> Poop! Why do I get sucked into these debates!
>
> I see no reason why the dynamic chamber principle would be any different
> with peripheral porting. Ken, you correctly defined the basic principles
> all the way through your message then totally destroyed the thing in your
> conclusion. Perhaps it may help if we define a plenum chamber. I have
> obviously failed miserably in my attempts to define a dynamic chamber.
>
> As I understand it (others feel free to jump in here) a plenum chamber is a
> reservoir of air (or fuel/air mixture) which is INTENTIONALLY DESIGNED to
> damp out reflected waves, pulses, etc., reflected from closing intake ports
> (or exhaust overlap). It is the same function as a capacitor serves in an
> electrical circuit. These are used on engines (like V8s) where the
> reflected waves from the various cylinders are as likely to hurt the other
> intake cycles as they are to help.
>
> Note that it is a fortunate happenstance that the timing is correct to help
> all the time on a two rotor wankel engine. A dynamic chamber on a V8 would
> not work, nor would it work on a 20B three rotor (which is why Mazda didn't
> use it there). Note that there are no big chambers (plenums) in the intake
> system illustrations. Also note that the High & Low range labels are
> backwards on the illustrations.
>
> Is any of this making sense?
>
> Tracy Crook
> tcrook@rotaryaviation.com
> www.rotaryaviation.com
When I raced Fiats we were limited to the stock DMSA100 two hole carb. One
27mm and one 23mm hole. A plenum the size of a goose egg was below the carb.
The connecting ports were drilled, so the undersides were sharp corners. The
runners from the four cylinders faced each other in pairs with little
attention paid to smooth transitions anywhere. I enlarged the plenum and made
the runners end in trumpet shaped bells. Some improvement, but way less than
I expected. I then shaped the under side of the carb spacer block into
trumpet shapes. Very impressive. I then cut the floor out of the plenum so as
to include the volume of the water jacket into fuel air plenum. With the
plenum volume nearly doubled it went back on the dyno and lost 5 or so HP.
Back almost to the baseline stock manifold.
My conclusion was that the effect is strong when the reverse pulse exits a
smaller runner into a small volume plenum directly aimed at the runner with
the open intake valve. That the small radius on the runners at the plenum was
not a problem at lower RPMs where the effect is used (and most apparent) to
boost torque. At high RPMs there is no detectable affect but the
exhaust/intake overlap becomes very strong. This requires the correct length
exhaust pipes for a specific RPM. This also demonstrates the importance of
understanding the connection between systems. Also, what works great on the
flow bench does not always work on the dyno.
I built up an E production engine that had individual runners to two (I
think) DCNF dual throat carbs with 30mm venturies. So this thing should
scream right out of the box, right? Wrong. It would stand a column of fuel
air mixture above the carb throats. It would recarburate and run over rich up
to 4,000 RPMs and then take off. It would pull no load at all down low. I
changed everything, one piece at a time until I got it going. I did the big
plenum trick on it so as to include the water jacket, and it ran fine. We
were required to run stock rods, so were limited to 7,500 RPMs,
but ran up to 7,800 RPMs on the dyno to get a good curve. In this case the
runners were 90 degrees to the plenum. It was just a log looking thing. The
small venturies should have made it tractable in the lower ranges with almost
any cams, but not so.
The large plenum calmed it down. So why did it work? I don't know. A
possibility is that on plenums where the runners face each other the small
plenum works best, and on plenums where the runners are parallel the large
plenum works best. My best power was 153 HP for the dual carbs and 148 for
the DMSA 100 (little carb). That same
engine today with aftermarket rods makes over 215HP at 9,000 RPMs. (the
national champion). If you make a mistake in design, make it after you have
made all inlet runners smooth and trumpet shaped. You will be well over half
way to the best possible flow for any system. If anyone is interested, I will
diagram a flow bench. They are easy to build, cheap and provide the biggest
advance in performance.
Lynn E. Hanover
I am always interested in this kind of stuff.
Paul Lamar
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