I originally had
planned on longer ‘tuned’ runners, Dave Atkins made all the arguments to me
about his short manifold being just as good, etc.,etc. I wanted to
believe because I liked the idea of a short compact manifold for my
installation; so I designed and had made a very nice short, compact
manifold.
I have since done
the dyno runs on my engine, and compared data from different sources. I
haven’t delved into the theory as much as Ed; I’ve come to a conclusion.
High rpm operation; say 7000+ likes a short compact manifold because at
those rpms the tuned length is very short, but also because the ease of
breathing is what matters. Dave Atkins doesn’t know the theory well at all,
he knows about performance of drag racing engines and others; and I’d say
mostly at higher rpm. And, of course Lyn is right as well, racing at
9000 rpm, short manifold is best.
In the case of my
engine the hp curve just kept going up to my end point of 7000 rpm where it
reached 95 hp per rotor. But I was disappointed (just a little bit) by
the hp at 5500 of 78.5/rotor). It’s really hard to find good real dyno
data for comparison, but my educated guess is that I might get 3 - 5
more hp per rotor somewhere in the 5000 – 5500 rpm range with longer
tuned runners. I’d probably get about the same, or a little less at
7000.
Dave also argued
that the 9:1 compression rotors that he put in (for normally aspirated) were
just as good as 9.5:1. I never did really believe that; but again, the
data shows that at higher rpm he’s probably right. Curves that Mazda
generated show that above about 6800 or so, that compression ratio doesn’t
matter; but at 5500 rpm it could mean another 3 – 4 hp per rotor. So,
in my case, tuned induction (done correctly) and the 9.5:1 rotors might give
me 85 – 87 hp per rotor at 5500.
So for a N/A
engine and a 2.17:1 redrive ratio these factors matter. For
turbocharged and higher rpm it doesn’t; just make it breath easy. Any gain
from tuned induction effect can easily be achieved with just a bit more
boost.
At least that’s
my take.
Al
Hi, Al....and
thanks for the education and work on the dyno. I guess I'm kinda
thinking that I would be pretty happy with 78.5 hp per rotor at 5500
rpm. That's nearly 160 hp on auto fuel with a bullet-proof engine that
I can rebuild for less than the cost of one cylinder kit for a
Lycoming....hmmmm....not too shabby.
It a tuned intake might give another 3 to 5 hp increase per rotor, I'm not
entirely sure I'd go through the extra effort and expense, versus a simple,
straight bolt-on manifold that fits just right inside my cowling, with no
modifications, plus it is very light weight. My aircraft, being a
canard, is pretty clean aerodynamically, and I think that 5500 rpm might
give me reasonable fuel burns properly leaned out, and the engine won't be
working all that hard at those rpm's? I'm far more interested in
fuel economy at cruise than high performance climb-outs. I figure I
spend 95 percent of my flight at cruise, and only 5 percent on take-off and
climb, so my priorities are for low rpm, economical cruise
performance. After I receive this new manifold, I may have a couple
aluminum tubes bent and have longer intake runners, won't know until I
receive the new intake manifold. Will keep you posted. Thanks
again for all your input and information. sincerly, Paul
Conner