Rotary Rev Limits
   by an anonymous contributor

   Expensive noises can come from overrevving a reciprocating engine, but not a rotary. If more revs equal more horsepower, then what stops a rotary from reaching astronomical rpm and producing more power?

   First, it's important to understand that the motion of a Wankel's rotors is not purely rotary -- they do spin, but in a quasi-elliptical path defined by large rotor-mounted ring gears that revolve around stationary gears mounted to the engine's side housings. It's this out-of-round action that, in turn, defines the characteristic pinched-oval shape of the rotor housing, known as a peritrochoid curve. This motion also drives the offset lobes of the eccentric shaft (analogous to a piston engine's crankshaft) through plain bearings in the rotors to produce "pure" rotary motion at the flywheel.

     So, according to Jim Tanner, general manager of rotary specialist Racing Beat, it is the flexing of the output shaft from the "non-rotary" components of rotor motion that ultimately limits rpm -- and can destroy the engine. Two things can happen when the shaft starts flexing: the rotors can start wobbling slightly and come into contact with the engine's cast-iron side housings; or the tips, or apexes, of the rotors can come into contact with the chrome-plated periphery of the rotor housings. (Normally, only the spring-loaded apex seals, the Wankel's equivalent of piston rings, touch that chrome plating.) On a modern RX-7's stock 13B engine, a brief stint at 10,500 rpm could spell the end (of course, a rev limiter intervenes just past the redline of 8000 rpm).

   For racing rotaries, steps can be taken for survival at higher revs. The rotors themselves weigh anywhere from 9.7 to 11.5 lb., depending on the year, and at least one pound of metal can be removed through judicious milling. Racing Beat is [at the time of this article] experimenting with a third, center-mounted main bearing that will considerably limit whipping of the eccentric shaft; in a stock 2-rotor Wankel, the shaft is supported by two bearings, one at either end of the engine. Hardened stationary gears are another precaution, as is securing the rotor gear with a large retaining ring that fits into a custom-machined groove. This technique prevents the gear from walking out of its recess and scuffing against the side housing, a phenomenon that can occur with the stock system that retains the gear with several steel roll pins.