Gunner, I notice the higher torque off of idle myself. However, the accelerator pedal is not full throttle. At part throttle the faster burning of the biodiesel gives a better burn. It is when I am full throttle up a hill that I notice a significant decrease.

After writing the above I felt I should add a bit more. The above comment pertains to an ignition that is already too far advanced.

I don't know what the torque curve is on Gunner's vehicle. However, most engines, including diesels, have a torque curve of some descripton. The torque curve is based upon the speed of the engine and the resulting twisting force at the crankshaft.

When an engine is at peak torque, that is when it is volumetrically the most efficient. That is, the engine is getting the best cylinder filling. At low rpms the valve timing may have a detrimental effect because of valve overlap between the exhaust phase and the time after bottom dead centre that the intake valve closes. The wave dynamics in both the intake and exhaust system also play a large part in the volumentric efficiency at any given rpm.

When the volumentric efficency is the highest is usually when the ignition timing is set. I say usually as if you slightly advance the timing you can pick up a bit on each side of the peak with only a slight decrease in the peak torque.

So how does this relate to biodiesel? If the timing is too far advanced at peak torque, then you will pick up power (full throttle and part throttle) on each side of the torque peak. However, at peak volumetric efficiency you will be trying to compress a mixture that is rapidly trying to expand, the result is a loss of work or power.

If on the other hand the timing is too far retarded, then the loss of power either side of peak torque will be felt even more. The torque peak will also be down and tend to be more pronounced, that is you will tend to feel it when the engine has some power.

Biodiesel burns a bit quicker than petrol diesel, thus at peak torque the timing should be retarded a bit.

You can play some games with this timing gambit. You can leave the timing advanced, but you could reduce the amount of fuel that you pump into the cylinder (leaner mixture) to reduce the rise time. This slows down the flame front and thus the cylinder does not come up to peak pressure as fast. Of course, the engine will not have as much power, but you will increase you fuel mileage.

I set my timing for a mixture of biodiesel and petrol diesel. Thus, most of the time I am in the ball park. If I end up running straight biodiesel or petrol diesel, again the timing is at least close.

Forget about setting ignition timing by a factory manual. I recommend setting the timing by the seat of your pants method. See what feels the best.