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Posted for "Al Gietzen" <alventures@email.msn.com>:
a 4% increase in power with 9.7 vs 9.0 at 5000 rpm. It may also
improve some as the engine wears in - this is an engine with only a
couple of hours of running since overhaul.
I won't argue the case for/against the longer so-called 'tuned'
induction runners, but I conclude that performance benefits are
minimal. Tuning for a given range may give slightly higher power in
that range, but I'll have to see the dyno results to believe it. If
you're looking for a flat torque curve (great for a car) this setup
achieves it.
I agree with you that the RPM of interest is limited to around 6250 rpm max
but I reached very different conclusions on some things. I'll have to dig
it up in my Mazda archives but power varied very little with compression
ratio from what I recall. I was thinking it was in the range of 1% rather
than 4% for a 9.0 to 10.0 CR change.
I have a plot of Mean Effective Pressure vs RPM for WOT performance, for
compression ratios of 9.2 and 10.0. I believe that hp is directly
proportional to MEP. At 5000 RPM is shows a 5% difference in MEP between
the two compression ratios. By doing interpolation on the chart, I come up
with 4.1% difference between 9.0 and 9.7. I'd be interested in further data
that you might have.
I looks like the biggest difference is at about 4500 rpm, and that the
curves come back together at about 6100 rpm. Does this data apply to my
engine? I can't be sure because there are a number of other variables.
On the other hand, I think the improvement at 6000 rpm to be had with
manifold tuning is much greater than you do. Your dyno figures indicate a
HP of 255 at 6000 rpm. This equates to a power of 169.8 for a two rotor
engine. Good, but not great. I have no dyno data but I'm making around 185
HP based on top speed comparisons with 180 HP Lycoming RVs. I think 15 HP
is significant. Other factors to consider in the comparison are my intake
ports which are small compared to yours and my manifold execution is crude
compared to the work of art you came up with.
I don't dispute that there is some gain to be made at cruise rpm or at 6000
rpm by using a properly tuned intake. I suggest it may be small; definitely
less than 15 hp. Knowing that there are a number of variables from one dyno
test to another, I respectfully suggest that you have only a wild guess at
what your horsepower is based on comparing speed of a different airplane,
with a different prop, with a completely different engine. And it is only a
guess that the Lyc would put out 180 hp. Get some dyno data on your engine
and we'll compare, or better yet, I'd like to see some dyno data on my
engine with a tuned intake.
My data shows a couple of HP more per rotor than the four rotor engine with
tuned intake, at the same rpm, tested on the same dyno.
Now for the 'other factor'. As a practical matter, most of us are stuck
with fixed pitch props. If the prop is pitched for a maximum in-flight
engine rpm of around 6200, takeoff & climb rpm will be in the range of
5100 - 5300 rpm. This is where significant gains or losses in aircraft
performance are to be found. If the torque curve can be boosted at this
rpm it makes a big difference in takeoff & climb performance.
I agree. The operative word is "If". There is a complex relationship
between the tuning, the pressure drop in the runners, the velocity (inertia)
at the port; and whether or not there is an operative dynamic chamber
(reinforced pulse from one rotor to another). Most of the so-called tuned
induction I've seen I'd say is a guess at best. I would also say that I did
analysis that shows that on a 3-rotor, with a dynamic chamber, the optimum
runner length is about 13.5". That's about the effective average length on
mine; and although the ports can communicate one rotor to the other, I don't
have anything close to an optimum dynamic chamber. Just didn't have room in
the cowling.
I'd be the last to suggest that my design is optimized. The configuration
is reasonably well thought out for the constraints that I had; and the
performance is good. It is at least 5 hp less at 6000 rpm than I had hoped,
hope based mostly on speculation. I haven't seen any data that shows it
could be better with a different induction system. I agree with Paul Yaw; until you've run it on the dyno, you don't know what
you've got. Al
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