Subject:Re: (rx7-fb) Lake Cities Manifold
Date:	Tue, 6 Jun 2000 21:23:53 -0600
From:	Paul Yaw <yawpower@...>
Organization: Yaw Power Products
To: Felix Miata <mrmazda@...>

Felix Miata wrote:

> Brad Page wrote:

> > Jenaro Rodriguez wrote:

> > > This question is for anybody running a lake cities manifold(sidedraft
> > > carburetor). Did you port match the manifold to the engine?

> > Yes.  Port Matching the manifold is always a good thing to do when
> > installing a "new" intake manifold.  More flow, less restriction.

> This is merely common sense, not necessarily proven fact.

> > The ports on
> > > the manifold are much smaller than the stock intake ports. I'm wondering
> > > if it was designed to work this way or if it was meant to be port
> > > matched.

> > There wasn't much port matching to do on the manifold flange facing the
> > motor, but it was a tad smaller, which would increase velocity a bit.
> > BUT I don't like the idea of a large port on the motor sucking air/fuel
> > thru a smaller port on the manifold.  Why do that?  Why not have it all
> > matched up?

> I'm not a physicist, nor do I have a flow bench or dynamometer to prove
> the theory, but here's a theoretical reason why enlarging the manifold
> runners might be counterproductive:

> At the most elemental level, any mass in motion tends to stay in motion,
> at rest to stay at rest, inertia defined; and inertia is what makes any
> mass want to continue in a straight line rather than change direction.
> Those of us who drive our cars with spirit understand that the faster we
> drive our cars, the more difficult the job of our tires to make the car
> go the direction we want when that direction is anything but straight.
> Slowing down is a fundamental way to make any mass change direction more
> easily.

> In the intake system, there are no tires of course, but there is a way
> to make a change in the direction of the intake charge easier: vary the
> cross sectional size of the flow path. Increasing velocity can be
> achieved by reducing area, and the converse is also true. The latter is
> the key, as shortly after the intake charge leaves the intake manifold
> is when it is forced to make some very abrupt changes in direction to
> leave the port and enter the working chamber.

> The transition from port to chamber is a very complex phenomenon, not at
> all apparent to anyone who merely looks at a port all by itself instead
> of as it looks while doing its job. The port itself looks like it ought
> to provide a smooth and efficient direction of the charge into the
> chamber. It does get some help because the manifold cross section is
> normally smaller, providing velocity reduction as the charge approaches
> the port. The ultimate question is how much is best. Paul Yaw is
> probably the best person many of us are familiar with to provide the
> closest thing we might get to the ultimate answer to the question, but I
> doubt anyone not buying it from him would get much help here, as this is
> what makes all the time he has invested valuable.

> The really complicating part is that most of the time, the rotor
> obstructs part of the flow. As the end of each port open cycle is
> approached, there is a scissor effect, closing the port from the bottom
> (narrow end) toward the top (the part street porters "extend" to extend
> the RPM range and improve HP). The port is also opened in a scissor like
> action, but from the outer side (manifold) toward the center. Except
> when wide open, halfway between the the port open and port close events,
> the rotor is acting as a dam, catching a portion of the charge in a
> basin.

> If I am right in supposing that an extraordinarily sophisticated and
> expensive computer model would be required to evaluate the effects of
> port and runner changes in the context of the morphing port opening,
> which I doubt exists outside the confines of Mazda or Moller, if there,
> then the only way to really get a handle on the effects of changes is a
> herculean trial and error effort that includes a very large supply of
> housings and manifolds to port and test, and a *lot* of engine building,
> with only one single change each and every time.

Congrats to Felix for using his gray matter.  I really like his approach
which seems to be "I don't know for sure, but how about using logic to make
an educated guess."  Why is it that most rotary enthusiasts are smarter than
the so called experts?  Okay, I'm getting way off base here.

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!)

If you decide to port match the manifold, open it up to the shape of the
runner a ways into the port, NOT the entrance of the runner which is flared.
This will leave a small anti-reversion dam, and will not hurt the forward
flow a bit.  If I remember correctly, (I haven't seen one in years.) the
problem with that manifold is the abrupt direction change where the
secondaries meet the engine.  I believe it is in excess of 30 degrees.  The
primaries flow pretty well.  If you want to go a bit further, get inside the
manifold entrance where the runner separates into two, and grind that sharp
edge down to a nice fat radius.  The fatter the better!

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