Erik Werner Hansen, since Roger has not responded I will give it a shot.
I do NOT believe that Roger's statement ("if you have a ground adjustable prop on a 914 you lose all its benefits that a turbo can offer") is accurate. I believe it would be much more accurate to say "without a constant speed propeller, you cannot take advantage of all that the turbo can offer."
The basic problem is that the atmosphere thins out as you gain altitude, but the turbo charged engine does not lose power as you gain altitude. These two factors combined can set up a condition where you inadvertently overspeed the engine.
Let's look at an example. Suppose you have a normally aspirated engine like a 912 and you set it so that at 1000 ft MSL your prop turns 5750 rpm at wide open throttle (WOT). As you climb, the air becomes thinner and so it offers less resistance to the propeller. You might think that because of this the propeller will spin faster at the same WOT setting. But there is something else going on at the same time. As you gain altitude and the air thins, the engine is now being fed less of the fuel/air mixture that it runs on, and so the power drops off (2% - 3% per 1000 ft). The net effect is that even though the air is thinning, the propeller will turn more slowly because of this loss of power.
Now, a different example. Suppose you have a turbocharge engine like the 914 and you set it so that at 1000 ft MSL your prop turns 5750 rpm at WOT. As you climb, the air becomes thinner and so it offers less resistance to the propeller. This time the propeller DOES spin faster because the turbocharger compensates for the thinner air, and the engine continues to put out full power. Before long you reach an altitude where full throttle will result in a condition where the prop is turning faster than allowed (see Line Maintenance Manual 05-50-00 page 10 for ramifications of engine overspeed).
The third example is the 914 and a constant speed propeller. Now you can increase the pitch of the propeller and that will allow you to go to full throttle, at altitude, without overspeeding the engine.
The advantages of a 914 are: 1) more horsepower 2) horsepower is maintained at altitude. Both of these are true, regardless of what type of propeller you have. But with a ground adjustable prop, you will need to choose the pitch setting that works best for the type of flying you do. If you live in Florida you may never have a need to fly over 1000 - 1500 feet MSL, and you can adjust your prop so that it does not overspeed at that altitude, and you won't be losing much at all when you return to sea level. If you live in Alaska and regularly fly from Anchorage to Denali you will need to set your prop much coarser so that you don't overspeed the prop while touring the mountain at 13,000 MSL. When you return to Anchorage which is basically at sea level, full throttle will give you significantly lower rpm, which in turn will mean less power.
So if you have a 914 but don't have a constant speed propeller, you will need to adjust your prop pitch for the type of flying you do, and you might even need to set it differently for different missions. The important thing is that you do not exceed the rpm limits as specified by Rotax in the Line Maintenance Manual.
