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Re: E60 535D - BMW Fastest Diesel 353 HP (!!!)
Well, ok, now, i know that the torque/power curve (it's basically the "same" thing) is important, but ultimatly I have to go with something I read when I started "knowing about" ICEs:
"It's better to make torque at high rpms than low rpms because you can advantage of gearing."
That 35d is amazing, yes, but it only revs to 5000rpms. The M5 gets 8000rpms.
This means that provided the gearing is set for the same max speed (this being, max speed in each gear), the M5 can have a 1.6x advantage from gearing (8000/5000). Let's say the 35d has a 1:1 ratio all the way to the wheels (fiction, it's never like this, but keep with me for now...).
AT the torque peak:
35d has 723Nm at the wheels. M5 would has 832Nm (520 * 1.6)
Outside the torque peak the values would be different, but the M5 would always have a more favourable gearing.
Two aditional things:
1. When you increase the rpm of a diesel it starts to be less like a diesel and closer to a gasoline engine. The fact is that diesel burns SLOWLY and thus it's hard to make high rpms because the fuel doesn't have enough time to burn (if you don't burn it, no power). Also, the high eficiency is reduced by the use of higher rpms because of frictional losses and because the p*v diagram get's further away from an ideal compression ignition cycle.
2. Although i never tested a 35d so can't say for sure, it probably suffers from the same as all diesels. 1st gear is useless! why? well, it's generally put there so you can start the car and let go of the clutch at 5km/h (auto equivalent is not to have the torque converter sliding for a lot of time from a dead stop). This means it makes way more wheel torque than the tires can handle. In fact, for a decently powered, manual turbodiesel, you can acelerate quickly (speed wise, not distance wise, and yes, there is a diference!) by starting from 2nd gear.
About speed wise vs distance wise, i think for now it's enough stuff, but if you want I'll write a post about it.