what Ive done to my Intake manifold
Magnum Manifolding
The dry manifold is an entirely different animal than its counterpart.
The reason being that air traveling by itself in a dry manifold can make all
sorts of turns that would be impossible for an air/fuel mixture. Wet
manifolds having abrupt runner turns or irregularities separate the fuel from
the air. This in turns leans out a given cylinder. Trying to get the same
air/fuel ration in each cylinder tuned with a lean runner condition like this
is next to impossible. Builders will then epoxy in popsicle stick dams or dig
trenches on the plenum floor that lean down all the runners just to create a
balance. The end product is far less than perfect.
Fuel separation maladies just plain don't happen in a dry manifold. The
injector, suspended from the roof of the port, aims at the valve head,
insuring the correct air-fuel ratio in each cylinder. Most dry OEM manifolds
have large plenums to supply the cylinders with enough air and long runners to
generate torque. This combination of long runners and big plenums is great
for gas mileage and bottom end power, but does not lend itself to performance.
Even though the OE Magnum manifold falls in this category, it's easily
modified and shows substantial horsepower improvements when worked on. One
inexpensive revision that aids upstream air density is the fabrication of a
lifter valley heat shield. Magnum manifolds have a flat steel cover bolted to
the underside machined surface. It sits just above the lifters. The plate
becomes absolutely torrid when the engine is up to operating temperature.
NHRA truck class racers bend up a piece of steel plate the length of the
manifold cover and weld it into place. This shield acts as a heat buffer by
isolating hot splattering oil and rising heat away from the manifold's bottom
cover plate. Because the factory cover is not getting direct exposure with the
shield in place, air in the manifold stays cooler (denser). Cash outlay
should run under 25 bucks at your local welding shop (plus new gaskets). All
you need is a flat piece of steel stock, 1/16-inch thick, bent at the ends,
and welded into place. Have the shield welded 1/2 to 9/16 of an inch off the
valley cover and you're all set for the dance. Though it won't up your HP
figures, it will allow you to run more consistent times on hot summer days.
This mod also helps prevent pre-ignition in street trucks that see engine
temps soar when hauling heavy loads.
While your V6/8 Magnum minifold is off, the next performance step is to
shorten the runners as much as possible and cut two inches out of the plenum
divider. This mod will add about 10/12 HP to your engine and raise your shift
points about 300RPM. OE castings are made from some pretty tough aluminum,
and buzzing away with a grinder can get tedious. A better way to go would be
to have a machine shop mill the runners flush to the castings port floors,
then get in there with your grinder to clean things up. Finish touch the
runner mouths with a polished radiuses so turbulence is minimal. None of
these grind/olish/weld V6/8/10 modifications require a high performance
computer because factory computers require a great deal of latitude to cover
everything between lean high altitudes to rich seaboard environments.
Lastly, there are two Magnum precautionary notes. It's common for Magnums
to blow the gasket that seals the cover to the manifold. You'll know when
that happens because the performance will slip off par, or you may begin
hearing pre-ignition knocks. Lift your hood, remove the air breather lid and
look down the throttle body. If the venturis are dark, sooty, or oily
looking, your engine needs a new cover gasket. What the heck, you can modify
the intake while it's off. When you finally replace the cover and gasket, be
sure to put silicon on the cover side of the gasket and use a fixative on the
bolt threads. The other concern centers on the lifters. They're real heavy
and overcome the strength of the lifter bore casting at high RPM. Spinning
the Magnum to 5400 RPM is borderline, 5800 RPM too much, and above 6000 RPM
can be the kiss of death. What happens is the heavy lifters crack the
casting, start to wobble, and the valve train self-destructs. When the valve
train starts to go bonkers, you'll notice an upper RPM vibration, almost like
the engine is out of balance. Either limit your shift points, buy a
Fluidampr, or pray somebody develops a lightweight lifter for the Magnum.
Magnum plenums are extremely large. By taking up some of the volume,
resonating effects (powerful pressure waves) are increased, augmenting low end
torque. And, because the Magnum's not a high RPM engine to begin with the
high end is not affected. This is an involved process. The high dollar route
would be to buy four quarts of Devcon and fill each corner of the intake.
Besides being expensive Devcon is heavy. Another approach is to scrounge up
some 1/4 inch plates of quality aluminum, make some cut outs from cardboard
that, when heliarced in, box up plenum space in the manifold. It's time
consuming, but seems to work. Magnum V8's really respond to this plenum
reduction plan. Even though the V6 has a smaller plenum to begin with,
resonation is much greater than a V8. Nonetheless, power increases with a
plenum reduction.
The intake manifold is probably the most underestimated piece of
performance equipment on the engine today. When modified with even simple
changes, it responds unbelievably well. Though some of these alterations can
get quite expensive, a bucks-down performance aficionado can still do quite a
bit on a Saturday afternoons tie span. Though the new millennia will
undoubtedly bring further developments in the intake manifold, on thing's for
sure; as long as the internal combustion engine is around, it will always have
need of an intake manifold.
Mopar Muscle--Inductive Reasoning July 1997
Eric
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