Well, not to burst anyone's bubble when it comes to the IO-520 vs IO-550 conversions, the biggest difference is, in fact, the RATING scheme used to certify the respective engines.
The old rating scheme that Continental used with the IO-520 required that the engine make plus or minus 5% of "Rated Power". So, every engine coming off the line, at least theoretically, had to make AT LEAST 285 horsepower.
Now comes the 550, which was certificated under a different requirement: That engine is rated at 300 horsepower, plus 5%, minus ZERO. In other words, every 550 must make at least 300 horsepower.
The Continental Tech Rep who explained this to me then pointed out that you simply cannot have an engine fail that criteria, because if that happens, the factory has to pull a number of engines, not just the one that failed.
So, if all your 300 hp engine has to do is make at least 285 on the stand.....you shoot for "about" 300. If the engine the QC guy pulls only pulls 295 on the dyno, no sweat.
BUT, now, with the minus zero criteria, if that engine pulls 299 on the dyno, that engine and a bunch of others may be rejected.
According to the Tech Rep, the way you deal with that is you build a 300 hp rated engine that actually consistently pulls 310. And, the upper air induction engines, like the N, which are rated for 310....those actually pull about 320, again to prevent failure in quality control.
He also noted that BTW, Lycoming has always applied that tighter rating approach, which I verified when I was at the Lycoming Piston Engine Service School.
Ever notice that Lycoming engines of a certain rated HP often seem to pull harder than most comparably rated Continentals?
I ran 206s on floats, with IO-520s, then one with a 550, and there was no comparison as noted above in the 185 comparison.
But, that's not only a more powerful engine, but a LOT more efficient propeller as well.
You can't legitimately compare two engines running significantly different propellers.
It is, after all, thrust that matters.
MTV