Originally Posted by
DHfan
The motorcycle engine idea, in theory, would probably work better now as 750cc 'bike engines now as opposed to then are very different. Probably lighter and certainly vastly more powerful.
The point being missed by people advocating this sort of thing (along with suggestions that many car engines of 200-300bhp would be cheap lightweight alternatives to "expensive, heavy and archaic" aero engines, is that a 200bhp car engine (or a 100bhp bike engine) is designed and stressed to develop this power only for very short periods, and its "max continuous" rating is usually less than half and often barely a third of this. Car and bike engines only use full power for accelerating from a standing start or climbing *very* steep hills - the rest of the time they chug along at 30-50bhp (or 10-20bhp for the bike engine). Aero engines are required to run at a much higher cruise power level so the mechanical and thermal stresses (and things like oil cooling) are very different.
For example the most powerful version of the 3.2litre Porsche Flugmotor was rated at 240bhp, whereas the same basic engine used in sports cars was offered in versions that produced over 380bhp. The geared cams and twin ignition systems needed for aero application weren't responsible for the power deficit.
At another extreme, the 1040ish BHP produced by one of last season's F1 turbo-hybrid engines (1.6litre V6, mandatory 16k rev limit) was indeed a continuous rating, with some circuits having over 60% of the lap at full power (and achieve better than 50% thermal efficiency while doing it, which is frankly mind-blowing). But these engines have a TBO of less than 15 hours, and as with some aero engines, many of them never reach the TBO...
PDR