Originally Posted by
AdamFrisch
Big Pistons - I also fly geared engines, but the GTSIO are especially highly strung. I always quiz my old Commander mechanic (he knows serial numbers by heart after 30 years of only doing Commanders) if the old GTSIO-powered 685 would be something I should step up to at some point, fully knowing it's his least favourite of them all. Always gets him going. He says they rarely make TBO and often have to have some top work done before hitting 1000hrs. Sluggish take-off as well, even though they have all that power. They do perform like turbines up high, though.
I think the part of the recommendation in the manual you talk about is probably due to gearbox limitations rather than a cooling thing, no? The gearbox chatters and doesn't like to be pushed by air. I always try to fly mine with positive drive and have been cautioned to never pull to idle ever, except in the flare. It sometimes means she's little harder to slow down, but if you dump gear early, it's normally not a problem.
Unlike the geared lycomings which uses a planetary reduction gear the Continental GTSIO 520 has one just massive spur gear to provide the reduction in prop RPM. It is pretty unbustable, the problem with this engine is the cylinders. They are basically the same as the 285 hp I0 520. Pulling another 90 hp (Cessna 421) or worse another 115 hp (Commander 685) is asking a lot. High CHT's or shock cooling will just kill this engine. However the last 421 I flew was sold with 1400 hrs on both engines and both had all original cylinders. It will hold up just fine if you treat it right.