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4.6L / 5.4L Headers and Exhaust Systems
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Like the intake manifold, the exhaust manifolds (or tubular headers) are designed to not only allow,
but also actually aid, the exhaust flow out of the combustion chamber. The most common
misconception about exhaust systems is that bigger is somehow better (a common theme for
everything from throttle bodies to cylinder heads). Using this logic, we see that a set of 13⁄4-inch
(primary tube diameter) headers will outflow a set of 15⁄8-inch headers. Though this is certainly the
case, does this mean that replacing the 15⁄8-inch headers you currently run on your mod motor
with a set of 13⁄4-inch headers will result in a gain in power? The answer to this question (like most
of the dynamic equations involved with an internal combustion engine) is: it depends. You see the
power potential of a given set of headers has much more to do with their overall design (primary
length, tubing diameter, and merge points) than the sheer size. Just like an intake manifold, the
runner (port) length plays a much more important role than the absolute airflow.
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Oddly enough, headers made less of a
difference on the higher-horsepower
supercharged motors than on the
naturally aspirated versions. The
tuning effect offered by the exhaust
pulse scavenging was less
pronounced on the blower motor.
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Exhaust manifolds (or headers) are not actually designed to maximize flow. If that were the case,
you’d simply build short, large-diameter pipes that offer the least amount of flow resistance. The
zoomies used on Top Fuel motors are a good example of maximizing flow without concern for the
effect of scavenging. Proper header design will actually evacuate the residual exhaust and even
help draw in the fresh induction charge, in effect helping to supercharge cylinder filling. This
improved cylinder evacuation (and filling) happens by means of both kinetic energy and reflected
pressure wave scavenging. Kinetic energy scavenging occurs by means of the release of pressure
from the cylinder just as the exhaust valve opens. The elevated cylinder pressure (from the
expansion created by the power stroke) finds the opening created by the recently opened exhaust
valve (as the piston approaches BDC). The compression wave created by this flow of the spent
gases rapidly displaces the existing column of gas occupying the port. This compression wave
increases pressure on the front (leading) side and reduces pressure on the back (trailing) side.
Since the compression wave travels through the port (primary tubing in a header) faster than the
gas discharge speed (out of the cylinder from the upward moving piston), the low pressure on the
trailing side of the compression wave actually helps draw out the remaining spent gases from the
cylinder (in essence helping the piston do its job). In addition to improved exhaust evacuation, the
low-pressure side of the compression wave also aids in intake flow since the intake valve has
opened before the piston reached TDC.
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The new 4.6L 3-valve will benefit from
long-tube headers as well. These JBA
headers featured long primary lengths
to enhance low-speed and midrange
torque production.
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You may be wondering why long-tube headers are so much more effective than the traditional
cast-iron (or even short tubular) exhaust manifolds. While the actual flow rates may be comparable
between the two types of exhaust systems, long-tube headers improve power production in the
same way long runner intakes improve the volumetric efficiency on the intake side. Reflected
pressure wave scavenging occurs when the compression wave (that occurred when the exhaust
valve opened to release the elevated cylinder pressure) arrives at the end of the primary tube
(usually in the collector). Due to the increase in tubing diameter, the compression wave is allowed
to expand and spread in all directions. The depression created by this expansion causes air to rush
in from the surrounding area, forcing the negative pressure wave back down the pipe to the
awaiting exhaust port. This negative pressure wave helps further scavenge exhaust flow and aid
induction flow into the cylinder during overlap. Naturally, the length of the primary pipe has an effect
on when (in RPM) this scavenging becomes most effective, as the event should be timed so that
the primary reflected wave will arrive at the exhaust port when the piston has just past TDC. Since
the reflected pressure waves travel at the speed of sound, the length of the primary pipes
determine when the low-pressure wave will coincide with the proper piston position, thus headers
are tuned for particular engine speeds.
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Kooks supplied a number of headers
for testing, including these 1-5/8-inch
headers for the 4.6L 2-valve motor.
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One of the common misconceptions about headers is that high-horsepower blower motors will
require large-diameter headers. If 15⁄8-inch headers (of a given primary length) work well on your
naturally aspirated 4.6L 2-valve or 4-valve, then shouldn’t your supercharged motor work best with
larger (free-flowing) headers? The logic seems right, but the reality is actually otherwise, as testing
has shown that even on 600+ hp supercharged 4.6L 4-valve Cobra motors, the smaller 15⁄8-inch
primary headers produced a better overall power curve than the larger 13⁄4-inch versions. The
smaller headers produced slightly better peak numbers, but picked up significant power in the
midrange compared to the larger 13⁄4-inch headers. This header test was run on a ’03 Cobra motor
equipped with a Kenne Bell blower upgrade. If any combination would respond to the larger
headers, you would think that a 600+ hp blown Cobra would, but testing revealed otherwise. Check
out Test 6 in this chapter for a rundown on the test results of the 15⁄8-inch versus the 13⁄4-inch
headers on this blown Cobra motor.
While I have harped on the fact that exhaust flow takes a backseat to scavenging, in some cases
flow is important. If space (or cost) prohibits you from running a tuned header length, you can install
a set of “shorty” headers in place of the factory exhaust manifolds. These shorty-style headers
offer little or no actual tuning (scavenging effect), but they will improve the flow rate over the factory
manifolds. The Ford Racing shorty headers we tested for this chapter showed impressive power
gains over the factory manifolds. That they are much easier to install than the traditional long-tube
headers can mean a lot to a do-it-yourselfer. The other area where flow is important is in the
exhaust system after the headers. Obviously restrictions imposed by the catalytic converter and/or
cat-back exhaust system will have a negative effect on power. While regulations usually prohibit
messing with the catalytic converters, you can replace the cats with an X-pipe for racing.
Aftermarket cat-back exhaust systems are definitely beneficial on higher horsepower motors. Our
testing on the Bassani cat-back proved just how restrictive the factory cat-back was. While
performance is important, many enthusiasts purchase exhaust systems strictly for the improved
sound quality. This is where an X-pipe really shines, as nothing sounds better than a supercharged
mod motor with an X-pipe exhaust.
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Stock Manifolds vs. Hooker
Long-Tube Headers
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Stock Manifolds:
269 hp @ 5,200 rpm
Hooker Headers:
307 hp @ 5,200 rpm
Largest Gain: 38 hp @ 5,000 rpm
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Early 2-Valve GT: Stock Manifolds vs. Hooker Long-Tube Headers (Horsepower)
The Hooker headers were worth some serious ponies on this mildly modified 1998 4.6L. Imagine
adding as much as 38 hp to your early GT with just a header swap.
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Stock Manifolds:
319 ft-lbs @ 3,600 rpm
Hooker Headers:
345 ft-lbs @ 4,100 rpm
Largest Gain: 36 ft-lbs @ 4,800 rpm
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Early 2-Valve GT: Stock Manifolds vs. Hooker Long-Tube Headers (Torque)
The Hooker long-tube headers improved the torque output from 2,500 rpm all the way to 5,500
rpm. The largest gain was an impressive 36 ft-lbs.
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Previous | Next
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This has been a sample page from
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Building 4.6/5.4L Ford
Horsepower on the Dyno
by Richard Holdener
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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.
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Click below to view sample pages
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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
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8-1/2 x 11"
Sftbd.
208 pgs.
340+ b/w photos
Item # SA115
Price: $22.95
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This is a great book and a
must have for anyone
considering modifying a 4.6 or
5.4 Ford for more power!
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Click here to buy now!
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Other items you might be interested in
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How To Build Max Performance 4.6 Liter Ford Engines
With Mustang buy-in prices dropping and aftermarket support booming,
there has never been a better time take your 4.6-liter’s performance to
the next level. In How To Build Max-Performance 4.6-Liter Ford
Engines, renowned engine builder and tuner Sean Hyland gives you a
comprehensive guide to building and modifying Ford’s 2- and 4-valve
4.6/5.4-liter engines.
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Price:
$22.95
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High-Performance Mustang Builder's Guide 1994-2004
The availability of high- performance parts for the 1994-04 Mustang is
unparalleled. You can build your Mustang for drag racing, road racing,
or improved street performance – and the High-Performance Mustang
Builder’s Guide will show you how! Author Sean Hyland uses over 300
photos to explain how to upgrade your Mustang’s engine, suspension,
chassis, transmission, rear end, brakes, and body.
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Price:
$22.95
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