Interesting. Back when I got into biodiesel in the early days I was reading on sites like Journey To Forever that you needed to retard your timing 1-2 degrees for biodiesel, now personally I do not adjust timing between the two fuels, in fact I have been saying here that I like more advance with fuels similar to biodiesel because it has fewer BTU/unit than petroleum based diesel and a higher cetane number as you indicated. A well respected member here has made the observation that for same engines, European specs were always slightly more advanced than American specs and that was because they started trying to clean up diesel emissions, use ULSD fuel and Biodiesel blends years or decades earlier than we did here. When the ULSD came out we had a lot of people complain of low power and poor fuel economy. We had been advocating 8.5* BTDC at 2000 with the pulse method and thanks to our fellow OB and his and another members calculations we decided we could make up for some of the lost power with advancing the timing to 9.7 BTDC, those who have tried it seem to think it made a big difference. Now I personally use luminosity timing and of course that measure directly the explosion in the cylinder, so it is ATDC timing. I wanted to put mine right at 1* or maybe even at TDC at 1400, but after finally getting a matched set of injectors and stopping all the leaks 2.5 ATDC already clatters pretty hard, so for the time being I'm leaving it alone. My fuel economy has already improved some. I've been getting 12.7-13.8 mpg in town with mostly stop and go city driving which I'm not real proud of but I think is acceptable for the truck in my sig, bear in mind no OD and an X-cab. My thought is lower cetane/higher BTU=more retarded timing, higher cetane/lower btu=more timing. My understanding was that the suggestion to retard timing with bio was solely to offset the slightly higher NOx emissions which can be observed with using bio, but from a conservation standpoint, fuel economy, CO2 emissions and emissions harmful to human health must be accounted for, and for CO, CO2 and many other pollutants it seems like I've read retarded timing spews more of them out....at least with diesel, I suppose it COULD be different with bio but I don't see why. The NOx makes sense because NOx production is worst with high peak cylinder temps(or is it pressure? or both?) which come from advanced timing. However very good writeup, and you may be 100% right for all I know....would be interested to see what kind of fuel economy in terms of US MPG you get, but the fact that you have OD and probably do less city driving might be confounding variables.