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