Hi, Rusty.....I purchased the short
intake manifold from Dave Atkins. He claims he has more than adequate power
with his, and that idle is also good. Look at all the race cars and
racing motorcycles that have the carbs or throttle bodies right next to the
engine. They seem to generate a lot of power that way. For what it's
worth. Paul Conner
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