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Subject: [FlyRotary] Re: Intake questions

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