[Diy_efi] The Hunt effect

[Adam Wade]

--- Geoff Harrison <geoff_h at smartchat.net.au> wrote:

> In my humble opinion, the first requirement for a 4
> stroke hemi is a true crossflow head, not one where
> the valve stems are parallel to each other
> as in the Aussie 6 cyl Valiant / Chrysler Hemi.

Well, as I understand it, the combustion chamber shape
in the Chrysler Hemi is much closer to that delivered
by a wedge-shaped chamber roof.  If anyone has any 3D
modeling, animation, or even isometric cutaway views
of the piston crown and chamber roof on a Chrysler
Hemi engine, I'd really love to see them.  Please
email me if you have.

Your above comment reminds me of an engine you would
call a "true hemi", and it actually does a great job
illustrating my point about the combustion chamber
roof shape *in isolation* being a basically useless
thing to specify (of course it will determine valve
size as well as valve and port layout -- or vice versa
-- but I'm specifically talking about the combustion
chamber shape at the moment of combustion).  The
example is the Porsche 911.

In Bruce Anderson's wonderful book about modifying and
maintaining the Porsche 911, I believe it is, he shows
a great number of different heads and pistons, many
used specifically for racing over the years when the
911 was really a 911 model by internal designation,
and the motor was entirely air-cooled.  In fact,
anyone hoping to learn some of the "soft science" of
knowing in general what changes in combustion chamber
shape and port layout will do to fuel economy and
power production would do well to study this book very
closely.

The air-cooled 911 has always had crossflow heads with
a hemispherical combustion chamber roof.  Over the
years, as displacement and compression ratio
increased, things progressed as you suggest, with the
piston crown being raised, some squish area being
provided in some cases, and the crown being contoured
with valve and spark plug pockets.  The ultimate
development, in racing terms, was a piston that so
protruded into the combustion chamber that the chamber
at TDC was basically two thin slices, the size of the
valve heads, with another by the dual plugs in the
center, connected by very narrow gaps.  It lookd
rather like a lumpy capital V, but inverted. 
Emissions and fuel economy were both horrendous on
this motor, although it allowed compression ratios
that more than made up for this inefficiency. 
Further, titanium connecting rods and lighter
valvegear allowed them to rev higher, thus increasing
power output in that fashion as well.  Scavenging was
excellent due to the open roof shape, included valve
angle, fairly straight ports, and higher engine speed,
but more could have been done with less (less except
for engineering and metallurgy, that is; I meant less
in terms of refueling time and cost at the racetrack),
as we now know.  They took the easier way, which was
also much less expensive and much more likely to be
engineered, given the time period.

> But it was probably as close to a hemishephical
shape
> as possible with that valve layout.

No true hemi chamber roof design, even with something
like those 911 racing pistons, could meet today's
automotive emission standards, I think.  Swirl,
turbulence, and more complete combustion are the order
of the day, and very different combustion chamber
designs are required to make that happen.

> BTW, what happened to the "Hunt Effect"

>From <http://www.handprint.com/HP/WCL/color4.html>:

Hunt Effect. As light becomes more intense, the
perception of colorfulness or chroma also increases.
This is called the Hunt Effect, after the vision
scientist who measured and described it in 1952 by
asking participants to match the chroma of two colors
presented separately and at different luminance levels
to the two eyes. It applies both to lights and to
surfaces.

It is easy to observe the Hunt effect. First find a
large, brightly colored image in a magazine or book,
but do not look at it closely once you have chosen it.
Instead, go outdoors on a bright day, wait a few
minutes for your eyes to adapt to the light, then
leisurely study the image and the color contrasts it
contains. Then go indoors to a windowless room
illuminated by an incandescent (tungsten) light, let
your eyes adapt for five minutes, and study the image
again.



I have no idea how it would apply to fuel injection,
though.  ;)

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