Mike,
This phenomenon has been bothering me for quite some time. I
doubt anyone has noticed this in a car and I have not convinced myself that it
should be happening in our installations.
Tracy
once remarked that the throttle body might not be the most restrictive point in
the intake. I have the Renesis. The stock intake tubes are about
1.125 in diameter where they are cut in my installation. The stock
throttle body in the Renesis is 75MM and is electrically controlled. When
Tracy got
delayed in developing his throttle control, I purchased an aftermarket throttle
body that is, I think, 76 MM.
So, the tubes have about 4 square inches of area, and the throttle body
somewhere around 7 depending on what is lost by the presence of the
butterfly. Tracy
was correct! (big surprise :>))
I know of one bypass of the throttle body in the stock intake and there
may be others that I don’t know about. The one is filtered air just
prior to the throttle body that goes to a 5/16 tube at the bottom center of the
intake. It feeds two small tubes that are inserted into the primary
ports. The purpose is to give a blast to the fuel during idle to help
break up fuel droplets. I asked Tracy
what to do about this opening and he said I could use it or block it didn’t
matter. I am using it as a vacuum port for my fuel pressure regulator.
(but planning to change that to its original use)
This port will move lots of air at idle and very little at WOT.
(depending on the intake restriction) it would improve the top end, but
probably very little.
This port could be used to find your total intake system restriction.
If you connect a manometer or magnehelic guage to this port and run the engine
at full throttle, the inches of water will equal the total restriction.
(if you use a manometer, all the water will be sucked out anywhere close to
idle. You will need a valve to protect it)
Maybe in the car the restriction prior to the throttle body takes care
of the difference in sizes of the tubes/throttle. Maybe there are other
bypasses that I don’t know about???
This is an area of a lot of learning opportunity for those of you who
have a running engine (I don’t right now [:>( )
My manifold pressure guage correlated well with my several altimeters
with the engine off. When my engine was running, prop blast would
give me about 30.5 inches of manifold pressure at WOT measured after the
throttle body and prior to the 4 primary/secondary tubes. I never
measured the pressure at the primary intake ports.
I think a lot could be learned by measuring the intake port pressures
at different throttle settings.
Publish anything you find!
Bill B
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.