump and injectors-increasing the cetane rating won't alleviate that.

A low cetane rating will not decrease 'responsiveness' in itself-in fact it will raise EGTs and help spool the turbo faster.

Magnum mega fuels tend to perform better because the purchaser wants them to and subconsciously skews the 'experiment' to deliver the preconceived result.

It's even funnier when we get to spark ignition engines. Users start saying silly things like, 'you won't notice a difference in economy until you run 2-3 tankfuls through the engine' and the ECU 'learns' that it has different fuel in'. Or even better: "you won't notice an improvement in fuel economy in a low performance 1.2 but you will in a high performance 3.0 engine". Or how about: "My engine has a compression ratio of 11.24 whereas yours 'only' has a compression ratio of 9.86, therefore mine will benefit from high octane fuel and yours won't.'?

Riiiight. So an engine that is switched to run on lower octane fuel will sit there pinging away for 2-3 tankfuls before the ECU notices anything is amiss?

A 1.2 litre engine operating with the throttle almost wide open is far more prone to detonation than a big 3.0 loafing away with the throttle almost entirely closed.

Static compression means nothing. Factor in the inlet valve timing and you'll find most petrol engines fall somewhere into the 9s-regardless of advertised compression.

Virtually all of this obvious yet completely missed by readers of glossy car magazines.

Increased performance and economy could be achieved by reducing internal friction

Indeed. That's why most OEMs stipulate 5W oils nowadays. This avenue is obvious and has been thoroughly explored. I have no idea what 'reconditioning fluid' is. Internal wear in a properly adjusted engine should be minimal. If the rings or main bearings are worn, a cheap 20W-50 will act as a band-aid to "seal up the gaps" for a short while.

Interesting so what is the answer untrottled ?

I use BP ultimate in a TDCI engine.Not to save on fuel but a higher quality fuel for the engine on short runs.

If this is a waste of money I fill up with the standard diesel and go from there.

It would be interesting to compare the state of the turbocharger nozzle ring (particularly one with variable geometry) on an engine that had been run exclusively on "premium" fuels with one that had been run on Tesco's finest after say 75 000 miles.

Are any figures available for ash and Conradson Carbon Residue for road fuels?

Like Roly93's post above, over the years I have tried pretty much everything in my Passat PD130 and new Mondeo 2.0 TDCi, from Shell Diesel Extra, FuelSave, V-Power, BP Ultimate, Esso Energy, to supermarket fuel.

Also additives including Stanadyne, Millers and a super Cerium-based catalyst and ....... none made ANY measurable difference to performance or economy. Certainly no difference that couldn't be explained by ambient temps or driving mix.

Sure, I thought I felt a difference in performance or throttle response or smoothness, but the calculator and stopwatch said no.

Now I just use whichever fuel is cheapest (Shell or Tesco, if I had a 5p- off voucher) and give the car a weekly blast through the gears to clear the soot :)

I believe the cleanliness of a filling station's storage tanks and pumps (i.e. if there are contaminants or water in the fuel when it's stored) will make a bigger difference to a car's overall running than exactly which brand you choose.

Edited by craig-pd130 on 11/05/2011 at 11:34