# Rotax 914 power setting question

#### ckurz7000

##### Senior Member
As many already know, I am flying an AC20 ArrowCopter with a Rotax 914 and a constant speed prop. Back when I did the test flights for the certification process, I did an exhaustive suite of test flights on cruise performance. Just recently I revisited the results and a few questions popped up.

Here is the fuel efficiency calculated from fuel flow and IAS for a range of power settings.

The way to read this graph is to pick a MAP (e.g. 30 inHg) and an RPM value (e.g. 5200 rpm). Near the point, where the vertical line from 30 inHg and the horizontal line from 5200 rpm cross, is a red line labelled "14.6". This means that at a power setting of 30"/5200 you need 14.6 liters of fuel for every 100 km travelled (corresponding to 17 mpg, in imperial units).

Rotax, in their engine manual for the 914, recommend only three power settings. They are marked as the blue dots in the graph and correspond to 35"/5500, 31"/5000 and 29"/4800. It is probably safe to infer that any power setting on the blue line connecting those three dots will also be OK according to Rotax.

As you can see from the chart, not surprisingly, the highest fuel economy is achieved at high MAP and low RPM. Ideally, I would want to cruise at 35"/5000 and would only use 13.6 liters per 100 km. At which point I would be zipping along at 175 km/h with a range (including 30 min reserve) of 455 km.

The problem is that, according to Rotax I would be overboosting the engine. However, I have received various indications from knowledgable people that Rotax is using a veeery conservative margine of error for the recommended power settings.

My question, therefore, is this: does anybody know how far into the regime of high MAP and low RPM the Rotax 914 can safely be flown without damaging the engine in the long run? And how would I know that I am getting close to pre-detonation ("knocking")? Would that show up on the EGT gauges?

Thanks for any comments, -- Chris.

P.S.: For completenes' sake I attached the other three contour plots which show fuel flow, range and endurance as a function of power setting.
All values are quoted at the MTOW of 560 kg and a density altitude of 500 feet.

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#### Smack

##### Re-member?
different animals

different animals

Chris,
The two events are either "pre-ignition" or "detonation".
They are different, separate, and cannot be combined.
Mike Busch explains it way better than I can; see his article here:
https://www.savvyanalysis.com/articles/detonation-and-pre-ignition

I have a Rotax 912 with an electric pitch adjustable prop on a Kitfox (airplane). The various combinations of RPM and pitch provide a lot of interest to me. I typically fly at 4800 RPM, but I haven't noted manifold pressure (normally aspirated). 4800 RPM gives me an indicated fuel flow < 4 gph which I consider pretty good economy. I've been running my 912 like this for 300+ hrs with no sign of trouble.

Brian

#### Vance

##### Gyroplane CFI
Staff member
Good Morning Chris.

Good Morning Chris.

I have not had specific experience with running a Rotax 914 at low rpm and high manifold pressure.

I will try to share my understanding of the compromises.

I recommend following the factory recommendations.

Any engine is a series of compromises and a turbocharger adds to the complexity.

More engine rpm is hard on things that have to reverse direction as the engine turns over. This includes rings, valves, valve train, connecting rods and wrist pins. The oil system needs to be carefully designed to manage these things. A little more rpm makes a lot more load on these components. A Rotax 914 is made to run at elevated rpms compared to a Lycoming so the designers have carefully addressed these things.

Less rpm and higher BMEP (brake mean efficiency pressure) is hard on rings, valves and rod bearings. Octane rating and quality of the fuel becomes more important. It is possible to be more efficient with more cylinder pressure.

A turbocharger compounds these challenges by making life even harder on the exhaust valve and adding the turbocharger to the things that can be damaged by heat. There tend to be hot spots in the engine that make life harder on the oil. These limitations are why they don’t simply run more boost to make more power. Any engine can be boosted to destruction and that is why the boost is limited in some way on turbocharged engines.

Detonation (uncontrolled combustion) can destroy an engine very quickly and the elements required for detonation are time, pressure and heat. A slow turning engine with a turbocharger has more of all three so the fuel quality and octane rating becomes even more important.

This is a long way or recommending that you follow the factory recommendations because they have studied this at length and understand the compromises.

If you suspect you have low quality fuel I recommend you use more rpm with less manifold pressure to make the required horsepower.

#### ventana7

##### Gold Member
Vance,

I am not very knowledgable about engine interiors and so greatly appreciate your input. I have sort of the same question as Chris but for different reasons.

On my Xenon with aftermarket turbo I also installed a manual wastegate control with a vernier knob. I do not have a constant speed propeller or a fuel flow gauge. Chris's tests were done at 500' density altitude whereas I fly closer to 10,000' density altitude.

With my controlable MP knob I have an infinite combination of RPM/MP settings and like Chris, I really never know am I better at lower rpm and higher MP or the other way around or in the middle or what?

Also since I am at high altitude if I set MP at 31" am I boosting much more than Chris would be at sea level? Does this mean I am more at risk for detonation? (I installed and intercooler so my airbox temps are always well below Rotax recommendations).

I appreciate any input.

Rob

#### ventana7

##### Gold Member
Chris,

I've been looking at your graphs- that must have been a massive amount of work to run all those tests, then compile and graph the data.

I have had similar questions but did not really know where to get answers. Lacking any other data I would like to interpolate what I can from your graphs but i do see a big difference in our set ups.

I have a question for you and Vance-
You are controlling MP by your prop pitch setting.
I have a fixed pitch ground adjustable prop and am controlling MP by a vernier closing the wastegate- would the engine know the difference?

I assume I am at more risk of overboosting than you would be- is that correct?

Thanks for any suggestions.

Rob

#### Vance

##### Gyroplane CFI
Staff member
Good Morning Rob.

Good Morning Rob.

Vance,

I am not very knowledgable about engine interiors and so greatly appreciate your input. I have sort of the same question as Chris but for different reasons.

On my Xenon with aftermarket turbo I also installed a manual wastegate control with a vernier knob. I do not have a constant speed propeller or a fuel flow gauge. Chris's tests were done at 500' density altitude whereas I fly closer to 10,000' density altitude.

With my controlable MP knob I have an infinite combination of RPM/MP settings and like Chris, I really never know am I better at lower rpm and higher MP or the other way around or in the middle or what?

Also since I am at high altitude if I set MP at 31" am I boosting much more than Chris would be at sea level? Does this mean I am more at risk for detonation? (I installed and intercooler so my airbox temps are always well below Rotax recommendations).

I appreciate any input.

Rob

Speaking is a very general way more cylinder pressure (higher BMEP) makes more horsepower per pound of fuel burned. Higher manifold pressure is one way to get higher BMEP so in theory the engine can be more efficient and burn less gas for a given power output. This is assuming the mixture is constant and uncontrolled combustion is avoided. I have found that making assumptions is often a bad idea.

My feeling is a prop pitched for less rpm and using higher manifold pressure to make up for the loss of horsepower will burn fewer pounds of fuel per horsepower and be at greater risk of uncontrolled combustion. Higher manifold pressure is also hard on rings and exhaust valves. More manifold pressure makes more heat so cooling becomes more critical.

In my opinion the engine’s cylinders are filled by manifold pressure and it doesn’t matter how high the engine is. In other words 31 inches of manifold pressure at sea level and 31 inches at 10,000 feet MSL has the same effect on the engine.

#### Mike G

##### Junior Member

Does anybody know the 914 turbo manufacturers (Garrett??) type/designation number?
Most turbocharger compressor and turbine performance maps are available on the internet and I'd like to see where on the map the 914 is operating.

I suspect that at sea level and 35" of boost the compressor efficiency isn't as good as at 10,000' and technically speaking Chris at 500' needs the intercooler more than Ventana at 10,000'!

Ventana, Vance is right you are not overboosting your engine by running at 35" at 10,000', the engine doesn't know the difference, however your airbox temperature may be lower (even without the intercooler) than Chris's under the same conditions because the inlet temperature is probably lower to start with and the compressor is (probably) more efficient and heats the air up less.

Strictly speaking the best efficiency is with the throttle wide open (WOT) and dragging the speed down with the prop pitch. This is because any pressure drop across the throttles is energy wasted and that is sort of the definition of efficiency.

Charles Lindberg demonstrated this to the AAF (or the Marines??) with the P38 Lightning in WW2. He flew on missions in the Pacific with the squadrons and when he got back there was always plenty of fuel in his tanks and everybody elses were empty. The difference was he knew that the most efficient way to fly was with WOT (max boost) and max pitch, the other guys were flying per the manual. That started a debate with Allison (engine manufacturer) saying it would wreck the engines. After testing it was found that Lindberg was right and hence the range of the P38 was dramatically improved by adopting his method. There's a documentary about it somewhere on youtube.

Chris I seem to remember your curves when you did your testing demonstrated the Lindberg effect quite well. Your problem might be more a question of airbox temperature at high boost pressures, hence my desire to look at the performance curves.

So Chris I doubt that Rotax will do the testing for you (you're not Charles Lindberg;-)) and the customer isn't the Pentagon so the only way to find out is to suck it and see.

Ventana if you wanted to take advantage of the Lindberg effect you could try running at WOT and setting the rpm with your vernier boost control. I don't know how practical that would be because I have no idea how sensitive your system is but it would be interesting to see if you gain in fuel consumption.
Mike G

#### pancho

##### Member
From a fixed wing guy's perspective. Without a turbocharger as you reduce rpms the manifold pressure goes UP. I would never run an engine at a higher MP than the manufacturer recomends. Nor would run higher rpms. If you run your engine too slow you can over boost the cylenders with even less than 30 inches MP. Because of the time it takes to run the cycle. (Detonation) You cant stick a probe up inside a running engine, so there is no way to be absolutely certain about the internal pressures until you poke a borescope in there. That is why we watch for exterior signs such as EGT and CHT. I would never run the engine for any length of time outside the published paremeters.

#### Jason O

##### Junior Member
Power

Power

When running a turbo without a constant speed propeller, what happens when you increase the boost? I would think the rpm would have to increase to absorb the additional power available, but in doing that the MAP would drop a little and the engine would find a new balance. Is this what happens? I have never really though about it and the relationship between throttle position, boost and RPM with a load that only varies with RPM.

Jason

Vance,

I am not very knowledgable about engine interiors and so greatly appreciate your input. I have sort of the same question as Chris but for different reasons.

On my Xenon with aftermarket turbo I also installed a manual wastegate control with a vernier knob. I do not have a constant speed propeller or a fuel flow gauge. Chris's tests were done at 500' density altitude whereas I fly closer to 10,000' density altitude.

With my controlable MP knob I have an infinite combination of RPM/MP settings and like Chris, I really never know am I better at lower rpm and higher MP or the other way around or in the middle or what?

Also since I am at high altitude if I set MP at 31" am I boosting much more than Chris would be at sea level? Does this mean I am more at risk for detonation? (I installed and intercooler so my airbox temps are always well below Rotax recommendations).

I appreciate any input.

Rob

#### PW_Plack

##### Active Member
Vance mentioned it briefly, but fuel octane is very important here. Rotax's ratings for HP and TBO are published based on fuel which meets the recommended octane requirements. It might be possible to achieve higher BMEP at lower RPM using 100LL, but you'd be in uncharted territory.

A hint can be taken from traditional aircraft engines, which need at least 100LL to make about 0.5 horsepower per cubic inch without the risk of detonation. The reason? They are making their power at lower RPM, and the longer time spent at high cylinder pressures makes detonation more likely.

Many of the current euro-config two-place machines simply need more displacement to get the power they need for good takeoff and climb performance at max weight with expected durability. The aftermarket has recognized this with "big-bore" kits for the 912 and 914, similar to products developed a half-century ago for air-cooled VWs. Rotax is late to the party, but the forthcoming 915s is a 914 with larger displacement. It still won't be enough for many.

The 914 is already remarkable, able to make 1.35 horsepower per cubic inch at 5,500 RPM for over 2,000 hours. At some point, if you need to pound it harder than that, you just need more displacement.

#### eddie

##### RAF, turbo subaru 230hp
Air fuel ratio is most important,Turbo engines have to run really rich,if not the pistons will

melt rather quickly,follow the manufactures recommendations, 14.7 to 1 is the perfect air

fuel ratio for normally aspirated engines, turbo engines use a fuel mixture of 11.0 to 1 or

even richer to prevent knock or piston melting.

best regards,

#### Rick E

##### Active Member
I have an Ivo inflight adjustable prop on my Cavalon and I follow the POH in regard to MAP / engine rpm settings but I did note that there is no limit to MAP settings when above 5100rpm.
I generally cruise two up at 5000rpm and MAP at 31" which gives me a fuel burn around 18-20 litres an hour at 75 knots.

#### ckurz7000

##### Senior Member
...
This is a long way or recommending that you follow the factory recommendations because they have studied this at length and understand the compromises.

Thank you, Vance, for trying to educate me about engines and what they like and don't like. I have a basic understanding of engines but nothing much beyond that. Your reply very was helpful.

Just to make sure I got it right: Vance and others pointed out that there are a number of reasons why running an engine (particularly a turbocharged engine) with lower RPM and higher MAP than the manufacturer recommends is a bad idea. Things like air temperature and fuel quality also play a role in determining how far to uncontrolled ignition you are.

Since I know of no reliable way to warn me of impending pre-ignition (unless somebody wants to chime in here), I will stick with the recommendations of the manufacturer.

Thanks to all, -- Chris.

#### ckurz7000

##### Senior Member
I have an Ivo inflight adjustable prop on my Cavalon and I follow the POH in regard to MAP / engine rpm settings but I did note that there is no limit to MAP settings when above 5100rpm.
I generally cruise two up at 5000rpm and MAP at 31" which gives me a fuel burn around 18-20 litres an hour at 75 knots.

Rick, the Rotax recommendations are in the Rotax engine manual. 31"/5000 is the recommended 75% power setting.

-- Chris.

#### ckurz7000

##### Senior Member
From a fixed wing guy's perspective. Without a turbocharger as you reduce rpms the manifold pressure goes UP.

The Rotax 914 has an automatically controlled waste gate. If I reduce rpm, MAP stays the same.

-- Chris.

#### Vance

##### Gyroplane CFI
Staff member
You understood the point Chris.

You understood the point Chris.

Thank you, Vance, for trying to educate me about engines and what they like and don't like. I have a basic understanding of engines but nothing much beyond that. Your reply very was helpful.

Just to make sure I got it right: Vance and others pointed out that there are a number of reasons why running an engine (particularly a turbocharged engine) with lower RPM and higher MAP than the manufacturer recommends is a bad idea. Things like air temperature and fuel quality also play a role in determining how far to uncontrolled ignition you are.

Since I know of no reliable way to warn me of impending pre-ignition (unless somebody wants to chime in here), I will stick with the recommendations of the manufacturer.

Thanks to all, -- Chris.

You have caught the point of what I was trying to communicate Chris.

The factory engineers know that better fuel efficiency lives at higher manifold pressure and lower rpm and have chosen their recommendations carefully.

One of the challenges with uncontrolled combustion is it tends to feed on itself and damage components very quickly. Detonation makes more heat and pressure; two of the three things needed for detonation.

I lost an engine at the Isle of Mann TT in 1981 because I received some bad gas. I heard it rattle about three revolutions coming out of a slow uphill corner (low rpm, high manifold pressure) before it split the cylinder from top to bottom. I was not the only one. The whole shipment of gas was mislabeled.

A knock sensor (a cheap microphone screwed into the cylinder head looking for the sound of detonation) is not going to warn you in time to prevent engine damage.

Exhaust temperature is also too slow to help.

#### Stan V

##### Active Member
The Rotax 914 has an automatically controlled waste gate. If I reduce rpm, MAP stays the same.

-- Chris.

Chris, with an automatically controlled waste gate and a constance speed prop, short of pulling the throttle back to idle, does your MP ever change? Doesn't your prop control change engine RPM or MP? I believe you need an in flight over ride on the auto waste gate so you can better manage your power. The way I understand it, Rotax doesn't want you to run your 914 below 75% power. I run my Piper Arrow (non turbo) at WOT and control the mixture for peek EGT at 10,000'. This yealds around 65% (or less). At this setting my IO360 is burning 8 gal per hr.

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#### ventana7

##### Gold Member
When running a turbo without a constant speed propeller, what happens when you increase the boost? I would think the rpm would have to increase to absorb the additional power available, but in doing that the MAP would drop a little and the engine would find a new balance. Is this what happens? I have never really though about it and the relationship between throttle position, boost and RPM with a load that only varies with RPM.

Jason

Jason you are correct- If I increase boost then RPM goes up. So I can get the same RPM and MP numbers at varying throttle positions. I am guessing I should pitch the prop so near max MP and Near Max RPM occur at near Max throttle position. That would give me best speed I am thinking. When i do long XC trips I would prefer more speed.

On Chris' graphs the fuel burn was not significant to me so I would personally prefer a bit more speed even at the cost of a bit more fuel.

Rob

#### ventana7

##### Gold Member
Thanks to everyone for the great input.

Rob

#### Vance

##### Gyroplane CFI
Staff member
A few words of caution Rob.

A few words of caution Rob.

Jason you are correct- If I increase boost then RPM goes up. So I can get the same RPM and MP numbers at varying throttle positions. I am guessing I should pitch the prop so near max MP and Near Max RPM occur at near Max throttle position. That would give me best speed I am thinking. When i do long XC trips I would prefer more speed.

On Chris' graphs the fuel burn was not significant to me so I would personally prefer a bit more speed even at the cost of a bit more fuel.

Rob

More manifold pressure makes more power which wears the engine out faster.

It is instructive to note that Rotax decided to limit 39inches to five minutes. This suggests to me that during some tests they found what they considered excess wear with too much boots for too long.

I doubt they felt that was a feature and it requires complexity (\$).

The 914 in the white Cavalon I borrowed had some senders that would reduce the manifold pressure to 29 inches after flying along at 32 inches if they saw too much of something, probably heat. I don’t know what or where they were measuring it. After flying along for a while at 29 inches the boost would increase again. I never had it happen with the red Cavalon (Puff). Rotax may be experimenting with what they feel could reduce the life of the 914 or it may just have some bad sensors. It seemed to do it more with mo gas so it could also be a knock sensor.

Your turbocharger system doesn’t have those controls so you as the pilot in command are expected to use good judgement and understand the engines limitation.

I suspect Rotax engineers felt the engine could be pushed too hard with a turbocharger and that is why the peculiar and seemingly conservative boost limits.

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