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.