Nerding Antireversionaries

[QSHIPQ@aol.com]

Bruce and Graydon:  For the record, you are both partially correct, just
addressing different theories of exhaust pulses.  (Let's go with turbo
practical application for simplicity, then back up to N/A)

Since most of the gas velocity exiting the valve is pressure and little of the
energy is gas movement, a small diameter port from valve to turbine is best.
The general rule of thumb here is that the effective valve area should not
exceed the cross sectional area of the port.

The "Pulses" in a turbo exhaust are generated (and vary greatly) by the
reflection of the exhaust pressure wave as it hits the collector at the
turbine (Diameter of sum of ports < diameter of throat area at turbine), IOW,
not all the pressure creates turbine spin, some comes back up to the valve.
That reflected wave, depending on the length of the port, can come back and
arrive at the valve, at the same time the valve is opening to create another
pressure wave.  If this is timed exactly right, or in specifically here,
exactly wrong, the energy of the 'second' pressure wave decreases (Tp2 = p2
out - p1 back pulse).  The bad effect and timing of these primary and
secondary pulses can either create a reversion during the scavenge period
(this would all but stop a 2 stroke motor) or increase piston work needed to
scavenge the chamber.  The very short ports (like the audi manifolds) 'can'
actually increase the velocity of the primary pulse since the primary and
secondary waves can be superimposed.  

Carrying this theory further, 3 cylinders have the best phase angle, with
240degree exhaust valve openings (highest energy at the turbine is on a turbo
that is attached to 3 cylinders).  One of the reasons 6 cylinders (and all
other divisible by 3) have the best turbo output numbers vs the other turbo
cars.  

>From these basics we can see that some simple things we can do to reduce
reversion.  The best and cheapest is to make an antireversion "step".  That
would mean NOT port matching the exhaust manifold, but rather making the
exhaust manifold ports slightly larger than the head ports.  This minimizes
the energy of the reflected pulse wave, before it gets to the valve.  This
theory works in a N/A car as well as a turbo car, the significance in both is
the same.  The only difference is that there is a turbo at the "collector" in
one, and just a smaller single exhuast in the other (actually in both, just
that reversion in a turbo car occurs sooner).  The reversion of the exhaust
gases occurs at the collector, turbo or N/A.

Bob Dupree already has an anti-reversion ramp cast into his LT1 exh ports,
btw.  :)

HTH

Scott Justusson