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Hi Folks,
Well, I have now conducted
12 ground runs with my new variable length intake manifold. This was a
prototype using some old manifold components I had laying around for the lower
manifold (see photo of slab lower manifold). The maximum port to
port length of the manifold was restricted to 47.8" as a results of using some
of these make-shift components. My design calls for one able to extend
this distance to 51-52" which I could not do due to limited space and these
components. The turn from the my lower manifold into the block is not
optimum as it makes a 90 deg turn with no inside radius - certainly not
conducive to good airflow there.Also my throttle
body plenum has less that an optimum air path from throttle body to the
runners.
Now with the caveats out of the way here is the
bottom line.
My 91 street ported turbo II has Tracy's 2.17
gearbox and swings a 68x72 Performance Prop - this is the same set up that Tracy
Crook has on his Rv-4. My nominal static engine rpm is (not too
surprising) is the same as Tracy has reported 5200 rpm. So that is what I
take as my baseline.
With the variable length manifold at its full
extension of 47.8" inches (port to port) the distance from intake port to
throttle body is 1/2 that or 23.9" for those who might be wondering how I
stuffed 47" under the cowl.
At that setting and with the OAT at 65F my static
rpm increased from 5200 to 5600 rpm or a 400 rpm increase. That of course
equates to approx. 200 rpm increase in static prop speed. I believe
that had I been able to extend it to 50" I would have been able to pick up
another 100-150 engine rpm. My fuel flow while the engine howled at 5600
rpm (really had the RV bouncing and a rocking) was between 16.5 - 17
GPH. My best estimate based on the numbers is that the engine picked up
12-17HP. When I decreased the length of the intake from its 23.9" to
21.4" the rpm dropped back to 5200 rpm indicating (as I would have expected)
that the shorter length raised the EDDIE RPM out of the reach of my static
rpm range.
My equations indicated that assuming a 55F increase
in air temp from OAT (65F) to a manifold temp of 130F (I had previously used
180F for the calculations but am now convinced that's a bit on the high side for
the air temp inside the intake manifold) that for the full extension of
23.9" length, that the EDDIE RPM would occur at an rpm of 5650. This
appears to correlate well with the 5600 rpm I observed. The minimum
length of 21.4" would have an EDDIE rpm of 6500. 6500
rpm is of course much higher RPM than I can reach on the ground (with a
prop {:>)). It appears that the 5650 EDDIE rpm was close enough to pull
my static rpm up into that "EDDIE bucket".
However, it was not all peaches. I had hoped
that my aluminum tubes sliding inside aluminum tubes might not require Nickel
plating as I originally planned. In the work shop the tubes slide inside
each other without any problem. On the aircraft, however, after 12 ground
runs, I noticed that the tubes were beginning to fret in places. It
appears that the pounding on these tubes caused by the DIE pulse as it races
from intake to intake is considerable, either that or perhaps the vibration
combined with the sliding action is the cause. In any case there is
fretting, the end results is that the tubes began to show signs of unequal drag
as my small 1/90 HP motor started groaning. The rectangular tube that
houses the movement mechanism also needs to be mounted more rigid. So
while I would have liked to have make a couple of flights to evaluate the higher
rpm operation before going on to the next design, it was not to be.
So although disappointed I was unable
to test the EDDIE at higher rpm, I am satisfied with the results
of the prototype and plan to go to the next stage of building with a new lower
manifold with better airflow and to lower it a bit so I can get the extra couple
of inches in.
My initial thoughts for the next manifold is to use
one of the aftermarket IDA Webber type manifolds that I believe Paul Conner and
Rusty used. This would replace my "slab" lower manifold providing improved
airflow in that region. I believe it would also lower the profile of the
intake permitting me to add another couple of inches to the runners. The
Webber manifold I am currently using has the centers of the tubes 90mm
apart. At the top of the tubes where the throttle body is attached is the
path is also 90 mm (approx. 3.5") apart which I believe does two
undesirable things. There is insufficient room for a good airflow route
from the throttle body to each individual tube and the manifold path is curved a
tight 180 deg to round the top and connect the two parallel tubes. My
suspicion is that when the pulse starts to round the curve this causes
additional pressure on the walls of the path in that area as the
outside of the pulse is now being pushed by the outside of the path wall to
change the pulse direction rounding the curve and going to the parallel tube for
the other port. Since my air inlet from the throttle body is in that same
region, I suspect that some of the pulses energy is blasting out the throttle
body port. I believe that may be one reason why Mazda has their manifold
"round the curve" before the throttle body opening.
The IDA Webber aftermarket manifold has a 120mm
(4.7") distance between its openings which would provide more room for a better
airflow path from the throttle body and a less drastic 180 deg curve for the
pulse.
So the bottom line is I was not able to fully test
the prototype as I had hoped and I clearly will need to have the tubes nickel
plated to hopefully prevent the fretting and galling.
If anyone has one of the IDA "Webber" type
manifolds, I would appreciate it if you would provide me two measurements.
1. The vertical distance from the
block-mating surface of the manifold to the center of the opening of the
intakes of the manifold - looks like it might be 2-3 inches.
2. The distance from one of the rear center
block attachment bolt holes to the front surface of the front of the manifold -
should be on the order of 8-10 inches.
Thanks
Ed.
Ed Anderson
RV-6A N494BW Rotary
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Matthews, NC
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