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 article.

Engine Basics

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 article.

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 at http://www.keveney.com/otto.html.

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 this article.

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.

Engine Pricing

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 engine.

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 comes from.

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 chamber.

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 damaging detonation.  

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 variables.

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 catastrophically.

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

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 revving engines.

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 fasteners.


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.

Engine Balancing

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.

Vibration Damper

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.

Oil Pump

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 Pan

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.

Windage Tray

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.

Cylinder Heads

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 characteristics.

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 lower emissions.

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 all conditions.

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 turbocharger.

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

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 rich.

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 more expensive.

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 tappet lifters.


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 cylinder head).

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 cam.

Valve Timing

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

PC 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.