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)