As of 15 years ago rotary's were a lot less durable than piston engines. About 100K miles was the most that could be expected before a rebuild. Not sure where the current state of the art lies, but I would be surprised if they have caught up to piston engines.

A wankel has a higher ratio of surface area in the combustion chamber relative to displacement, as compared to a piston engine. Hence there is greater heat loss than in a piston engine. This translates into lower efficiency. As a result, a rotary engine delivers lower fuel mileage than a piston engine of equal power. A quick comparison: the 2006 Corvette has 400HP and delivers MPG of 18/28. The smaller, lighter RX-8 with only 238HP delivers 18/24. Guess which ones larger, faster, and gets better fuel economy?

In my opinion the real attraction of the Wankel was to the designers and stylists. It is a short, compact engine that opens up new opportunities in the packaging, styling, and design of a vehicle.

Lastly, wankel's are wankers. They don't make much torque, so you end up with a car that will accelerate hard flat out, but is a gutless wonder at middle RPM. The current RX-8 makes 158ft-lbs of peak torque. For reference that's less than the current crop of 4-cylinder Accords, Camrys, and Altimas - not exactly paragons of power. Floorboard the throttle at 3K RPM, and essentially nothing will happen. The 158ft-lbs comes at 5500RPM, just above the point at which 1ft-lb equals 1HP (5252RPM). So at 5500RPM, the RX-8 is making perhaps 165HP. From there the power curve presumably climgs steeply to the power peak at 8500RPM. Basically what 'ya got is a car that's dead below 6000RPM.

Basically, the Wankel is the opposite of a diesel. The former has little power throughout the RPM range with a strong burst at the top end. A modern diesel has a flat torque curve with strong power anywhere throughout its more limited RPM range. Really, the diesel is a better engine for street driving - it's more efficient and it delivers instant, flexible thrust at any RPM.

Geez, that got long,

- JimY