Is engine efficiency proportional to the compression ratio?

Well, to get 100 hp from the 912 block instead of the original 80, making it a 912S, Rotax did increase the compression ratio - making 95 octane fuel unusable in the process. They did a few more tricks, too, though.

Well, the main difference between the 912 and the 912S is that the displacement was increased from 1.2l to 1.35l. The rest is probably the compression ratio, the carburetors, air intake system and whatnot but 11.5% more displacement should account for the majority of the extra power.

Why they didn't manage to squeeze out a single extra horse power out of their more-expensive, heavier, complex 912iS remains a mystery to me.

Power is torque times rpm times a constant. The power of an engine rises linearly with rpm, but only if the torque remains constant. Given that its harder to fill the cylinders as well in a shorter time the power rises with rpm for a while, then at some rpm the increase in power due to rpm is equal to the decrease due to reducing torque. That is the rpm for peak engine power.

Typically an engine's peak torque rpm is lower than the peak power rpm, which makes the power versus rpm curve relatively flat. If you try to extract more power from an engine like that, one way is to change the cam etc to shift the torque peak upward in rpm. That makes a fixed pitch propeller more problematic, particularly for an aircraft with a wide airspeed range.

Higher compression ratio has the advantage of not greatly affecting the 'power band' as well as making more power without increased fuel consumption.

Why they didn't manage to squeeze out a single extra horse power out of their more-expensive, heavier, complex 912iS remains a mystery to me.

Fuel injection does not help power output much for an engine that does not require much low rpm power, as with driving a propeller. In terms of power/weight, fuel injection is great if you need an engine that is smooth and torquey at low rpm as well as making power at full speed. If you can live with less tractable performance at low rpm, you can make as much peak power with carb (s).

@achim: you might wish to ponder why the turbo-charged 914 is based on the 912 , not on the 912s. Not sure I could put it into words myself, but I kind of feel the story behind that.

Also, I am not convinced that increasing the displacement was the main change between 912 and 912S - someone with more courage than me could calculate the individual effect of the various changes.

And if they did not try to use the injection advantage to get more power from the 912i, it seems obvious to me that it was obvious to them that better profit could be made in other domains, whether reliability or longevity or fuel economy or what else. Mind you, for the typical 450 kg two-seater (which must account for at least 70% of their output) 80 hp is largely sufficient; many outperform the ubiquitous C152 in every way.

Well, once you own 120% of a market like Rotax does and unless that market is growing like crazy, your focus as a business should be on addressing new markets. The P2006T is a dog and it could be a really great airplane with a few more HP. Others could follow.

Fuel injection does not help power output much for an engine that does not require much low rpm power, as with driving a propeller.

5800rpm is low? I didn't want to imply that fuel injection is a way to increase the engine power but there are always ways to squeeze out a bit more. Imagine an update to a Mercedes AMG or BMW M didn't produce more horse power with the same base engine, that would be a shame. Rotax should offer a bit of extra power in return for extra money and extra 6kg.

Mercedes AMG produce a 2.0l 4-cylinder engine that delivers an absolutely incredible 355hp. A lot is possible with the right brains and monies.

A lot is possible with the right brains and monies.

... and marketing / communications team.

Mercedes AMG produce a 2.0l 4-cylinder engine that delivers an absolutely incredible 355hp. A lot is possible with the right brains and monies.

With a large enough turbocharger an enough RPM you can get any kind of power from any kind of displacement. Not much brain required, just buckets of fuel and deep pockets to pay for frequent repairs... When Formula One allowed turbocharged engines during the 1980ies, BMW squeezed 1250hp from 1,5 litres. (and even for my little 2l Fiat there are chiptuning-kits on the market that claim to get up to 650hp out of the engine. Not that I have the slightest ambition to try that out...)

Exactly, Max. The Porsche venture may be a good illustration: I think nobody will argue the quality or quantity of (technical) brains at Porsche KG, and the budget must have been ample (rumoured around 75 million US$) yet it was a complete failure commercially. That engine produced some 200 HP from a block normally making 50-70. Check for more details.

With a large enough turbocharger an enough RPM you can get any kind of power from any kind of displacement. Not much brain required, just buckets of fuel and deep pockets to pay for frequent repairs.

There is a difference between a one-race formula 1 engine and a commercial product that meets Euro 6 requirements and is durable. Have a look at the engine, it's really an engineering masterpiece. If GA had any relevance and budgets, this is the kind of engine we'd be running today...

That engine produced some 200 HP from a block normally making 50-70.

Nonsense other than the brand name it has nothing to do with the 1950s designs. I happen to be at the airfield where most of the PFM3200 fleet is being serviced so I know a bit about the engine and its history. There was and is nothing wrong with it, it is very reliable but it was a commercial failure. This also had to due with it being released exactly at the time when GA went downhill. Remember that in 1986 Cessna stopped making piston airplanes. The technical problems were around the gearbox and resolved.