In the comments section of Erin's post on the diesel motorcycle, I mentioned that I didn't think it was a great idea, despite my love for both diesels and motorcycles. A couple of readers challenged me on this, and my reasoning has to do with the power-to-weight ratio of a diesel engine. Frankly, it's usually not all that hot; Mercedes' newest CDI engine barely achieves a power-to-weight ratio of 0.5 HP/lb, while GM's LS7 pushrod V8 produces about 1 HP/lb and modern motorcycle engines manage something maybe around the 1.5 HP/lb mark. Not a fair comparison, you say? Fair enough - how about a look at an aviation diesel? DeltaHawk's two-stroke turbodiesel is 327 lbs and produces 200 HP. That's an impressive 0.625 HP/lb (with a future model said to be in the 1 HP/lb range) - still not enough to get me excited about oil-burners for sportbike applications, but such a powerplant might indeed be at home in a big touring bike where the performance/economy compromise skews in the opposite direction, and where the longevity of a diesel has a better chance of being exploited. And could such an engine also be appropriate for small cars? I don't see a reason why not.