Subject: [FlyRotary] Re: intake
ideas?
Russell, you will be on as solid ground as anyone else
to simply use the
Atkins "short intake". I've visited with him
extensively, visited his shop
and asked every question I could think of, and here's
what Dave says and I
believe to be so on intake
manifolds:
We have 2 choices: 1) Long tuned manifolds that
have more resistance to
mass flow of air, or 2) the short untuned manifold
(used by Mazda Racing??)
which gives higher mass flow.
- They both generate about
same torque and HP but 2) has less space
problem.
- Dave is a competive racer
and is one of those guys with max
experience with rotaries - and he is quite comfortable
with the short
manifold.
I'm going to start off with the Atkins short manifold
- or one similar that
fits the RX-8 Renesis engine.
This topic has been hashed and
re-hashed over the years, and it’s a little like the old ‘blind man describing
the elephant’ story. When it comes to hands-on experience and knowledge
of the rotaries, Atkins is probably ‘da man’. When it comes to something
departing from his experience and into the realm of analysis or theory, Dave
is definitely ‘anti’. And from a racing standpoint, where you may be
running 7,000 – 10,000 Rpm; short is good.
On the other hand,
both the analysis and dyno results make it conclusive that there is
performance benefit to tuned intake runners, and it is especially applicable
to aircraft application where we will generally operate over a small rpm
range, say 5000-5500 Rpm. This is a different question than where you
put the injectors, and there is some reason to believe that having the primary
close to the port, or in the housing, is good. Having the secondary a
bit further out may have an advantage at higher air flow allowing more
thorough vaporization and mixing of the fuel.
The question of
tuned runners is ‘how much is the benefit?’, and ‘what do you want to give up
for it’. Could maybe gain 4-7%. Compactness inside the cowling can
be an important factor. I went with short manifold on my 20B based on
wanting a compact configuration, and Atkins convincing me that short was just
as good. That was before I did much analysis and studied dyno data.
I’m happy with my installation, and the dyno results show the
performance is fine; but if I were starting over I might look a bit longer at
fitting in tuned runners.
My very flat
torque curve is great for a car, but a bit more peaking in the 5000-5500 to
improve cruise economy in the plane would be nice. What I have instead
is the hp curve still going up in a nearly straight line at 7000 rpm.
The short manifold may be great for 2.85 : 1 redrive. More pics and dyno
results at http://members.cox.net/alg3/airplane.htm
FWIW,
Al (obviously not
at Fun-n-Sun)