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4.6L / 5.4L Intake Manifolds
As with any other type of engine, your intake manifold is one of those key elements that can make
or break your modular engine combination. For the carbureted contingent, adding a stock
dual-plane or worse yet, a cast-iron 2-barrel intake to your 12.0:1, roller-cammed stroker will be a
sure recipe for lackluster power. For the mod-motor fuelie Ford fanatics, the same thing happens
when you stick a short-runner manifold on your stock or even mildly modified 4.6L 2-valve. Just as
a manifold can literally ruin an otherwise good combination, the right intake can help you produce
impressive power when used in conjunction with the right cams and cylinder heads. More than any
other single component, the intake manifold (most specifically the runner length) will determine
where the motor will make effective power. Match the runner length to produce power in the same
operating range as the cam profiles and you are a long way toward making an impressive
Mustang Bullet 4.6L intake manifold with high flow accufab throttle body
The correct intake manifold can make
or break the power curve offered by
your motor. This Bullitt intake from
Ford Racing offers improved flow over
the factory PI intake. Check out the
single blade Accufab throttle body.
Generally speaking, intake manifold design may be broken down into three distinct elements:
runner length, cross section (and taper ratio), and plenum volume. These elements are listed in
order of importance or in the order they most affect the performance of a given manifold. This is not
to say that all of the elements are not important, it’s just that proper care should be given to the
elements in accordance with their eventual effect on performance. Perspective intake designers
should take note of this, as I have seen fabricators spend countless hours altering the plenum
volume in an attempt to change the effective operating range when they should have been
increasing (or decreasing) the runner length. Manifold design is sometimes limited by production
capability, or rather, ease of construction. Building a set of runners with a dedicated taper ratio and
a compound curve is difficult, if not impossible, for the average fabricator. Despite the fact that this
design produces the best power, it simply isn’t going to get produced unless a major intake
manufacturer steps up and pays the cost of such a complex combination.
Reichard Racing high RPM 4.6 liter 2 valve intake manifold
This system from Reichard Racing is
one of the very few aftermarket intake
manifolds available for the 4.6L 2-valve
motor. The shot runners relegated this
intake to effective use on high-RPM
4.6L motors only, as the factory PI
intake out-powered it all the way to
5,900 rpm.
In my opinion, the first element in intake design is the runner length. The overall intake runner
length actually includes the head ports, but the discussion will be limited to those in the manifold.
Unlike their carbureted counterparts, fuel-injected intake manifolds seem to be broken down into
two distinct groups, long and short. This obviously isn’t very scientific terminology, as it doesn’t
describe a complete manifold design. The reason for the simple long and short designations is that
generally speaking, the longer the runner length, the lower the effective operating RPM. The
opposite is also true; shorter runner length will improve top-end power. If you take a given intake
combination (like our 4.6L PI intake) and decrease the runner length, the motor will definitely lose
power at lower engine speeds and possibly pick up power at higher engine speeds. It’s possible to
design an intake manifold that will offer better low-speed torque and top-end power than a stock
manifold, but at some point, compromise is the name of the game. It should be pointed out that the
“ideal” intake design will vary with engine configuration, as the power gains offered by a given
design on a stock motor will most likely be different than on a wilder combination.
4.6L truck and Mustang GT intake manifolds compared
Here is a shot of the 4.6L (aluminum)
truck intake versus the 4.6L
(composite) Mustang GT intake. The
truck manifold featured longer,
smaller runners to help promote
torque production. The truck manifold
also featured a dual-plenum
resonating chamber to enhance
low-speed power production.
The next element in intake design is cross section, or port volume. A related issue is taper ratio, but
we will cover that shortly. The port volume or cross section of the runner refers to the physical size
of the flow orifice. Suppose you have an intake manifold that features 14-inch (long) runners that
measure 1.5-inches in (inside) diameter. It’s possible to improve the flow rate of the runners by
increasing the cross sectional area. Suppose we replace the 1.5-inch runners with equally long
1.75-inch runners. Naturally the larger 1.75-inch runners would flow a great deal more than the
smaller 1.5-inch runners, thus improving the power potential of our motor. The increase in cross
section will retain the same volumetric efficiency, but it will just occur at a slightly higher engine
speed. This differs from a change in runner length in that the longer runner (with a constant
diameter) will actually increase the volumetric efficiency at lower engine speeds. Taper ratio refers
to the change in cross section over the length of the runner. Typically, intake manifolds feature
decreasing cross sections, where the runner size decreases from the plenum to the cylinder head.
The decrease in cross section helps to accelerate the airflow, thus improving cylinder filling.
FR500 Variable Geometry intake manifold from Ford Racing features dual runners to help produce a broad power band
Since long runners enhance
low-speed power production and short
runners do the same for top-end
power, why not combine them in one
intake for the best of both worlds?
This FR500 Variable Geometry intake
from Ford Racing features dual
runners to help produce a broad
power band.
The final element is plenum volume. Plenum volume refers to the size of the enclosure connecting
the throttle body to the runners. Typically, the plenum volume is a function of the displacement of
the motor. Most production intake manifolds feature plenum volumes that measure smaller than the
displacement of the motor (somewhere near 70 percent), but this depends on the intended
application. As a rule of thumb, the plenum volume is increased with the RPM potential of the motor,
but as one of our tests demonstrated, increasing the plenum volume offered changes in low-speed
power only. A number of manufacturers including Ford and Porsche incorporate devices in the
intake manifold to alter the plenum volume to enhance the power curve. We tested Ford’s version
on the dyno with interesting results. Increasing the plenum volume does increase the air reservoir
allotted to the motor, but the real change comes from the resonance wave. When excited, the area
in the plenum resonates at a certain frequency. Changing the plenum volume changes the
resonance frequency. The Helmholtz resonance wave aids airflow through the runner (sometimes
referred to as acoustical supercharging). Where this assistance takes place in the RPM band is
determined by (a number of things), but primarily by the plenum volume.
Dyno testing graph of 4.6L truck intake manifold verses PI car intake manifold Truck Intake vs. PI Intake
4.6L Truck Intake:
356 ft-lbs @ 4,600 rpm
4.6L 2-Valve GT PI Intake:
372 ft-lbs @ 4,200 rpm
Largest Gain: 28 ft-lbs @ 5,900 rpm
4.6L Truck Intake vs. PI Intake (Horsepower)
Obviously, the PI intake out-powered the 4.6L truck manifold (365 hp to 340 hp), but note that the
long runners used in the truck intake made themselves known up to 4,000 rpm. It’s a simple matter
of the intake working for a specific application. Since most truck owners never see 6,000 rpm, the
truck intake is a better choice, especially for heavy hauling. For wide-open-throttle runs in a 4.6L
GT, the PI manifold is definitely the choice.
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This has been a sample page from

Building 4.6 / 5.4L Ford Horsepower on the Dyno Building 4.6/5.4L Ford
Horsepower on the Dyno
by Richard Holdener
The 4.6- and 5.4-liter modular Ford engines are finally
catching up with the legendary 5.0L in terms of aftermarket
support and performance parts availability. Having a lot of
parts to choose from is great for the enthusiast, but it can
also make it harder to figure out what parts and modifications
will work best. Building 4.6/5.4L Ford Horsepower on the
Dyno takes the guesswork out of modification and parts
selection by showing you the types of horsepower and torque
gains expected by each modification.

Author Richard Holdener uses over 340 photos and 185
back-to-back dyno graphs to show you which parts increase
horsepower and torque, and which parts don’t deliver on
their promises. Unlike sources that only give you peak
numbers and gains, Building 4.6/5.4L Ford Horsepower on
the Dyno includes complete before-and-after dyno graphs,
so you can see where in the RPM range these parts make
(or lose) the most horsepower and torque. Holdener covers
upgrades for 2-, 3-, and 4-valve modular engines, with
chapters on throttle bodies and inlet elbows, intake
manifolds, cylinder heads, camshafts, nitrous oxide,
supercharging, turbocharging, headers, exhaust systems,
and complete engine buildups.
Click below to view sample pages
Chap. 1 - Throttle Bodies
Chap. 2 - Intake Manifold
Chap. 3 - Cylinder Heads
Chap. 4 - Camshafts
Chap. 5 - Nitrous Oxide
Chap. 6 - SOHC Supercharging
Chap. 7 - DOHC Supercharging
Chap. 8 - Turbocharging
Chap. 9 - Engine Headers
Chap. 10 - 4.6 Engine Buildups
8-1/2 x 11"
208 pgs.
340+ b/w photos
Item # SA115P
Price: $28.95
This is a great book and a
must have for anyone
considering modifying a 4.6 or
5.4 Ford for more power!
Click here to buy now!

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