remapping for mpg?

[grainqey]

Originally Posted by fierceteapot

Don't diesels just slam home whatever fuel they need? Surely running lean would just mean less revs?

lol theyre a bit more sophisticated than that

[Jim314]

Originally Posted by grainqey

its quite easy to justify, at 10% throttle you will have a greater amount of power/torque, then before mapping so require less throttle and you'll inject a more exacting amount of fuel as it will be optamised

The original poster was talking about a turbo diesel. In that case in my simplistic notion of how these engines are controlled, I can imagine a remap raising the torque at a given rpm. Since power is torque times rpm times a constant, one could develop the same power at lower rpm. This could possibly lower some of the parasitic drag in the engine, and hence increase efficiency, which would raise mpg.

But for what my opinion is worth this could make only a negligible improvement in mpg. The same power would be made by injection of more fuel in each stroke (to increase torque), but lower rpm would mean fewer strokes per unit time. And, higher torque would increase the stresses on engine internals and transmission and would increase certain aspects of engine wear.

But, grainqey, your reasoning is unconvincing. To me it seems like you are just saying in a round about way that efficiency is increasing. You are just stating what is to be proved.

I think we all agree that remapping increases the maximum power output from the engine, but you seem to be saying that this would allow the engine to use less fuel when it is operating at some set power level which is a fraction of maximum power. This does not follow from anything you have offered.

It is as if you are implicitly making an analogy with a human improving his maximum strength as determined by the maximum weight he can lift. We know that if someone gets stronger he can 'more easily' lift a moderate weight and can do more repetitions of lifting a moderate weight before getting exhausted. So we imagine he is more efficient at lifting the moderate weight. There is no reason to think this is a correct analogy with changing the fuel map of an automobile piston engine.

[grainqey]

just look into remapping, i know a mapper personally and what you can do to increase power and efficiency is just set the ignition and fuelling correctly for a set grade of fuel, just by doing that you can increase the efficiency of the engine because it knows exactly how much fuel to use and with how much timing advance/ignition advance to apply, thats why sporty cars say to run on 98 ron or similar because they are mapped to run most efficiently on that grade or higher, you get a higher knock threshold so can advance the timing, you can map for power or if you wanted you could detune an engine and set it for economy, it's only like the 163 and 185 euro4 D5's theyre the exact same engine and ancillaries but with different maps

anyway if you need an even more precise answer then i can copy and paste if you want?
thanks
alex