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A Practical Approach to High Performance
Induction Systems
Common Terms
Plenum
Carburetor Spacers
The plenum area is where the intake
runners meet. There can be one plenum that all
runners meet, or two smaller plenums with 1/2 the
runners meeting in each. The plenum volume is a
very importing tuning aid. As high velocity gasses
flow through the carburetor or throttle body, the
plenum give the gasses a chance to slow down, as
the velocity is reduced the pressure rises. Higher
pressure means that the air will be denser, and of
course that means more power.
As rpm goes up you need a larger plenum,
but a larger plenum will reduce throttle response and
low-end power. A plenum also reduces peak air
velocity through the carburetor (or throttle body).
The induction pulses in an intake cause velocity to
rise and fall with every pulse. The plenum helps to
reduce them by acting as an air capacitor. Average
velocity will remain the same, but the highs and lows
will be closer together. Since you need a carburetor
that will flow enough air at peak velocity, a larger
plenum will allow you to run a slightly smaller
carburetor without losing airflow, but it will also
reduce the peak signal strength, which is why large
plenums tend to reduce low-end power.
These are probably the most misunderstood
things there are. It seems that almost everyone
installs one on his or her engine. Most people know
that it helps top-end power, but they don't really
know why. The answer is, it increases plenum
volume, which reduces the induction pulses at the
carburetor and brings the peak velocity through the
venturi down.
Most manifolds are made with plenums that
are too small, so adding a spacer will usually help.
Manifold companies know that the plenums are too
small, but it is easier to add a spacer if it's too small,
than to remove space if it's too big. Just about every
engine design will be offered at different
displacements. So a company must design a
plenum to work well with the smallest displacement
engine available or make sure that is marketed
toward larger displacement or higher revving
engines.
Individual Runners (IR)
Individual runner manifolds have no plenum.
There is one throttle bore per cylinder and nothing
connects with anything. These offer the best signal
strength at low rpm, because the have the highest
peak velocity through the throttle bore, but are very
hard to tune in and induction pulsing at high rpm is a
big problem.
Due to the high peak velocity, IR set ups
need a lot of airflow capacity. The basic carb sizing
formula does not apply here. There could be 2500
CFM on top of a 350 cubic inch engine and it could
run fine. This is because each throttle bore gets an
induction pulse once every two engine rotations, so
it's only in demand about 25% of the time. Plenum
type set ups will allow other cylinders to use that
throttle bore while other cylinders do not need it, so
you don't need nearly as much airflow capacity.
Helmholtz Resonator
A Helmholtz Resonator is the theory behind
what happens in the intake (and exhaust systems).
Induction pressure waves can have an effect on how
well the cylinders are filled. Carburetors that have
velocity stacks in each barrel are taking advantage
of this; it can help (or hurt) power in a narrow rpm
range. For more information see the Helmholtz
section below.
Intake Runners
These are the connections between the
cylinder head and the plenum area. They must flow
enough air at peak rpm to support the horsepower
your engine is capable of, but not be so big that they
have extremely low velocity at low rpm. The runner
length is also very important if the induction pressure
waves are to be used to increase volumetric
efficiency. Runner taper is also important to
consider (see the Tuned Port Section below) for
more info).
Tuned Port
When a port is the correct length to add
volumetric efficiency by utilizing the induction
pressure waves, it is said to be tuned. This can only
help over a narrow rpm range (see tuned port basics
below for more info).
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Manifold Heat
signal strength than a smaller one, but will also flow
more air. If the venturi is too big, it will have a hard
time metering fuel at low rpm, if it is too small, it will
be a restriction at high rpm. This is why larger
carburetors need larger idle feed restrictions and
jets. Not necessarily because the engine needs
more fuel, but the lower signal strength needs larger
passages to flow the same amount of fuel.
Most production manifolds will have some
sort of exhaust or coolant passage in it to heat the
intake. This helps fuel atomization, but hurts top-
end power. Cooler air is denser and denser air
makes more power. Any kind of performance
engine should not use manifold heat. Manifold heat
does help low-end and fuel mileage by aiding in a
more efficient burn.
Dry Flow Intake
Venturi
With fuel and air traveling through the
intake, sharp corners can lead to problems as
velocity increases. Air is lighter than fuel and can
take sharper turns. As an air fuel mixture goes
around a sharp turn, the fuel separates and flows
along the outside of the turn.
Getting intake runners long enough to help
low to mid range torque is hard to do with limited
hood clearance. Multi-port fuel injection lets us
inject fuel right at the intake port of the head, which
leaves the rest of the manifold flowing only air. By
doing this, we can have some sharper bends. Air
still flows better in a straight line, but not having fuel
separation is a big plus.
The GM TPI manifold is a good example of
a dry flow manifold. There is no fuel in the runners
until right before the heads. The runners come out of
the plenum and cross to the opposite side of the
engine, making them long enough to help low-end
and still give hood clearance.
An hourglass shape in a carburetor that
causes the air to increase velocity as it passes
through the narrower section. As velocity increases,
pressure decreases. This is how a carburetor flows
fuel. The pressure in the venturi will be lower than
the pressure in the fuel bowl, so the higher pressure
will push fuel through the carburetor. This is the
simple principal of pressure differential, which
relates to many things in an engine.
Booster Venturi
This is where the fuel enters the venturi and
it is fact another smaller venturi itself. Its main
purpose is to further increase the speed of the air
and in turn lower it's pressure even more to gain
more signal strength. There are many kinds of
booster venturi; the ones that give the best signal
strength and atomization are usually the most
restrictive to airflow.
Wet Flow Manifold
Signal Strength
They flow air and fuel of course.
Carburetors and throttle body injection are wet flow
systems. The intake runner shape is much more
critical because it must minimize fuel drop out. Wet
flow system designs are much more limited due to
this.
This directly related to venturi size, shape,
booster venturi, and air speed though the carburetor.
The signal strength is how much the venturi can
reduce pressure. A large venturi will have less
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Types of Intake Manifolds
Dual Plane
This type of manifold has a divided plenum
(or two smaller plenums). It is a good choice for low
rpm power and gives better throttle response than
most other manifolds. The small plenum area gives
good carburetor signal strength and low-end
drivability. Dual plane manifolds generally can
tolerate larger carburetors than similar open plenum
manifolds.
range. I have seen many back-to-back dyno pulls
where a tunnel ram beat single plane manifolds.
Individual Runners (IR)
This type manifold has one throttle bore per
cylinder. It enhances low and midrange power by
increasing peak velocity through the venturi. There
is no plenum to dampen the induction pulses, so it is
difficult to get them to work at high rpm (It is
common for fuel to splash out of the throttle bore at
high rpm).
The carburetion is also very critical, a IR set
up will need each throttle bore to flow enough for
peak airflow. This means a 350 cubic inch engine
can have almost 3000 CFM and not be over
carbureted. If this setup is used with dual 4 barrels
(Holley dominators are common), you'll need to
make the linkage a 1:1 ratio so the secondaries
open at the same rate as the primaries.
Single Plane
Also known as 360° manifolds or open
plenum. All intake runners come form a common
plenum. The open plenum smoothes out the
induction pulses better than a dual plane manifold
and can give better top-end power, at a cost of low
rpm power. The open plenum reduces peak velocity
through the carburetor, which reduces signal
strength. If you have a high revving engine, a single
plan would probably be the better choice.
Tunnel Ram
Cross Ram
Mostly used on bigger cars to help low to
mid rage torque. The long runners can help low-end
power. Hood clearance can be a problem with long
runners, so by crossing the runners to a carburetor
located on the other side of the engine, they can be
longer but not higher. This was common with older
Mopars and worked very well for its time. Fuel drop
out was a problem, so these set-ups generally ran
rich at low rpm and sucked up gas. Long runners
with a wet flow system give the fuel more time to
form into large droplets at low rpm.
Really this is just a more exotic version of an
single plane. All the intake runners are straight and
meet at a common plenum (the tunnel). This type of
manifold gives excellent fuel distribution and flow for
top-end power. The large plenum area reduces
signal strength and throttle response, so it takes
some good tuning to make these responsive for
street driving.
When tuning in one of these, you'll need a
quick accelerator pump and more ignition timing
down low. In most cases, you can lock your
distributor to total advance. You might need a retard
box to retard the timing while you start it, but for the
most part tunnel rams run best with a lot of advance
at an idle. If you want an advance curve on a street
tunnel ram set up, use a vacuum advance and hook
it directly to manifold vacuum. The poor mixture at
low rpm requires a lot of timing at idle and cruise
conditions.
Many people will argue that tunnel rams are
a race only, high rpm manifold, but this is not really
the case. They have worked very well on street
engines and when tuned right will almost always out
perform a single plane manifold across the rpm
Tuned Port
Tuned Port manifolds can come in various
shapes and forms. They are usually associated with
fuel injection, but the Tuned Ports idea is not related
to EFI at all. Tuned port simply means that the
intake runners are tuned to a specific rpm range.
Most factory tuned port set up are sized to
help mid range torque. The Chevy TPI works very
well in the 3000-3500 rpm range. The problem with
them is they run out of air by 4500 rpm due to the
small runners.
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Intake Manifold Basics
Function
You will need to cc the ports often and
measure flow often to get good results. If you don't
have access to a flow bench, it's best to remove as
little metal as possible. Most pocket porting jobs
give very good results when less than 5 cc's of metal
is removed. More than that, you need a flow bench
to see if what you're doing is helping or hurting.
The basic function of the intake manifold is
to get the air form the carburetor or throttle body
directed into the intake ports. It may seem like a
simple thing, but what really goes on inside is quite
complex. The design of the intake manifold will
have a significant effect on how the engine runs.
Port Shape
Airflow
Any sharp edges or corners make a
restriction to airflow. Air is light, but it does have
mass and will flow better if it does not have to
negotiate sharp corners and around obstacles. With
a wet flow manifold (fuel flows through the manifold
as well), sharp turns in the manifold will cause fuel
separation at higher rpm. Fuel is heavier than air,
so when a fuel are mixture flow around a corner, the
heavier fuel will not be able to turn as good as the
lighter air.
If you look at a basic 4-barrel intake
manifold, the area directly under carburetor has a
sharp turn. The air flows straight down through the
carburetor as then has to take an almost 90° turn to
get to the cylinders. At high rpm the fuel has a hard
time staying mixed with the air and can puddle on
the port floor.
Another thing that causes fuel separation is
low velocity. This is especially a problem with large
ports at low rpm, the lower the velocity is, the more
time the fuel has to drop out. Fuel is heavier than
air, so the longer it has to separate, the more it will.
Getting high velocity is very easy, but getting it
without making a restriction is a little more difficult.
You need large ports to flow well at high rpm, but
large ports will decrease velocity and hurt low-end
power.
Getting air into an engine is the key to
making power and there are many ways to increase
the air flow into the engine, some are obvious and
some are not. Other than forced induction and
nitrous, there are 3 ways to increase airflow. The
first is obvious, better port and valve shapes to
improve flow.
The second and less realized is harnessing
the inertia of the airs velocity to better fill the
cylinders. This is why cams keep the valves open
before TDC and after BDC. If all the induction parts
are matched to the same rpm range air can continue
to fill the cylinder even as the piston begins to move
upward. This is due to the speed of the intake
charge giving it inertia to resist reverse flow, to a
point.
The third and not known to many people is
induction wave tuning, this is related to inertia
tuning, but is more complex and more difficult to
tune to a specific rpm range. Induction wave tuning
is why tuned ports work so well.
Porting Goals
Your goal with any port modifications should
be to get as much flow and velocity as you can with
as little restriction as possible. When working on a
flow bench, pay close attention to how much metal
you remove and how much the port flows. If you
have a 100 cc port that flows 100 CFM, then you
modify the port by grinding 5 ccs of metal away and
the port now flows 110 CFM, you gained flow and
velocity (a good thing for a street engine). If your
modified port flows 103 CFM, you gained a little flow,
but lost velocity.
Port Polishing
Polishing the intake ports can show slight
improvements in airflow, but can hurt power. A
rough texture will make some turbulence at the port
walls. Fuel has a tendency to run along the port
walls, especially on the outside of turns and the
floor. A rough texture will help keep the fuel
suspended in the air. Unless you really know what
you’re doing, don't polish the intake ports.
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