Actual Power

Max horsepower is delivered at 5800 rpm, and at 5500 rpm max cruise 94% of the horsepower is delivered. This is based on Rotax's standard exhaust configuration.

As you say there is no way to know which exhaust system delivers more power than another. But there is more involved in determining how an engine will perform. The propeller performance can vary based on length pitch and the number of blades. And then there is the airplane it self. An RV12 with a 912 100hp will deliver 120 knots at 5500 RPMs, while there are very clean composite planes with retractable landing gear which reach 172 knots with an 912 100hp.

Hi Sean,

I read somewhere that Rotax tests their engine HP in a chamber that simulates 2000 feet MSL at standard temperature and pressure. I'm not an expert, so bring on the corrections if this is incorrect. 

I believe that for most installations density altitude has a much bigger effect on the HP than the minor differences to the exhaust or intake system.  Moreover, in my opinion the HP available at takeoff RPM is more important than the max HP the engine will produce at 5500 or 5800 RPM. If you have an adjustable pitch prop then this is less of an issue because you can dial it up to full RPM.  But a lot of us don't, and thus we are stuck with the HP we can get at take-off RPM with our prop adjustment, probably somewhere around 5100 RPM.

Edge performance has a fun series of videos of dyno testing various Rotax 912, 914 and 915 series engines. A couple of these videos test the HP of the stock engine before they do their performance mods. Looking at my engine, I was impressed with how much of the engines total HP the 912iS Sport made at 5100 RPM.  On the dyno, the 912iS produced 97 HP at 5100 RPM, much better than the RPM/HP graphs in the operator manual would indicate. This is a testament to the changes made by Rotax on the Sport version to produce the maximum torque earlier (about 5000 rpm).

Here is the link to the dyno video for the 912iS Sport, you can easily search youtube for the others.    

Rotax 912iS Dyno Testing And Engine Comparrison At Edge Performance AS (youtube.com)

Hi All

The Rotax factory tests with Eddy current dynamometers.  The factory stock airbox is used and a factory stock exhaust, the same as they sell in the parts system.  The amount of backpressure allowed, to get best power, is found listed in your installation manual.  if you have a different exhaust you should get full power as long as you do not get more back pressure that whas is called out.  

To be clear, no propeller is used.  HP is a formula, T X RPM/5252 = HP.  Full power for the 4 strokes is at 5800 (max time allowed 5 mins) This is the advertised power and the output is corrected for standard pressure and temperature.  Every engine is tested at the end of assembly with a hot test and those numbers have to meet spec or it is not released.  

Cheers

Ronald Lane:

Sure a fixed pitch prop pitch may limit maximum Hp  or not ie this is at the discretion of the owner. The exhaust/induction configuration is wished on the owner by either the aircraft manufacturer or the (experimental) builder. In both cases the limits of the cowling/engine bed will dictate exhaust configuration which may or may not be optimal.

Jeff B:

Maximum Hp is always at max permissible RPM ie 5800 for 5 mins. While many owner/builders will choose to have a prop pitch which limites max achievable RPM & thus their particular installation HP, this does not change the aforementioned.

RW:

"The factory stock airbox is used and a factory stock exhaust, the same as they sell in the parts system."

I accept that the airbox can be "stock" because the factory offering can be installed as is. Not so the exhaust system?

What performance differences may be expected for the basic/individual conical filters induction system??

As you know - The Rotax exhaust system, for the UL & ULS, comes as two hollow "donuts" two straight pipes & sundry bits. This assortment must be cut & welded into four header/extractor pipes, which terminate in a muffler (The muffler tail pipe may also be a custom job).

A factory may elect to install their own custom bent exhaust system that optimises the engine power output but the home builder may only have the option to make a system (from Rotax parts) that fits his unique aircraft.

The resultant exhaust configuration is unlikly to be as per "stock". Just the fact of having very diffrent length headers between front & rear systems, is likely to have some (?) impact.

Do we have any data on tuned/extractor type exhaust systems fitted to Rotax 9, naturally aspirated, engines?

RW,

Now that you brought math into the discussion we have to consider where the constant of 5252 comes from in the HP equation.  Ok, we don’t have to, but why not!

One horsepower is the ability to lift 33,000 lbs one foot in one minute…and…the outer end of a 1-foot long lever travels 6.283 feet in one complete revolution (picture a 1-foot long wrench turning a crankshaft by the center bolt).  Now divide 33,000 by 6.283 and you get your constant of 5252.  It makes sense when you think about it 

Another interesting factoid is that the nature of the HP equation is such that the HP and torque lines in the dyno graph (when equally scaled) must always cross at 5252 RPM. That is to say that the HP and torque will be equal at 5250 RPM if this RPM can be reached.  However, this refers to the speed of the output shaft, and with the geared Rotax engine this RPM will not be reached at the prop shaft, so the graph won’t show this crossing.