When we first started our Cobra project, we had little knowledge about engines. I understood the basics about how a four stroke engine worked, but knew little about the details. Of the books and magazine articles I found, none provided a real overview of engine design and operation – they all either assumed a working knowledge of engines or talked about improving power by bolting this or that gadget on without really explaining how it worked.
So, after spending the past several months reading
various books and magazines and spending lots of time perusing the Club
Cobra archives, I’m starting to understand just enough to be
dangerous. Here’s an overview of what I learned, particularly as it
applies to the engine we’re building for our Cobra.
The Four Stroke Engine
Almost all modern automobile engines are what are
called four stroke or four cycle engines. This refers to the four
different operations that occur during each set of four strokes each
piston makes before repeating the process. The four strokes are referred
to as INTAKE, COMPRESSION, POWER, and EXHAUST. These four strokes repeat
over and over as the engine runs. On the INTAKE stroke, the piston moves
down and sucks the air/fuel mixture into the cylinder.
As the piston moves back up the cylinder during the COMPRESSION
stroke, the air/fuel mixture is compressed. At approximately the time
the piston reaches the top of its motion, the spark plug ignites the
compressed air/fuel mixture and forces the piston down for the POWER
stroke. On the next up stroke, the EXHAUST stroke, the spent mixture is
forced out of the cylinder making way for fresh air/fuel mixture on the
following INTAKE stroke.
I should note that the above description is a
simplified view of engine operation. Real engines actually have some
overlap in the cycles in order to improve performance and account for
the acceleration and deceleration of the air/fuel mixture as it moves in
and out of the cylinders. This will be discussed in detail later in this
The engine block is the big chunk of metal that
holds everything together and forms the core of the engine. It has
cylinders for the pistons to move up and down in and holds the
crankshaft and camshaft (in a pushrod engine) in place. As the piston is
pushed down, it pushes on a connecting rod which pushes on the
crankshaft and forces it to turn. As noted above, with a four stroke
engine, each piston has a power stroke every other crank rotation, so
for a V8 engine, a different piston is pushing down on the crank every
90 degrees of crank rotation.
A cylinder head is bolted to the block above the
cylinders and pistons. On a V8 engine, one cylinder head is used for
each bank of four cylinders. The cylinder head serves a number of
purposes – together with the top of the cylinder and the piston, it
creates a combustion chamber for each cylinder; it holds the intake and
exhaust valves and provides a method to actuate them; and it provides a
path for fresh air/fuel mixture to enter the cylinder and spent mixture
to leave the cylinder.
In a push-rod V8 engine such as the Ford 302 and
Windsor 351, a camshaft is mounted in the block above the crankshaft.
The camshaft is driven by the crankshaft through a timing chain or gears
that cause the camshaft to rotate one full turn for every two
revolutions of the crankshaft. In this way, the camshaft makes a full
rotation for each set of four strokes (two up, two down) of each piston.
The camshaft has lobes that are used to control the
valves. Think of each lobe as an elongated circle (ellipse) mounted on a
shaft such that if you put your finger on it as you rotated the
camshaft, your finger would move up and down (or more accurately, away
from and toward the center of the camshaft). The camshaft has a separate
lobe for each valve that it controls. In the case of the above Ford
engines, the camshaft has sixteen lobes – one to control an intake
valve and an exhaust valve for each of the eight cylinders.
The engine block has sixteen small holes in the top
(in between the cylinder banks) that fit lifters (also called tappets).
The lifters look like cylinders a couple inches long and about a half
inch in diameter. The lifters slide through the holes in the block and
rest on the lobes on the camshaft. As the camshaft rotates, the lifters
slide up and down as they follow the profile of each lobe.
Each cylinder has an intake valve and an exhaust
valve. The valves look like disks 1.5” to 2” in diameter with rods
(approx 3/16” in diameter and a few inches long) called valve stems
mounted to the center of each disk. The valves push up through the
bottom of the cylinder head so that when they are inserted all the way,
the valve disks fit into holes the same diameter (called the valve seat)
and close the valve. The valve stems stick out through the top of the
cylinder head. A heavy spring that fits around the stem holds the valve
in the closed position.
For each cylinder, the intake valve opens the
cylinder to the intake port and the exhaust valve opens the cylinder to
the exhaust port. So, for example, when the intake valve is open (pushed
down toward the cylinder), fresh air/fuel mixture can flow from the
intake port into the cylinder.
Mounted on top of the cylinder head is a set of
rockers, one for each valve. The rocker pivots in the middle so that one
end pushes down on the valve (forcing it to open) when the other end of
the rocker is pushed up. A rod (approx 3/16” diameter and several
inches long) called the pushrod fits through another hole in the
cylinder head directly under the rocker and rests on the top end of the
lifter associated with that valve. So as the camshaft rotates causing
the lifter to move upward, it pushes the pushrod, which pushes one end
of the rocker upward. The other end of the rocker pushes down on the
valve stem causing the valve to open. When the camshaft rotates further
and the lifter moves down, the heavy spring on the valve allows the
valve to close.
The intake manifold bolts to the inside of the
cylinder heads and allows air/fuel mixture to be directed to the intake
ports on the cylinder heads. For a carbureted engine, the carburetor
sits on top of the intake manifold and controls the flow of air into the
intake manifold and mixes fuel into the air. For a fuel injected engine,
such as we’re building, a throttle body controls the flow of air into
the intake manifold and fuel injectors spray fuel into the intake. In
both systems, the air flow is controlled by the throttle. There are
numerous configurations for fuel injected systems with different pros
and cons. I’ll describe the configuration we’re using later in this
An exhaust manifold or headers are bolted to the
exhaust ports of the cylinder heads and carry the spent mixture away
from the engine, to the muffler if one is used.
At approximately the end of the COMPRESSION stroke
for each cylinder, a spark plug is used to ignite the air/fuel mixture.
Very high voltage electrical current is sent to the spark plug to cause
it to spark at the right time. Most engines use a distributor to control
when the spark occurs. The distributor is essentially a rotary switch
with one contact per spark plug that rotates at the same rate as the
camshaft. The distributor is mounted in a hole in the top of the block
with a drive shaft that is driven by a gear on the camshaft.
A nice animation of a four stroke engine is shown
Significantly more detail about engine design
including a discussion of all secondary parts and a more complete
explanation of valve timing and camshaft design is provided later in
Small Block, Big Block, Short Block, Long Block
Sounds like a Dr. Seuss book, and almost equally
meaningless to me when we started this project. Small block and big
block refer to engine sizes; short block and long block refer to engines
at various stages of completion.
For Ford engines at least, engines built using the
Windsor or Cleveland 351 blocks or smaller are considered small block
engines. This is true even if the engines are over-bored and stroked to
a larger displacement. The larger Ford engines, built using the Ford FE
block (360 or larger), are considered big block engines.
Engines are available in various stages of
completion. You can buy a bare block and build everything up from that,
but this requires a fair amount of specialized machining to be done
correctly. A short block is a partially assembled engine that includes
the pistons, connecting rods and crankshaft. A quality engine shop will
do a fair amount of custom machining to the block to make sure
everything is squared off and within tolerances and that everything will
work with adequate clearances.
The next step up is a long block, which adds the
cylinder heads and valve train to the short block. The valve train
includes the camshaft, timing chain, timing chain cover, lifters,
pushrods, rockers, valves and springs. Most long blocks also include the
valve covers, oil pump, oil pickup and oil pan. Some long blocks include
a balancer, flywheel, water pump, sparkplugs and plug cables, although
most engine builders call this a “crate motor” and often also
include the intake manifold.
To create a complete engine from a long block, you
must add the intake system (intake manifold and carb or fuel injection
system), the exhaust system (headers, collector, mufflers), an ignition
system (distributor, coil, etc.), a computer (if electronically
controlled), a fuel pump, an alternator, and a starter. And if not
included in the long block, the balancer, flywheel, water pump,
sparkplugs and cables.
A complete new Ford small-block V8 engine will cost
anywhere from under $4000 to well over $20,000. We estimate that our
engine will cost about $18,000. The difference in price is determined by
the quality (robustness) of the parts used, the sophistication of the
engine, and the amount of custom machining required to assemble the
For just under $4000, you can find a more-or-less
stock carbureted 302 that will generate about 300HP. The engine will
likely be built with components that will provide years of reliable
street service as long as the engine is well taken care of and not
pushed too hard.
There are many bolt-on performance accessories for
the Ford 302 that will increase performance, but these will generally
add stress to the engine that the stock components were not designed
for. If you want
substantially more power from the engine, you’ll have to use stronger
components to maintain reliability. This is where much of the added cost
Moving to a 351 Windsor block is a relatively easy
and not too costly way to increase performance. A stock 351W engine can
be built for about $5500 and deliver about 385HP.
This will probably be a more reliable engine than adding $1500 of
performance add-ons to a stock 302 to get to the same performance level.
Switching to fuel injection provides a number of
benefits over carburetion, but also adds quite a bit to the cost –
figure at least a couple thousand dollars. A high-end fuel injection
system such as the one we’re using on our engine costs about $7000
including the computer.
The following sections will discuss the various
components of the engine and where the cost/quality/performance
trade-offs are made.
The standard Ford 302 block has a bore of 4.002
inches and a stroke of 3.000 inches. The bore is the diameter of the
cylinder. The stroke is the distance the piston moves in the cylinder.
The displacement of the engine is calculated by determining the volume
of the stroke times the number of cylinders. In this case, it comes to
301.9 cubic inches. The cylinder wall thickness is adequate to increase
the bore size to 4.030 inches without significantly compromising
cylinder wall strength. This is called over-boring and is used to
recondition used blocks and/or to increase engine displacement. The 302
block can also support a longer stroke although this puts considerably
more stress on the bottom end of the engine (rods, crankshaft, bearings,
and block). About the largest stroke that can be supported in the 302
block is 3.4”. Combined with an overbore to 4.030, the 302 block can
therefore support a displacement of 347 cubic inches. An engine with a
longer than stock piston stroke is called a “stroked engine.”
The 351 Windsor block is from the same engine
family as the 302 and many parts are interchangeable. The primary
difference is a larger “deck height” which means the top of the
cylinders is higher than on a 302, which allows for a longer stroke. The
stock bore is the same 4.002” and the stock stroke is 3.5” for an
actual displacement of 352.2 cubic inches. The 351W can also be over
bored to 4.030” and can be stroked up to 4.17” to increase
displacement to 425.5 cubic inches (these engines are often referred to
as 427 ci).
At the bottom of the block, the crankshaft is held
in place by a set of main bearings. A bearing is used between each pair
of cylinders (one from each side of the “V”). The bearings are held
in place by webs in the block above and by caps that bolt to these webs
below. These webs and caps take a tremendous amount of abuse as the
pistons push down on the crankshaft through the connecting rods.
The stock block (whether 302 or 351) is designed
for street use and is optimized for reliability and light weight. The
webs are relatively thin in order to save weight and the caps are only
large enough to hold the bearings in place. They are bolted to the block
using two bolts, one on either side of the crank. This is often referred
to as a two-bolt main block.
Ford makes racing versions of the 302 and 351
blocks that are considerably beefier but also heavier. The main webs are
significantly thicker to handle more abuse, and the caps are bolted on
with two bolts on either side or four bolts total. These blocks
therefore have four bolt mains.
There are two compromise blocks available. The Boss
302 block has four bolt main caps but still has relatively thin webs.
And the Sportsman block has two bolt main caps, but web thickness
comparable to the racing blocks.
One way to increase the strength of the block is to
add a main cap girdle. This is a machined aluminum or steel structure
that is bolted under the main caps using the main cap studs to add
additional torsional rigidity to the bottom end of the engine. While it
won’t replace thicker webs or four bolt main caps, it can add
considerable strength to the block.
For our engine, we decided to go with a Sportsman block with a main cap girdle. We felt this would be a good compromise between strength and cost and allow the engine to easily handle the 500+ horsepower we expect from our build.
Pistons are produced using one of three methods –
cast, hypereutectic and forged. Cast pistons are used on many street
cars and are not as strong as hypereutectic or forged pistons. As such,
they are really not well suited to high performance applications.
Hypereutectic pistons are also produced using castings, but they have a
higher silicon content (higher than 12% is considered hypereutectic)
that makes them harder and more wear resistant than traditional cast
pistons. Hypereutectic pistons are used in many factory performance cars
including 5.0L Mustangs since 1992.
Some engine shops consider hypereutectic acceptable
for high performance engines. Other’s claim that the added silicon
makes the metal brittle and conducive to catastrophic failure under the
heavy loads seen in a performance engine. This is a particularly
important consideration if the engine is run at a compression ratio near
the limits of the fuel octane level since pre-mature detonation can
result in forces more than twice what would be seen in normal operation.
Forged pistons are the best choice for a high
performance engine, but they are also the most expensive. Forging
eliminates material porosity and makes it more ductile. The smooth grain
flow pattern that results from this pattern makes the pistons stronger.
To prevent oil from leaking past the pistons into
the combustion chamber and prevent air/fuel mixture from leaking into
the crankcase, a set of springy rings are used which fit snugly around
the piston and slide against the cylinder wall. There are typically
three rings used – the top and middle rings are used to keep
combustion gases from escaping the combustion chamber, and the bottom
ring, often called the oil control ring, prevents oil from leaking into
The top surface of the piston can be flat, domed,
or cupped, and will often have notches to improve valve clearance.
Assuming the pistons have adequate clearance for the valves when the
valves are fully open, the choice of domed, cupped or flat is primary
driven by the desired compression ratio. A higher compression ratio will
result in more power, but also requires a higher octane fuel to prevent
Compression Ratio and Octane
The compression ratio is the ratio of the cylinder
volume when the piston is at the bottom of its stroke to the volume when
the piston is at the top of its stroke. At the top, the volume is
determined by notches or bowls in the top of the piston, the head gasket
thickness, and the size of the chamber in the cylinder head. So the
compression ratio can be adjusted by changing any and all of these
A higher compression ratio results in more power.
For example, a typical street tuned Ford 302 engine might make 250 HP
run at a compression ratio of 8:1 which would allow it to easily run on
regular 87 octane gasoline. Increasing the compression ratio to 12:1
will increase power to over 300 HP with no other changes, not that this
would make sense in a street tuned engine. But this engine would now
require high octane racing fuel to prevent engine damaging detonation.
The octane number is a measure of how readily the
fuel is to ignite. The higher the octane, the harder it is to ignite. As
compression ratio is increased, a higher octane fuel is required to
prevent the fuel/air mixture from prematurely igniting from the heat of
the engine. When premature detonation occurs, considerable force is
exerted on the piston, connecting rod and crank since the piston will
not yet be moving downward to adequately absorb the force. If this
happens enough, the engine will likely fail, potentially
For most Cobra enthusiasts, the key question is
what compression ratio can be used safely with pump gas. I haven’t
been able to get a straight answer to this question since there are many
variables including the type of cylinder head, the ambient air pressure,
etc. Aluminum cylinder heads will dissipate heat more readily than iron,
which most sources indicate is good for about 1 point higher compression
ratio. If you’re building your car with a computer controlled ignition
system with a knock sensor, the computer can retard the ignition when
pre-mature detonation is sensed, allowing the engine to be run a little
closer to the edge. So, bottom line, on 92 octane pump gas about the
highest recommendation I found was a 10.5:1 compression ratio and most
sources felt 10:1 was safer. We’ve decided to run our engine at 9.8:1
to provide a little more margin.
Note that supercharging will increase the effective
compression in the engine since the intake air starts at a higher
pressure. The engine compression ratio must be reduced accordingly if a
supercharger is used to prevent detonation.
Connecting rods are arguably the most critical
component in a high performance engine since they take the maximum
stress, sometimes in excess of 200,000 psi. But good quality H-beam and
I-beam connecting rods are readily available that are forged from high
quality alloys such as 4340. There
doesn’t seem to be any significant preference to H-beam or I-beam as
long as they are well made. The more expensive rods maintain high
strength with lower weight, making them more applicable to higher
The strength of the connecting rod bolts is equally
important. These bolts attach the rods to the crankshaft journals and
subsequently are under as much stress as the rods themselves. The use of
high quality fasteners, such as ARP, is therefore highly recommended.
Our engine is built with forged H-beam rods and ARP
The standard cast crankshaft used in stock 302 and
351 engines can apparently take a surprising amount of abuse. These
cranks are used in many high horsepower supercharged and/or nitrous
engines and are rarely the cause of engine failure. The major limitation
of a cast crank is RPM. For engines operating at 7000 RPM or higher, a
forged (or billet) crank is recommended. It is probably also worth
considering a forged crank for heavily stroked engines since this places
more load on the crankshaft. We used a forged crank in our engine since
the stroke was increased from 3.5 to 4 inches.
If you’ve ever looked a ceiling fan running at
high speed, you’ve probably seen it wobbling as it spins. This is
caused by a difference in the weight of the various fan blades. Now,
imagine if the fan blades weighed ten times as much
and were turning at
6000 RPM and you have an idea of what is going on inside the engine as
the pistons, connecting rods and crank are rotating at high RPM. Any
difference in the weight of the pistons and connecting rods, and any
imbalance in the crankshaft, will cause considerable vibration. To reduce
this, the engine is “balanced” by actually measuring the weight of
the different pieces and shaving a little metal off here and there until
the weight of the pistons and rods match as close as possible.
Some additional off-center weight must generally be
added to balance the complete rotating mass. This can be done inside the
engine, or weights can be added externally to the flywheel and vibration
damper to compensate. Ford 302 and 351 engines are usually externally
balanced and require either a 28.2 oz/in or a 50 oz/in counterweight. In
1981, Ford switched from 28.2 oz/in to 50 oz/in for production engines,
but if you’re building a custom engine, it may be different. Our
engine required a 28.2 oz/in counterweight even though it’s a
relatively new block.
When an engine is running, each power stroke
creates a torque spike on the crankshaft. On a V8 engine, this occurs
ever 90 degrees of crank rotation. These torque spikes actually cause
the crank rod journal to twist slight and spring back, causing
vibrations in the crankshaft. To reduce the damaging effect of these
vibrations, a vibration damper is mounted to the front of the
crankshaft. This damper uses elastomer or fluid to dampen vibrations,
and also provides the appropriate imbalance (as discussed above) to
balance the rotating mass of the engine.
The oil pump is necessarily one of the most robust
components in an engine since an oil pump failure would quickly be
catastrophic. The oil pump bolts to bottom front of the engine block and
pumps oil from the oil sump (through an oil pickup) through the oil
channels of the engine.
There are three types of oil pumps available –
standard, high pressure, and high volume. It would be easy to assume
that a high performance engine should have a high pressure or high
volume pump, but in most cases this is not recommended.
As a general rule, the oil pump should be sized to
deliver 10psi of oil pressure per 1000 rpm. The exception to this rule
is for racing engines that have very wide clearances or that are running
at high rpm (8000 or higher). These engines require higher oil pressure
and volume to maintain adequate oil flow over all surfaces.
A high volume oil pump has larger impellers than a
standard pump to increase oil flow. A high pressure oil pump has
standard size impellers but has a heavier pressure release valve spring.
For most high performance street engine
applications, the standard oil pump will work just fine. If slightly
higher pressure is required, a small washer under the pressure relief
valve spring will typically solve the problem.
Oil pans come in many shapes and sizes. The primary
considerations in choosing an oil pan seem to be providing an adequate
oil reservoir, making sure the oil pickup can get a good supply of
non-aerated oil (i.e. not too many bubbles), and providing adequate
ground clearance. We elected to have a custom oil pan made for us that
was uniformly deep across the bottom but was a little shallower than oil
pans with a deeper front or rear sump.
A windage tray is an optional accessory that mounts
under the crankshaft. It prevents the crank from whipping the oil in the
pan into a froth. This provides two benefits – it makes sure the oil
can be easily sucked up by the oil pump insuring better oil circulation,
and it saves the wasted power from the crank whipping up the oil
(although this is pretty minor). It also tends to reduce the bubbles in
the oil as it drips down from the crank.
DSS Racing sells a nice windage tray that is
designed to mount to their main cap girdle. We’re using one of these
on our engine.
The stock 302 and 351W engines come with cast iron
cylinder heads. These heads are probably the largest liability when it
comes to getting the most performance out of the Ford small block. The
primary purpose of the heads is to flow fresh fuel/air mixture into the
cylinder and flow spent mixture out of the exhaust. To do this, the
heads must have a large smooth channel from intake manifold to cylinder
and cylinder to the exhaust manifold or headers. The stock cast iron
heads have small valves (1.84” intake, 1.54” exhaust) and poor flow
In our engine, for example, we expect to reach
approximately 525 hp at 5800 RPM with high performance ported heads. If
we changed back to the stock heads (requiring a reduction in compression
ratio to 9:1), the estimated performance would drop to 325 hp at 4500
RPM, everything else about the engine being the same. These estimates
are based on simulations using Dyno2000 (more on this later). Of course,
adding high performance heads without addressing other aspects of engine
performance is not a particularly good investment.
There are numerous aftermarket cylinder heads
available for the Ford small block. Since these blocks use the same
cylinder spacing and bore diameter, cylinder heads are more or less
interchangeable. To my knowledge, all the aftermarket heads are made
from aluminum instead of cast iron. The most significant benefit of this
is a substantial weight savings (typically greater than 50 lbs), but
another benefit is the ability to run with a slightly higher compression
ratio with less danger of premature detonation. This is due to the
improved heat transfer properties of aluminum.
We are using Canfield aluminum heads with CNC’ed
combustion chambers of 65cc to reach our 9.8:1 compression ratio. Other
popular aftermarket heads are made by Trick Flow (Twisted Wedge),
Edelbrock, Brodix, and World Products.
Aftermarket heads generally have larger valves and
smoother, larger paths for intake and exhaust. For example, the Canfield
heads we’re using have 2.02” intake valves and 1.60” exhaust
valves. Aftermarket heads are generally quite good right out of the box,
but they can be further improved by “porting”. Porting refers to a
number of modifications made to the heads including changes to the valve
seats, enlarging and/or smoothing the valve pockets, enlarging and
matching the intake ports to the intake manifold, enlarging and/or
smoothing the exhaust ports, etc.
One of the most beneficial procedures that falls
under the broad definition of porting is a “valve job”. Out of the
box, most heads have what is referred to as a single angle valve seat.
The valve seat is what the valve seals against in the combustion chamber
when it is closed. The edge of the valve is usually ground at an
approximately 45 degree angle and the valve seat is ground to match. The
relatively sharp angles result in turbulence as the fuel/air mixture
flows past to enter and exit the cylinder. This turbulence slows down
the flow, reducing performance.
By grinding the valves and valves seats at multiple
angles, such as a 30 degree top angle, 45 degree middle angle, and 60
degree bottom angle, gas flow will be much smoother. This is called a
three-angle valve job and has become the standard of the performance
industry. With the right precision equipment, even more angles can be
used to further improve flow smoothness, although improvements are
minimal beyond three angles.
Most machine shops that can do head porting offer
several options at increasing cost, depending on what you’re trying to
accomplish. For example Panhandle Performance, the shop we’re using to
port our Canfield heads, offers three “stages” of porting.
Stage I – 5 degree valve job and blending the bowl to the intake port.
Stage II – 5 degree valve job, match the intake port to the intake manifold and fully port the bowl, fully port the exhaust.
Stage III – 5 degree valve job, CNC the
combustion chamber, fully port the intake and exhausts.
Fully porting generally implies that the metal in
the intakes and exhausts are ground away to increase cross sectional
area and improve the smoothness of the flow path as it goes from the
ports to the valves.
One important thing to keep in mind though is that
there is such thing as too much of a good thing. The velocity at which
gases flow through the ports is important to achieve maximum
performance. If the momentum of the intake gases is high enough, the
intake valve can remain open long after the piston starts its way up
during the COMPRESSION stroke and additional fuel/air mixture can be
driven into the cylinder. This will be discussed further when we talk
about camshafts and valve timing. A good head porting shop will know how
to optimize the porting job to have just enough flow restriction to keep
the intake gas velocity at the optimum rate (approximately 700 ft/sec).
This is generally achieved when the minimum cross sectional area of the
ports is about 0.85 times the valve area.
Fuel Injection vs. Carburetion
A modest sized carburetor and dual plane manifold
is by far the most cost effective way to achieve good performance. And
if you’re trying to achieve authenticity in your Cobra replica,
carburetion is the only way to go.
But there are many advantages to fuel injection
over carburetion. Carburetors
are finicky devices that don’t adapt well to changing conditions. If
you live in a climate with significantly changing weather conditions, or
you take drives with elevations that vary more than a few thousand feet,
a computer controlled fuel injection system will deliver better
performance and drivability. If well tuned, fuel injection can also
deliver more consistent power and torque over the RPM range (although
not necessary higher peak power and torque), better fuel economy, and
Many will consider computer control as a negative
for fuel injection since it means that the engine cannot be tuned with
simple mechanical adjustments. However, as I am quite comfortable with
computers, I found this to be a significant advantage.
Besides authenticity, the primary downside of fuel
injection is cost. When you consider the increased cost of the fuel
injection intake manifold, the cost of the injectors, fuel rails, fuel
pressure regulator, electric fuel pump, and computer, the added cost for
fuel injection is typically a couple thousand dollars or more.
We decided early in our project to use fuel injection, so I spent little time researching the carburetion alternatives. Our car also has limited clearance under the hood, so our choice was to use a 302 block and possibly go with a supercharger, or use the taller 351W block and limit our choices to a handful of induction systems that would fit under the hood. We elected to go with the 351W block and use the excellent fuel injection system from TWM Induction, which cleared the hood (with scoop).
Fuel Injection System
Fuel injection works by atomizing the fuel directly
into the air stream as it flows into the cylinder. The amount of fuel is
precisely metered by controlling the fuel pressure feeding the injectors
and by precisely controlling when the injectors turn on and off. To
support this, the fuel delivery system is a little different than for a
carbureted engine. Typically, an electric fuel pump is used to deliver
fuel at high volume (60 gallons per hour) and high pressure (45 psi).
The fuel is delivered to fuel rails, which connect to the injectors, one
per cylinder. A fuel regulator, also connected to the fuel rail,
regulates the fuel pressure in the rails by returning excess fuel back
to the fuel tank using a fuel return line.
An electronic fuel injection (EFI) computer
monitors engine RPM, throttle position, intake air temperature, block
temperature, manifold pressure, and exhaust gas oxygen content to adjust
the fuel injectors on a continual basis. The computer can automatically
compensate for cold or warm starts, warm up, and acceleration
conditions, as well as the engine volumetric efficiency at different
RPMs to provide smooth throttle response and maximum performance under
If you’re building a high performance engine with
many aftermarket components, your performance potential will be
maximized with a computer that can be customized for your engines
characteristics. There are a number of such systems available, as well
as systems that work in conjunction with the standard Ford EFI computer.
We selected the Electromotive TEC-II system, which provides a great deal
of flexibility, but is relatively simple to install. The TEC-II also
incorporates a distributor-less (coil per cylinder) ignition system.
The most common type of Ford small-block EFI system
uses an intake manifold with separate runners per cylinder which connect
to a shared plenum which in turn connects to a shared throttle body,
mass air sensor (usually) and air intake. The injectors are installed in
the intake manifold close to the cylinder heads. This kind of system has
good efficiency and pollution characteristics, as well as smooth
acceleration and broad torque and power curves. It has the added benefit
that it is relatively easy to install forced air induction since there
is a shared air intake that can be connected to a supercharger or
A popular lower cost fuel injection option is the
Holley Pro-jection system. This system makes a good retrofit to a
carbureted engine because it replaces the carburetor on a conventional
manifold and includes an EFI computer.
Another type of fuel injection system has short
intake runners with a separate throttle body for each runner. An air
horn is mounted above each throttle body. Kinsler, Hilburn, and TWM
Induction are all examples of this kind of system. These systems offer
excellent power and throttle response although the torque and power
curves are not as flat as other EFI systems. Because they include a
throttle body per cylinder and a much more complicated mechanical
throttle linkage, these systems tend to be more expensive than the more
conventional fuel injection systems mentioned above. As noted above, we
selected the TWM Induction system.
Fuel injectors come in various sizes – the right
size is important to get maximum power and smooth operation. Too small,
and the injector will not be able to deliver adequate fuel at high RPMs.
Too big, and the computer will not be able to precisely meter the fuel
delivery, particularly at low RPM where the mixture will often be too
Fuel injectors are sized based on their static flow
rate in lbs/hr of fuel assuming a fuel pressure of 43.5 psi (3
atmospheres). The amount of fuel actually delivered by the injector
depends on the length of time the injector is open. The EFI computer
opens and closes the injector once per POWER stroke. The percentage of
time the injector is open is referred to as its duty cycle. Injector
manufacturers recommend that their injectors are run at a maximum duty
cycle of 0.8 (open 80% of the time).
Injector flow rate requirements are calculated by
the following formula:
Injector Flow Rate = Max HP x BSFC / (No. cylinders
x FI duty cycle)
BSFC is the Brake Specific Fuel Consumption and is
a measure of the amount of fuel required to achieve one horsepower. For
a typical naturally aspirated engine, BFSC is 0.5. A more efficient
engine will have a lower BFSC.
Using this formula, we can calculate that a 500 HP
8 cylinder engine will require 39 lb injectors if they injectors are run
at 43.5 psi at a maximum duty cycle of 0.8.
We decided to use 36 lb injectors even though our
simulations indicate that we should slightly exceed 500HP, since we can
always increase fuel pressure slightly if needed, and we’re more
concerned about drivability at lower RPMs than getting the absolute
highest power at maximum RPM.
Driving the Camshaft
The camshaft is driven from the crankshaft using a
chain or gears such that the camshaft makes one revolution for every two
revolutions of the crankshaft. Most engines are built with a timing
chain, but after-market gears are available for the 302/351W as well.
Gears have the advantage of more accurate valve timing since they
don’t suffer from stretching and harmonics as chains do, but they are
also usually considerably noisier (some people like the gear whine) and
If you elect to use a timing chain, as we did,
roller chains (such as Cloyes True) incorporate rollers in the chain
that actually roll as the chain enters and exists the sprocket. This
reduces the friction and wear on the chain and gears.
Most timing gear sets and chain sets include crank
sprockets with multiple keyways so that the valve timing can be advanced
and retarded to optimize performance. If the camshaft is properly
designed, this shouldn’t be required, but it does provide a way to
tune valve timing without replacing the camshaft.
The pushrod V8 engine has a set of sixteen lifters
that ride the camshaft lobes to control the opening and closing of the
intake and exhaust valves. Lifters are available in four varieties –
solid flat, hydraulic flat, solid roller, and hydraulic roller.
The flat tappet lifter is the simplest kind of
lifter. It has a flat face that rides along the cam lobe. The roller
lifter, on the other hand, has a small wheel at the bottom end that
rides on the cam lobe. Both flat tappet lifters and roller lifters are
available in solid and hydraulic varieties. The hydraulic lifter
includes an oil cavity and check valve that allows the lifter to adjust
its length automatically while the engine is running to maintain zero
valve lash in the valve train. Valve lash is “slop” in the valve
train when the valve is fully closed.
The obvious choice for a high performance street
engine is a hydraulic roller cam and lifter set. Because of the
geometric relationship between the lifter roller and cam lobe, roller
cams can be ground with considerably higher valve lift acceleration than
flat tappet cams and lifters. This means that the valves will open and
close more quickly allowing the timing to be controlled more precisely.
Roller lifters also benefit from reduced friction against the cam lobes.
By using a hydraulic roller lifter, ongoing valve adjustments are
dramatically reduced as well.
Since the camshaft lobe profile is different for
flat tappet lifters and roller lifters, roller lifters should only be
used with a camshaft designed for use with them. Likewise for flat
As you’ll recall, each valve has a rocker
associated with it that is mounted to the top of the cylinder head and
pivots in the middle. As the lifter pushes up on the pushrod, the other
end of the pushrod pushes up on the inside end of the rocker (the end of
the rocker closest to the middle of the engine). This forces the rocker
to pivot pushing the valve stem down and opening the valve.
There are also a variety of rocker types to choose
from. Rockers are generally mounted on studs that are pressed in or
screwed into the cylinder head. Threaded studs are recommended for high
performance engines, but another alternative is a shaft-mounted rocker
(a shaft goes through the all the rockers from one end of the cylinder
head to the other). Shaft mounted rockers have a little less play than
stud mounted ones, although the rocker studs can be stiffened by using a
stud girdle (a metal bar that clamps to the top of all the studs on one
Rockers are available with and without bearings at
the pivot point, and with and without rollers at the end that rests on
the valve stem. Obviously, rockers with rollers and bearings will have
the least friction and smoothest valve train operation, although will
also be the most expensive.
Rockers come in a variety of arm ratios. This
refers to the amount of valve travel that occurs for a particular amount
of pushrod travel. A rocker with an arm ratio of 1.5 will open the valve
0.15” when the pushrod moves up 0.1”. A higher rocker arm ratio is a
way to get increased valve opening from a given cam. Rockers are
available with ratios ranging from 1.5 to 1.8. You should be able to get
plenty of valve lift from 1.6 ratio rockers if you’re using a roller
The most complex part of designing an engine is
figuring out the valve timing. Most of the books I read provide very
little information about how valve timing is determined, or what the
compromises are, so I hope this section will be very informative.
There are four timing parameters that define how
your engine will operate. These are intake valve opening (IVO), intake
valve closing (IVC), exhaust valve opening (EVO) and exhaust valve
closing (EVC). It is interesting that camshaft vendors do not discuss
these timing events when describing their products. It’s almost like
they are trying to keep camshaft selection a black art. But it is
relatively easy to derive these parameters from the specs supplied by
camshaft vendors (lobe center angle (LCA), intake centerline (IC),
intake duration (ID), and exhaust duration (ED)) assuming all these
parameters are specified.
IVO = ID/2 – IC
IVC = ID – IVO – 180
EVO = ED – EVC – 180
EVC = ED/2 – 2*LCA – IC
The simplified view of how a four stroke engine
works was discussed early in this article, but this description really
just scratches the surface. To really appreciate how an engine works,
and how to get the most performance, we must talk about wave dynamics.
But I should warn you that even this discussion is a simplified view of
engine operation. As gases move in and out of an engine, they are
constantly compressed and expanded, heated and cooled, with laminar and
turbulent flow. Each valve edge, bend in a pipe, gasket, fitting,
thermal change, etc. has an affect on how these gases flow and will
affect the behavior of the engine. Even complex computer simulations
cannot fully predict engine behavior, but they can come pretty close.
When valves open in an internal combustion engine,
gases don’t just flow smoothly into or out of the cylinder. There is
usually a significant pressure differential between the two sides of the
valve when it opens. This causes a sudden acceleration of gas molecules
that form a pressure wave. This is similar to an acoustic wave caused by
clapping your hands, but the pressure waves have thousands of times
higher pressure differentials.
But the pressure waves still behave in much the
same way as acoustic waves. Pressure waves can be positive compression
waves, or negative expansion waves (sometimes called rarefaction waves).
The behavior of these pressure waves in a pipe is very important to
understanding engine performance.
When a pressure wave traveling down a pipe
encounters a closed end (such as a closed valve), it will be reflected
back in its original form (i.e., a compression wave is reflected back as
a compression wave). But when a pressure wave encounters an open end
(such as open headers), it is reflected back “out of phase”, so the
reflected compression wave becomes an expansion wave. These reflected
waves can be used to great value in optimizing engine performance.
Valve timing events are referenced to TDC (top dead
center – the piston is at the top of its travel) and BDC (bottom dead
center – piston at the bottom). If a valve event is specified as 20
degrees ATDC, this means that it occurs when the crankshaft has rotated
20 degrees past (after) when the piston was at TDC. Likewise BBDC means
crankshaft degrees before bottom dead center.
In a simple engine model, we’d expect the exhaust
valve to open at the end of the POWER stroke when the crank was at BDC.
The piston would then force the exhaust our of the cylinder during the
EXHAUST stroke. It turns out that this valve timing is very inefficient.
By the time the crank has reached 25 to 30 degrees past TDC during the
POWER stroke, almost all the power has been transferred to the crank. By
opening the exhaust valve (EVO) during the middle of the POWER stroke,
we can take advantage of the residual pressure in the cylinder to start
to blow the exhaust our instead of forcing the piston to pump the
exhaust out. Of course, there’s a delicate balance between the power
wasted by opening the valve too early and the power wasted by forcing
the engine to pump out the exhaust.
But there’s an added benefit of early EVO. The
high pressure in the cylinder when the valve opens will cause a strong
compression wave to be generated out the exhaust port. This compression
wave will reach the end of the headers and reflect back as an expansion
wave. If this expansion wave reaches the cylinder before the exhaust
valve closes, and can further assist in removing the last remnants of
exhaust from the cylinder and even assist in starting with the intake of
fresh fuel/air mixture as we’ll discuss below.
I mild street cam generally sets EVO at 65 to 66
degrees BBDC, while an aggressive racing cam might set EVO as much as 85
degrees BBDC (although keep in mind that this is when the valve just
starts to open, not when significant flow can occur).
The next valve timing event to occur is the intake
valve opening (IVO). Note that this occurs before the exhaust valve is
closed. IVO is the least sensitive of the valve timing events, but an
earlier valve opening can benefit from a broad expansion wave from the
exhaust system to help accelerate the air/fuel mixture. If an expansion
wave is not present, early IVO timing will allow exhaust gases to flow
into the induction system since the cylinder pressure will almost
certainly be higher than the intake pressure. This is called reversion
and will have a damaging effect on performance by contaminating the
fresh fuel/air mixture and heating it up (making it less dense).
A typical mild street cam will open the intake
valve around 10-12 degrees BTDC. The IVO for an aggressive race cam will
be as early as 50 degrees BTDC. For a high performance street engine,
the benefits of going beyond 20-25 degrees BTDC do not seem to outweigh
the risks of reversion at lower RPM.
The next valve timing event is EVC, exhaust valve
closing. This determines the end of the overlap period (when both valves
are open) and, of course, the end of the exhaust cycle. If a strong
scavenging wave from the exhaust system is present, a later EVC can
provide significant help in drawing in the gasses from the intake. With
properly tuned headers, the scavenging expansion wave will be at its
peak at the RPM that delivers maximum power, further increasing power.
But at lower RPMs, this expansion wave will arrive early and will be
followed by a positive compression wave. If this compression wave
arrives before EVC, reversion will result, significantly affecting
performance. This is why “hot” cams that are designed to maximize
high RPM horsepower have such poor idle characteristics.
Exhaust valve closing typically occurs around 10
degrees ATDC with a mild street cam and can occur as late as 50 degrees
ATDC on a hot race cam. Typical high performance street engines will
have EVC at around 30 degrees ATDC.
The final valve timing event is the intake valve
closing. This is probably the most important valve event and the most
sensitive to the induction system used on the engine. The more fuel/air
mixture that can be forced into the cylinder, the higher the performance
will be. So IVC is normally delayed until well into the COMPRESSION
stroke. But if IVC is delayed too far, the building pressure in the
cylinder due to the piston upswing will exceed the induction systems
ability (through pressure waves and gas molecule momentum) to hold back
the pressure and fuel/air will flow back out of the cylinder.
As with the exhaust, a pressure wave will be
generated in the intake as well. In this case, an expansion wave is
generated although will less amplitude than the exhaust pressure wave.
The strength of this wave will be determined by the amount of suction
that can be created in the cylinder resulting from the piston downswing
and the exhaust scavenging wave.
When the expansion wave reaches the end of the
intake runners (or the top of the air horns in they EFI system we’re
using), it is reflected back as a compression wave. By the time this
wave reaches the cylinder, the intake valve is closed and the wave
bounces back out. This wave continues to oscillate in the intake system
until the next time the intake valve opens. Since the length of the
intake runners are typically significantly shorter than the exhaust
headers, the frequency of the pressure wave is considerably higher –
usually two to three times higher – so by the time IVO occurs, the
wave has bounced back and forth several times.
As with headers, the intake system must be tuned
for a particular RPM to deliver the most benefit from this pressure wave
oscillation. The air horns on some induction systems (Webers, TWM,
Kinsler) are designed to spread the reflection wave so that it will
provide benefit over a broader RPM range.
Intake Valve Closing is typically set at around 60
degrees after BDC on a mild street came, and as much as 85 degrees ABDC
(almost to TDC) on a very hot race cam. An engine with this kind of hot
cam will have a very narrow power peak and be designed to run at very
high RPMs. For a high performance street engine with a well tuned
induction system, IVC should be 65 to 70 degrees ABDC.
Open headers will produce a sharp pressure wave
reflection resulting in a strong scavenging effect. But because the
reflection is sharp, the resulting expansion wave will reach the
cylinder at exactly the right time only within a relatively narrow RPM
range. The reflection wave can be broadened by using a collector (i.e.,
a pipe with larger diameter than the header). Instead of a single sharp
reflection wave, a lower energy wave will reflect at the header to
collector transition, and another reflection (again lower energy) will
occur at the end of the collector. If the collector is approximately
half the length of the headers, the reflection wave fronts will tend to
act as a broad expansion wave and provide good scavenging across a
fairly wide RPM range. A muffler inside the collector will tend to
further dissipate the reflection wave and will reduce its effect but
also further spread the RPM range where it will be beneficial.
The optimal length and diameter for the headers is
difficult to determine without complex simulations, but is good ballpark
estimate is provided by the following formulas:
Header pipe length (in inches) = ((850*(360-EVO))/RPM – 3
Header diameter (in inches) = ((cylinder. disp. *
16.38 / ((hdr len + 3) * 25))) * 2.1
These formulas are from A. Graham Bell’s
Performance Tuning in Theory and Practice.
For a street engine, the RPM used should be the
peak torque RPM. For a race engine, the peak hp RPM should be used.
The quality of the nuts, bolts, and studs that are
used to assemble the engine have as much affect on the durability of the
engine as any of the parts. At a minimum, grade 8 rated fasteners should
be used. One company that specializes in fasteners for high performance
engines is Auto Racing Products, also known as ARP. ARP has a selection
of components which exceed grade 8 strength levels for almost every
fastener application in the engine.
Engine Simulation Software
software is now available to simulate engine performance, taking into
account virtually all the issues discussed above. A relatively low cost
package, called Desktop Dyno2000, is easy to use and very effective at
showing the relative performance of design changes. This package does
not have quite the flexibility needed to model every unique part, but it
will provide a general idea of what you can expect from your engine.
This package is available for about $50 from Motion Software, Inc.
A graph produced by Dyno2000 showing the estimated torque and horsepower for our engine is shown in the Car Details section of our site.
A more comprehensive wave analysis package is available from V.P. Engineering called Dynomation. But this package sells for considerably more ($600) and appears to be only available for DOS. We have not used this package.