I measured the stock manifold runner lengths from the engine to the dynamic chamber at 17".
Inside the dynamic chamber the outer loop adds about 6" per side, but maybe you don't count
that. The inner loop is more of a plenum.
My runners are much longer, due to shoehorning in the stock dynamic manifold, they are
approximately 28" long.  But now that I have my own plenum box in there, I can route the
runners to the rear side of the box instead of the top, get the lengths shorter again, more
direct, with fewer turns.

Perry

The 17 inch number is about what Mazda used for the Lemans engine at 6000 RPM
indicating the dynamic chamber is tuned for about 34 inches or 3000 RPM.

Here is a HP curve of a dynamic chamber engine. It is not clear
in the Mazda SAE paper this appeared in what it is being compared
to but I suspect it is a four barrel carb with a short manifold.

Note the bump in the torque curve down around 2500 RPM to 3000 RPM.
You don't need torque at 3000 RPM and the convoluted air path
inherent in a dynamic chamber is bound to hurt HP on the top end.

Therefore I suggest you shorten the manifold if you are going
to use the dynamic chamber or lengthen it if you are not going
to use it to tune for 5500 RPM. My suggestion is forget the dynamic
chamber and make all the runners the same about 18.5 inches
long and use bell mouths. Or make them adjustable and try different
lengths in that ball park. Also increase the size of the throttle
body if you can.

Paul Lamar

OK, that dynamic chamber replacement box I made is not a dynamic chamber, it's just a box of
about the same outside dimensions as the original dynamic chamber.
I'll route the runners into the rear end of the box instead of the top and try to get the
lengths down to 18" from 28".

Perry

 
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