914 Throttle/RPM No-Go Zones?

While Rotax is a little vague on the subject, let's do a little reality check.

In the Continental/Lycoming universe, the rule of thumb is to not go "Over Square".

What this means is to keep the MAP below the RPMs (in hundreds) at all times.

This works nicely because by sheer coincidence 2750 RPM and 27.5" of MAP are typical 100% power settings.

This also assumes a linear relationship down the chart.

2500@25", 2300@23" and so on.

Applying this same concept to the Rotax 914 world requires a correction factor due to the fact that 100% Power is 5500rpm@35".  Not nice convenient numbers.

A linear relation would require a 55/34 = 1.57 correction factor yielding...

5800@37 vs (the Rotax published 39)

5500@35  <<< Our arbitrary Reference.

5000@32  vs (31)

4800@30 vs (29)

4300@27  vs (28)

The correlation is Not too bad.

All of these values are still using al least a touch of Boost. So this is far from being conservative.

Even 4300@28"  could not be accomplished with normal aspiration unless you were flying very low over the ocean.  And a Normally Asperated engine loaded down to 4300RPM at WOT would be a bad idea in anyone's book,

The real question would be WHY would you want to do it?

The engine does not have a predetermined number of revolutions determining its life (~500 million???) any more than you have a predetermined number of heartbeats in yours (~3 Billion).

Higher Activity Levels(RPMs) at lower Stress levels(MAP) is the goal for both you and your engine.

Bill is correct in that higher RPM is less stress on the engine.  As we pass the peak torque point in the power band the cylinder peak pressure on a normal non-turbo engine drops off, less stress.  The issue with a turbo is we can boost to very high levels as it is possible to stuff the air into the cylinders.  The 914 is a boosted engine, not a sea level normalized like conventional turbo charged Lyc and Conti engines.  In your manual you see that there is a very steep jump in power at high RPM throttle settings.  The TCU looks at throttle position and as explained in the manual the internal "map", not to be confused with the MAP of the manifold, looks at all the inputs and this is what controls the wastegate for boost.  Simple tracking shows it is a 115 HP engine, 115 HP at 39 in HG and 5800 RPM.  This is good to know for setting WOT for take off with a constant speed prop.  The 5800 however should only be used for a max of 5 mins according to the manual.  We call that the 115% throttle position in the manual.  Once take off is achieved max continuous is 5500 RPM with an accordant reduction in boost by the controler to give us 100HP, this is the 100% throttle position.  You can run it there all day but you pay the price in fuel burn.  If you look at the data in the manual you will see that the controller assumes any throttle position above 108% open means you must want to take off so it jumps boost for that purpose. 

There is no advantage to using WOT and then loading the engine down to 5500 RPM, you simply waste fuel.  Pull the throttle out to the 100% point and adjust your load to 5500 for best operation.  The controller will also look at its internal clock and back the wastegate down to lower the pressure to avoid detonation.  (constant speed)  With the 914 it can maintain power up to about 5000 meters.  After that point the turbo is too small to maintain and you start to drop off as it can no longer stuff the air into the cylinders to maintain.  If you want to cruise and save fuel the best is normally just around 75% power, this fits nice as this is peak torque point in the engine and fuel burn is a lot less.  

Cheers

Hi guys so trying to get my head around all the different numbers and possible scenarios after reading everyone’s troubles.with map pressure 

I have a 914 with 30 hrs - yep only 30 hrs giving me grief , so I must be missing something, and I’m hopeful someone can guide me .

914 in a gyro

motor 4 years old 

I’ve had it from 10 hrs old with these issues , it sat in a shed with 10 hrs on it  before I bought it 

wot 5600 map 40 

fuel burn at 4800 /4900 26 ltrs ph

map 31 @ 5000

ive replaced floats , all good , no change , so dropped the needles to lean it out 

all good fuel burn I think should be in the ballpark 17/18 ltrs

BUT now my MAP has jumped from 31 to 35 at 5000

is 5000  rpm with a map of 35 excessive.? Fixed pitch 3 blade 
others I talk to with same planes refe  to 5000 rpm maps Of 26 and 17 ltrs ph 

Any suggestions?

Cheers