This 12A dyno curve was given to me by Dave Lemon at Mazdatrix.
It is a stock 12A four barrel carb. Typical of the 70's era
too sort and too small runner manifolds on the market like the
Atkins and the Racing Beat. Add about 10% to these numbers for a
13B. No porting was done and this was with a Racing Beat exhaust header
number 16433.
Note the torque is dropping off starting at 5000 RPM because
the breathing is rapidly heading for the toilet. Torque
and volumetric efficiency track almost exactly. Compare this torque
curve with the RX8 torque curves which are almost flat right up
to 8500 RPM. Only ten% more displacement for the RX8.
It wasn't until the advent of fuel injection in the 80's that Mazda started
tuning the manifold with the ultimate being the 1991 Le Mans engine P-port.
If your manifold has runners in the 1 to 1.5 inch diameter range and
is less than about 13 inches long and/or your engine is not ported it
is going to perform poorly. About like this dyno curve. Note what counts
is intake runner cross sectional area. The area of a runner is proportional to
the radius squared so a 1.5 inch runner is 1.76 square inches and a
2 inch runner is 3.14 square inches. Twice the air twice the power.
