RX7Club.com - Mazda RX7 Forum - ported intake manifold

I can do better than that :)

Here's the text from an old message on the FB RX-7 mailing list...

Quote:

I purposely make the manifold runner smaller than the port runner on all
engines that I build. A small offset makes very little difference in the
forward flow, but it makes a huge difference in the reverse flow. This is a
big deal because the charge in the manifold is moving in both directions
during each cycle. This is due to residual exhaust pressure in the chamber,
and the fact that the intake port closes well after BDC. At low rpm, a
large portion of the intake charge gets spit back into the manifold during
the period from BDC of the intake cycle, to intake port closing, which is
the end of the intake cycle. Anything that you do to increase the reverse
flow potential of the induction system will reduce low speed power.

If you don't believe me, and want a visual representation, all you need is a
12A with a stock manifold. From under the hood, open the throttle all the
way until the secondaries open. (Don't blow it up.) You will see a large
cloud of gas vapor spit out of the secondaries, BUT NOT THE PRIMARIES! The
stock primaries in the manifold are smaller than the ports in the motor by
nearly a half inch on both the top and the bottom. This limits the reverse
flow, and that is why the primaries do not blast a cloud of mixture out the
top of the carb. This spitback at high rpm is due to residual exhaust
pressure, so you can see that discouraging reverse flow can help throughout
the powerband. (Hey, I thought that was a trade secret!)

(stuff about Lake Cities manifold deleted)

As a side note, do not EVER port match the primaries of a stock 12A
manifold. You will lose low, AND high speed power. The reverse flow will
increase, and the forward flow will decrease. Here's why. By making the
port taller, you will increase the velocity differential between the "long
side" and "short side" of the port. This will lead to separation of the
airflow on the short side, (It's bad enough already!) and the majority of
the port will be a big swirling vortex, rather than a nice gradual flow
path. A drastic area change should never occur in the middle of a turn., Or
in the words of a very bright friend of mine (Who designed the water cooled
engine at Moller.) "You cannot diffuse and turn at the same time!"

I have made nearly 190 horsepower with the stock manifold and quiet street
exhaust without enlarging the primary runners at the exit. On an obnoxious
race motor, I have made in excess of 210, again without changing the height
of the primary runner exit.

Have you guys seen the bumper sticker that says "Friends don't let friends
drink and grind."

Paul Yaw