Subject: [alfa] head porting, flow reversal

[RonHorowitz <RonHorowitz@xxxxxxxxxxxxxx>]

Joe, et al.,

>  It's a given that to leave a step on the exhaust port can help
>reduce flow reversal with cams that have a large amount of overlap.
>Surely then the same would apply to the inlet port?

Sort of. One of the easier ways to "port match" on an intake is simply to
radius or fillet or "roll back" the ports in the head slightly larger than
the exits of the intake manifold.

> Also the sudden
>pressure drop at that point may induce more flow than a gradual
>taper would.

Nah. That's not an unreasonable guess, uninformed, but like many things,
once you understand what's what, you can see how wrong it is. The old
"gradual taper" concept is equaly wrong, for similar reasons.

For subsonic flow (momentarily neglecting losses, sources, and sinks and so
forth) the energy of the gases in the ports obeys the law of conservation
of energy. If you increase the port cross section, the "flux density"
decreases, thus, the pressure _must_ increase.

>Is what they say about the exhaust port/header interface true?

Yes. You might pretend it's like a sort of leaky (very leaky) check valve.
Really, ports just don't flow the same backwards as they do forwards. The
flow in the ports follows a certain flow pattern or mode. The pattern not
only changes with direction, it even changes with the CFM in a given
direction! A good way to think of this is to imagine that the port walls
don't so much tell the gases _where_ to flow, they tell the gases where
they _cannot_ flow.

Anyway, the more you can encourage the gases to flow forwards, and the more
you can discourage them from flowing backwards, the better off you'll be.
On the overlap period, the "straight shot" ports of our trusty old Alfa
Romeos are shitty even if you have _very_ high anti-reversion, but that's
for different reasons than reversion.

>If
>so, which way are they talking about stepping it--bigger header than
>port, or vice versa?

What you'd want in the intake isn't a step up in the port diameter, all you
really want is a small pocket with a rotating ring ("vortice") of gas
flowing at the port/manifold junction.

>And why does this reduce flow reversal

Long story short, because it increases the flow resistance.

> (which I
>assume happens because the exhaust manifold is at a higher pressure
>than the intake manifold).

Not exactly.... ah, hell... close the hell enough....

RON
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