Mike,

There are several reasons that an engine will produce max power at "less" than full throttle. First, and this can sometimes be hard to check, be sure that your full throttle position doesn't rotate the throttle plate past full open. I've seen this several times in butterfly throttle systems. If you are using a Mazda throttle body it is likely that the wide open position is controlled with a stop. You shouldn't have a over rotation problem with that. The second reason, and far more common, is that the design has more intake than it can use with the tuning or RPM that the prop limits it to. This isn't really a problem other than indicating that with development the engine could make more power. Often with the design constraints placed on us by a close fitting cowling or restrictive exhaust limit the best airflow to less than the throttle body can pass. The engine just can't "breathe" well enough to need a larger opening. The length of the intake or exhaust can be too long/short and the maximum amount of air that the engine can digest will be at the 3/4 throttle area or even less. Most engines can be tuned to best performance at a specific length inlet and the diameter is also important. Same with the exhaust. If either system is off a significant amount it can prevent you from making any more power past a given throttle opening. This won't hurt the engine at all, it just shows you that some portion of the system can be improved. If your exhaust gas temperature is correct then the injection is working properly and you need to check the other systems. Be sure to double check your exhaust for obstructions or a pipe collapse. We have had too many exhaust problems lately. To put your self at ease remember this, most modern diesels have NO throttle plate. The speed of the engine is completely controlled by the amount of fuel injected. This helps the efficiency at maximum throttle, but obviously there is more air than the engine can use at low speeds. A small amount of fuel is injected, compression ignited ,and all burns up. If all is healthy your engine is drawing all the air you can use by half throttle. You can probably make improvements and get more power or lower fuel burn by tuning.
Bill Jepson

-----Original Message-----
From: Mike Wills <rv-4mike@cox.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Sat, Oct 10, 2009 8:27 pm
Subject: [FlyRotary] where's the missing power?

I noticed quite a while ago and have mentioned several times here that my engine does not have a linear response to throttle. It reaches its max power before it reaches fully open throttle. I havent worried too much about this up until now because the airplane has sufficient power as is, has slightly better performance than my previous 160HP Lyc powered -6A, and my wood prop is actually a pretty good match for the current power level.

 

But I would like to understand what's going on here and eventually address it. I was flying yesterday, my usual boring holes in the sky directly over the airport. Decided to investigate just a little so leveled at 5,000 feet at full throttle. Started reducing throttle until I noticed a slight reduction in RPM and fuel flow. Then looked down at where the throttle was actually set and was shocked to see it slightly below half open. I dont have a regular manifold pressure gauge, just an industrial type vacuum gauge ( I really gotta get an MP gauge). Anyway, the vacuum gauge was indicating 4" of vacuum.

 

So I suspect I am giving up a substantial amount of HP. I think the most likely suspect is my throttle body. For a throttle body I copied Tracy's original design. Started with a stock late 80s Mazda TB and hacked off the third port and all of the extra stuff. The cowl in this area is very tight and I am unable to install any sort of air filter or any sort of bell mouth on the TB. And I'm sure the cowl's close proximity to the TB influences airflow into it as well.

 

Any comments on this or ways to test it are welcome.

 

Mike Wills

RV-4 N144MW