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I am not finding any reason not to run at 90% power.

Is there any?

Sorry if this is a repeat post, I tried searching but the search doesn't like me

I have the Thielert 2.0 motors and as far as I am concerned, you can run them at 100% if you choose. There is obviously a fuel burn/range penalty for doing so but the engines are certified for 2400hrs at max power without any restrictions. I believe the Austro engines can only be run at a continuous max of 92%. Not sure the reason but any setting higher than this makes the G1000 turn red and yell at you

I should hasten to add that I personally cruise at between 80% - 90% power which gives good performance & range and leaves me a mental comfort margin that the engines are not being flogged to death.

In the past I have entered air races where you go to 100% 5 seconds before the green light and only throttle back 2 hours later when you cross the finish line. There has been no problem with this and the engines sit very happily with all parameters in the green.

Thanks. Very reassuring. If u factor in engine replacement costs, at least in the USA, it seems cheaper to run at 90%

From data I collected during one year and 170 flight hours with my CD-155 engines, I can conclude that engines run at 100% with no problems in green area summer and winter, climb and cruise. Obviously, running at high loads means high consumption which is compensated with lower maintenance cost due to less flight hours. However, running at 100% doesn't give you so much benefit in speed to justify higher consumption - 90% or 85% or 80% somehow seem to be more justifiable. Also, there's a factor of range and necessity to land (which adds some risks and possible landing fees) which can add time and costs that can be avoided by running at 75% or 70%. E.g. yesterday and tomorrow I'm flying 3 short distances (1 hour each) at 70% to avoid refueling because on final destination the fuel is at half price. Flying at 100% would require refueling before last flight to be sure about reaching alternate and IFR reserve.

Steve D wrote: ↑Thu Jun 13, 2019 12:21 pm... I believe the Austro engines can only be run at a continuous max of 92%. Not sure the reason...

168 hp * 92% = 155 hp
155 hp * 100% = 155 hp

Both engines output the same max continuous power. The Austro provides 8% extra for takeoffs and go-arounds.

ultraturtle wrote: ↑Fri Jun 14, 2019 2:42 pm 168 hp * 92% = 155 hp
155 hp * 100% = 155 hp

Both engines output the same max continuous power. The Austro provides 8% extra for takeoffs and go-arounds.

Yes, but the Austro has a higher "critical altitude" on its turbo (about 12,000 feet), so it's able to maintain 92% power up to about 16,000 feet whereas (if I recall correctly) the Continental will provide full power only up to about 6000 feet.

The result is that the Austro does much better when flying up high where it's possible to obtain a higher TAS (about an extra 3 knots for every 2000 feet of additional altitude in a DA42-VI).

CFIDave wrote: ↑Fri Jun 14, 2019 4:39 pm whereas (if I recall correctly) the Continental will provide full power only up to about 6000 feet.

It’s actually 8000 ft by POH but in reality it’s usually 8500 or sometimes 9000 ft.

Austro itself doesn’t do much better (actually it does the same on same FF); it’s better aerodynamics of -VI which brings more advantage over CD-155 powered DA42.

To the original question, consider my cost per mile at 75% vs max continuous using some very broad cost estimates:
When reserves for engine overhaul and replacement of gearbox oil, filters, fuel pumps, and injectors get factored in, the per mile cost of flying the two speeds are essentially the same. It is because those cost get spread over an additional 1,400 miles flying the faster speed for 100 hours. Factoring in airframe reserve would probably make the faster speed even more economical.

My flying time is very predictable, and usually totals very near 100 hour per year, so I like to bring it in for annual as close to 100 hours as possible just for the sake of convenience. If I find myself needing a little more miles at the midpoint of the year, I may pick the speed up a bit, but generally cruise at 75%. I will be owning this aircraft for a very long time, and will play the long game by not stressing the engines more than needed just because the book says I can.

For what its worth, the -IV AFM states "The engine manufacturer recommends a cruise power setting of 75 %."

To bore everyone once again with numbers, while I'm not sure which engine the OP references with the "90%" question, the Austro AE300 recommendation of 75% power equates to roughly 126 hp, which would be about 93% power on the CD-135, 81% power on the CD-155, and 70% power on the AE330. Pretty much all of the engines we are discussing here (other than the very few 1.7 liter engines still flying) are based on the same 2.0 liter Mercedes-Benz turbo diesel. Physics is physics, and wear is wear. It is why the the AE300 and AE330 differ pretty much only by software allowing for greater power, the price for the additional power being greater maintenance requirements.

All very very interesting, but according to the FAA, the OP has a Lycoming IO-360…