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An Introduction to Sport Compact Cylinder Heads

If the engine block is the muscle, then it’s safe to say that the cylinder head is the brains behind the operation. This isn’t to speculate that it’s the more important component of the modern-day internal combustion engine, but it’s arguably the more complex and misunderstood of the two halves. Sure, the bottom end determines overall displacement and engine speed characteristics through crankshaft stroke, connecting rod length, and rod ratio, but it’s the top end that dictates how much, and when, both air and fuel will ultimately enter the cylinders for combustion to take place. You could have the biggest, baddest short block around, outfitted with the fanciest crankshaft, the most expensive connecting rods and top-of-the-line pistons, but without the proper camshaft, valvetrain and cylinder head combination you’ll likely just be spinning your wheels – figuratively.

As if talking automotive high performance doesn’t stir up enough controversy – what with all of the bench racers, so-called Internet chat room authorities and self-proclaimed experts – there are few more debated topics when concerning the internal combustion engine than that of squeezing more horsepower out by means of the cylinder head. Perhaps the reason is that top ends are simply misunderstood. Maybe, but that doesn’t explain the sometimes significant differences of opinion between many in the industry who’ve been messing with cylinder heads long before some of the cars in this book were even conceptualized. So what’s the point? There are often multiple ways to achieve the same end result and, in light of this, it’s important to keep an open mind. Some of the theories discussed in this book may contradict what you’ve learned in the past, but it’s important to hear them out. Now on with it.

Most are probably aware of what the cylinder head is: the detachable, upper half of the engine that creates a seal against the engine block’s cylinders. Most contemporary cylinder heads are constructed of aluminum and house the camshafts, valvetrain components, and spark plugs. Their design implements a series of ports on each side that coincide with the number of cylinders of the engine. Four cylinder engines will have four intake ports and four exhaust ports. Also integrated into the head is the combustion chamber, the place where the air entering the intake ports is directed. It all sounds simple enough but what may confuse some, however, are the inner workings of the head itself and how exactly the cylinder head does what it does it.

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The Cylinder Head’s Job

It’s been said before, and it’s worth saying again, the internal combustion engine is really just a glorified air pump. In fact, the four-stroke internal combustion engine spends half its time pumping air in and out. In the case of our air pump –or engine – in order to pump air, there are four required elements: the air itself, as well as fuel to burn, a spark, and the ability to compress everything together. The cylinder head plays a vital role here in ensuring that each of these four processes occurs and to whatever degree of success each will ultimately have.

Airflow Explained

The goal with any engine is to introduce as much air into the cylinders as possible and to release as much burnt gases out as possible with the least amount of mechanical effort. The second part of that sentence is key. While certainly not all engines are designed with maximum power output at the forefront, pretty much any serious engine we can think of was designed to produce its given amount of power utilizing the least amount of work possible. In respect to airflow, the larger and more optimally shaped the air passages are within the head, the less work the rest of the engine has to do, and less work usually translates into more horsepower. However, when small or poorly shaped ports are used to draw air in and out of the cylinders this creates additional work for the engine. This type of work that doesn’t contribute to spinning the internals, which in turn move the car, is referred to as pumping loss. To make it simple: pumping loss robs horsepower and is bad.

Volumetric Efficiency Explained

Equally as important as reducing the effects associated with pumping loss, volumetric efficiency is also the key to producing maximum horsepower. The volumetric efficiency of an engine is the percentage of an engine’s displacement that fills with air during the intake stroke. In other words, it’s a quantifiable figure of how well air is being moved into the engine at any given engine speed. The last part of the sentence is important in that an engine’s volumetric efficiency often changes throughout the RPM range. Take a 2,000cc engine for example, in order to operate with a volumetric efficiency of 90 percent, the engine must inhale 1,800cc of air during each intake stroke. The reason volumetric efficiency is important has to do with the direct correlation between the mass flow rate of air into the engine and its potential horsepower. Introducing more air into the engine allows more fuel to be burnt, which can, and usually does, result in more horsepower. It’s important to note here that despite equal volumetric efficiencies and engine characteristics, horsepower can vary greatly between two engines. This is because altitude and air density must also be considered. To make a long story short, the engine closer to sea level will produce more power because the air density is greater there or it has more mass.

As you might expect, volumetric efficiency is inversely proportional to pumping loss. That is, as pumping loss decreases, volumetric efficiency increases. Overall, it’s safe to say that the volumetric efficiency of an engine is one of its most important parameters as it dictates the engine’s power producing capabilities through quantifiably measuring its effectiveness of introducing air.

It’s important to understand here that volumetric efficiency measures the mass flow of air into an engine and, as such, can exceed 100 percent provided the air density is high enough. But most modern production vehicles fall into the 80 to 90 percent volumetric efficiency range. It’s easy to assume that through proper cylinder head and valvetrain modifications that 100-percent volumetric efficiency may be obtained. While this would be a true enough statement, it poses far too much limitation. In fact, volumetric efficiencies that well exceed the 100-percent barrier are not uncommon in the high-performance realm; the key lies in creating manifold induction pressure above ambient air pressure. In other words, bolting on a turbocharger or supercharger, which both create positive pressure within the intake manifold, will net a volumetric efficiency in excess of 100 percent. Even naturally aspirated powerplants making use of oversized valves and optimally shaped ports are known to eclipse the 100-percent mark. The key in these cases lies with beginning the intake process prior to the conclusion of the exhaust stroke and ending it after the compression stroke has already begun. The resulting inertia poses results similar to what is typical of forced induction.

Intake and Exhaust Port Shapes, Volumes and Their Effects

Unfortunately, engine designers are faced with a variety of obstacles besides optimizing airflow and are often forced to make compromises in the name of structural integrity, heat flow, fluid flow, and thermal expansion characteristics. In other words, the desire for the head to fit under the hood of the car, not crack, and not overheat generally supersedes that of performance. The results of these compromises often sacrifice the engine’s potential volumetric efficiency, but are in fact necessary evils most of the time. In order to achieve the most optimal flow however, some manufacturers design the ports first and then construct the head around them. This method almost always produces better results in comparison to constructing the head first and then implementing whatever port shape or size fits within the design.

As mentioned, the cylinder head exists to both permit and restrict air into the cylinders. It accomplishes this task through a somewhat complex valvetrain system consisting of oscillating poppet valves each controlled by a rocker arm which is in turn dictated by the camshaft (more on valvetrain and camshafts in later chapters). While the poppet valve is arguably the best design automotive designers have been able to come up with so far, the valve itself poses a restriction within the ports of the head. In closed position, the valves restrict both air and fuel (except for direct injection engines) from entering the combustion chamber during certain points in the engine cycle. But even when fully opened, both air and fuel are still unable to freely pass the valve without restriction; the valve itself remains an obstacle both air and fuel must travel past. The same is true for the exhaust side, where burnt gases are still restricted by the valves characteristics. A number of factors determine how well air is able to enter the cylinder: intake port shape, size and texture, as well as valve shape, size and placement.

In a perfect world, with a perfect cylinder head, the valve seat would be oriented at a right angle with the intake port’s centerline. If you’ve ever looked at a poppet valve, you know this is impossible and that the nature of the poppet valve requires its head to be located right in the middle of the port. This means that the head of the valve is the port’s biggest restriction. And since the valve can’t be removed the next best choice is to optimize the port to flow around the valve as best as possible.

Unless you have a concrete understanding of compressible fluid flow dynamics, the actual path air takes to travel around the valve head is quite different than one would expect. While assuming that air enters and travels within the port in straight lines, following the shape of the port along its walls, may sound like a good idea, it’s simply not the case. In fact, once the air hits a bend, irregularity, or bump, it loses all orderly behavior. This turbulent behavior causes the air to tumble, slow down, and separate from itself. With this understanding of how sensitive airflow is to the shape of the port, it’s important to note here that often times the port’s shape is often more important than its size. The truth is: the manner in which the ports are designed to accommodate the valves and other valvetrain components makes for a somewhat convoluted and restrictive shape. While some heads are certainly better than others, generally the air must travel past a somewhat steep bend once inside the port in order to make the turn down past the valve and into the combustion chamber. Better flowing heads tend to alleviate this bend or at least make it less pronounced.

Aside from various sizes, there are a number of different port shapes: square, rectangular, D-shaped and round, just to name a few. While no single port shape can be classified as the best, each certainly has its own advantages over the rest. Take the square or rectangular port for example, because of their shapes, they allow more area for air to flow through in comparison to the circle port. In contrast, circle ports allow for slower air speeds translating into a more stable, less turbulent airflow path, which, in turn, will allow more useable air mass through. D-shaped ports are a bit of a compromise. In an effort to create decreased velocity and increased pressure along the lower portion of the port, a square shape is implemented. The rounded portion of the top of the port creates the opposite effect. This comes in handy when a port makes a turn or a bend. As the port bends, the lengths of the top and bottom of the port itself will vary. Altering the volume, thus air speed, through the D-shaped design will trick the port into thinking both sides are the same length.

Finally, in discussing various port types, it’s also important to consider a port’s angle relative to the valve. In this case, straighter is better however, as you’ll see when we discuss valves in more detail, a straight port is impossible because of the valve itself.

Speaking of valves, now is a good time to discuss what happens to the air once it nears the valve head. In short, the air forms a cone shape as it breaks apart and travels around both sides of the valve. As the velocity of the air increases, the cone shape shortens. Once the air collides with the valve head it’s directed away from the valve stem thus forming its cone shape. Another cone is formed when the air hits the valve’s rim.

Porting and Polishing

Cylinder head porting is the process of reshaping either the intake or exhaust ports, often times enlarging them in the process. The goal is to reduce the effects of pumping loss through straightening the ports in relation to the air’s entry point and the valve seat. The result of a properly ported head is a more streamlined shape with a less turbulent path for the air to travel. The goal when porting is to avoid enlarging the ports excessively. Often times, all a head needs for improvement is some minor reshaping or straightening out.

Going too big will do two things: it will generate larger flow numbers, something that may appear positive on the surface, but it will do this at the expense of air velocity. As the air’s velocity drops so does its inertia, or its ability to have as much of an effect. As a result the cylinder will not achieve an optimum fill at lower engine speeds and may perhaps only benefit inside a narrow high-RPM window. As you’ll see with the ill-effects of oversized camshafts in chapter three, oversized ports can also affect drivability due to poor idle and low-speed responsiveness due to less efficient atomizing of the air and fuel. The goal then is to achieve maximum flow with minimal enlarging. This is definitely easier said than done and may require several hours of work as well as much flow bench testing. To make matters more confusing, remember that different engines will respond in different ways. While sacrificing velocity for flow is generally not a good idea, there are some engines that will require more flow than others without posing much of a loss as far as velocity is concerned.

As you’d expect, not only will different types of engines respond differently to various porting methods, an engine’s method of induction will also create different responses. Beginning with naturally aspirated engines, these have a tendency to respond well to larger ports that are capable of flowing more. As expected, velocity is sacrificed in this case but, when performed carefully and properly, the benefits found in the upper RPM range will outweigh the negative results found at the low end. This scenario is especially true for naturally aspirated drag racing engines, where low-RPM performance is of little or no concern.

Turbocharged, supercharged, and nitrous oxide-fed engines are an entirely different story. In fact, contrary to what some believe, they’re even different from one another. Turbocharged engines generally respond well when air velocity is maintained relatively high. The results are less turbo lag, which creates a more usable power curve. Port volumes on these engines are kept relatively small, whereas reshaping and straightening play more of an important role here.

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Supercharged engines are quite different. In this case, larger ports with more volume are a plus. The added flow of the larger ports reacts well with the more linear and overall flow volume provided by the supercharger itself. Nitrous engines are quite the same. Unlike turbocharged engines that generally have windows of flow that are significantly higher than they are at other points, like the supercharged engine, the nitrous engine features a more overall flow volume which can significantly benefit from ports that are able to take advantage of this by offering more flow capabilities. As for your street car, where drivability and low-RPM responsiveness are what you care about, it’s important to keep air velocity at a maximum by maintaining a relatively small port size.

Combustion Chamber Shapes, Volumes, and Their Effects

While all cylinder heads share the same task, there are certainly a number of varying types. Differences range in cylinder numbers and orientations, material selections, combustion chamber configurations, and camshaft orientation. A number of different methods and types have been introduced over the years, each with their own advantages and disadvantages according to the times.

Overhead Valve Engine Combustion Chambers

Camshaft-in-block engines, or OHV (overhead valve) engines, are few and far between as far as being mass produced today but thousands still roam the highways. The camshafts in OHV engines reside inside the engine block, just above the crankshaft. These engines rely on a series of pushrods to open and close the valves and are quite different in comparison to OHC (overhead camshaft engines. Without going into too much detail on these heads, it’s important to give a brief description of them in order to make a solid comparison against their more modern OHC counterparts.

There are really three types of OHV heads: the L-head (also known as the flathead), the I-head, and the F-head. The valves are placed within the engine block alongside the cylinders on flathead engines. The advantage of these heads in comparison to OHC versions is that they are arguably simpler in design. In comparison to its fellow OHV engines they don’t have pushrods which also lead to a simpler design. But aside from simplicity, these heads don’t offer much more. Their 90-degree ports, hence the L in L-head, put a serious damper on engine breathing. The only place you’ll find one of these heads offered today is in a lawnmower.

Next up are the F-heads, and no, F doesn’t stand for flathead. The intake valves are located above the engine block on these heads and are operated by pushrods. The exhaust valves however are located within the engine block, beside the cylinders, like the flathead. As expected, the intake characteristics of these heads were improved in comparison the flathead but they still fell short on the exhaust side.

The most successful OHV engine, and the one still used today, is the I-head engine. On I-heads, both the intake and exhaust valves are placed within the cylinder head and are operated by pushrods that oscillate parallel with the cylinders. In comparison to its predecessors, these heads flow better on both the intake and exhaust side. In comparison to OHC engines, they often times offer better, smaller packaging because of the block-located camshaft, simplicity and a slight power bump due to the absence of parasitic drag from a belt or chain-driven cam. The bad news for OHV engines in general is that engine speed is severely limited by the flexibility of their pushrods. Valve float, as well as broken pushrods, becomes an issue at engine speeds significantly lower than what OHC engines might see on a regular basis. Lastly, OHV heads are limited on the number of valves they can fit per cylinder – a big problem when the idea is to introduce as much air in as possible. Enough with the history lesson; since we’ll only be covering inline, aluminum, OHC cylinder heads in this book, we’ll limit further head classification to combustion chamber types.

The cylinder head’s combustion chamber is where the air/fuel is compressed and burned. Once ignited, the formed gases occupy far more space than in their liquid state and react against the piston, compressing it down the bore. This in turn spins the crankshaft and makes the car go. This recessed area within the cylinder head that contains the valves and spark plugs, referred to as the combustion chamber, plays a vital role in the four-stroke engine process. The combustion chamber’s shape and size both play significant roles in dictating the type of explosion, in turn controlling how forcefully the piston will be pushed down. The results are indicative of how much power the engine will ultimately produce. The combustion chamber’s shape is cast into the cylinder head and is formed with the piston’s shape in mind. When the piston is at TDC, the allowable volume of air/fuel within the combustion chamber is at its smallest point. Although a number of factors affect flame propagation, its role is just as important as that of spark plug orientation, airflow characteristics, piston shape, and valve location.

Hemispherical Combustion Chambers

Hemispherical, or just plain hemi, chambers are shaped like half of a sphere and cast to the underside of the cylinder head. Hemi heads allow for the valves to be placed around the outside of the chamber’s bore at varying angles in relation to the crankshaft’s centerline. They also allow for much larger valves in comparison to other designs since they’re placed on opposite sides of the head. The advantages of the hemi head are numerous. First, their flow capabilities are second to none due to the fact that the valve angles may be positioned most ideally, moving the valve away from the chamber wall to avoid turbulence. Second, the chances of exhaust gases contaminating the intake charge during valve overlap are greatly reduced with these combustion chambers. The exhaust ports on these heads can remain short, which is a good thing, and they generally feature centrally located spark plugs – a really good thing.

Hemi heads also allow for ample distance between the intake and exhaust valves to allow for reduced heat transfer on the intake side. The hemi combustion chamber solved a lot of problems its flathead counterpart faced: its surface-to-volume area was decreased and it was able to host larger valves. The relationship between the surface area of the chamber and its volume is an important one. The more surface area there is in relation to the size of the chamber, the more heat will be given up. This results in an unstable burn of the air/fuel mixture.

Pent-Roof Combustion Chambers

The chamber of choice for four-valve per cylinder heads has long been the pent-roof design, which shares most of the advantages of the hemi chamber and then some. In a pent-roof configuration, the intake and exhaust valve heads face in opposition to one another, forming a V shape. The configuration allows for four valves per cylinder, something a hemi head simply cannot afford. This design has also long proved ideal in that the spark plug is most optimally located right in the center of the four valves. Pent-roof heads also have a much smaller surface-to-volume ration in comparison to hemi chambers. This means that less heat is lost during combustion and flame front traveling distance is also shorter. But, all in all, pent-roof chambers differ little from the hemi design.

Wedge and Heart-Shaped Combustion Chambers

While the pent-roof chamber has proved to be the most optimal design for internal combustion engines to date, it’s worth mentioning some of its predecessors. The wedge head also gets its name from its shape. These heads don’t feature centrally located spark plugs, rather the plug is located off to the side of the valves, which themselves form a line. The shape of the wedge design is actually supposed to promote a swirling of the air down the cylinder and is perhaps why this is one of the more popular non-pent-roof designs used. Like pent-roof chambers, the valves in wedge chamber heads are not oriented vertically but they come down only from one side. Heart-shaped chambers account for everything else, that is, those that aren’t hemi or wedge-shaped and is still used by Chevrolet today on the Corvette LS1. Its heart shape allows for two squish regions and their spark plug orientation is almost centrally located.

Improving the Combustion Chamber

Unless you’re trying to lower your compression ratio, you’ll generally want to refrain from removing too much material within the combustion chamber. The larger the chamber, the more difficult it will be for the flame to travel and the more material there will be to heat up. The key is to remove only enough material to prevent valve shrouding (which we’ll discuss shortly) and to smooth out any sharp edges that may lead to detonation. Of course adding material to the combustion chamber is also an option. Doing this will promote higher compression, but must be taken on cautiously as combustion chamber shape will obviously be altered in the process.

Spark Plug Orientation Matters

Spark plug orientation is important and is often taken for granted since there’s not much you can do to change it. The spark plug exists to ignite the air/fuel mixture; it goes without saying that there is going to be an optimal location for this to all happen. That optimal location just happens to be the center. Placing it anywhere else will create a longer distance for the flame front to travel, thus prolonging complete combustion. Placing the plug in the center allows for the best compromise for the flame front coming from any direction. This makes for a fast burn and efficient combustion all with less spark advance but with equally impressive power output.

Valve Jobs and Airflow

In regards to airflow, the valve seat is probably the most crucial yet problematic area of the cylinder head. It’s at this point that the traveling air stream is forced to make the most abrupt change as it turns to make its way from the port down into the combustion chamber, or out of the chamber and into the port if we’re looking at the exhaust side. Needless to say, this is one of the best places to go looking for improvements when it comes to cylinder head modification.

Now is a good time to discuss valve jobs, specifically, what they are and the different types offered. As mentioned, the valves open and close in a timed fashion in order to introduce and release just the right amount of air and fuel at just the right times. But, interestingly enough, a good valve job, that is, the cut of the seat that the valve actually seals to, can have a significant impact on flow. That is precisely why we’ll discuss valve jobs at this time.

A proper valve job is important for a number of reasons. First, it’s important to understand that an engine’s valves remain fully open for far less time than they do closed or close to their seats at lower lift points. It’s for this reason that a good valve job can have such a positive impact on an engine’s overall performance. Without getting into the full details of the four-stroke engine process yet, which we’ll cover in chapter three, keep in mind that the valves spend their least amount of time at full lift. It only makes sense then to do things that will maximize their flow potential at the points in the cycle where they spend significant amounts of time. 

Factory valve jobs generally must make a compromise between cost effectiveness and performance. For this reason, most factory valve jobs leave plenty of room for improvement. Most of the time, the valve seat is cut at a 45-degree angle. Occasionally, additional cuts are made in the name of more airflow, but these single-angle valve jobs just aren’t the best way to go. These one-angle valve jobs that focus solely on the seat surface do so at the expense of flow. In addition to one-angle valve jobs, some factory equipped vehicles feature two-angle valve jobs, which are a little better. In these cases, a rough cut is made to the port itself where it comes into contact with the valve. Blending this portion of the port, referred to as the throat, with the valve seat is certainly a step in the right direction since flow is increased due to an ease in the airflow path, these still leave room for improvement.

Three-angle valve jobs are the norm as far as high performance is concerned. Like the two-angle valve job, these feature a cut where the port meets the seat. The degree to which these are cut is much more than that of the 45-degree seat cut, generally in the neighborhood of 60 degrees. The second cut is the cut found on both the one and two-angle valve job, the 45-degree cut on the seat. This is where the valve actually seals. The third cut is made just after the valve seat and is always a smaller angle in comparison to the seat cut, generally measuring in at around 30 degrees. A good valve job will alleviate any sharp edges by blending them together. Sharp edges can create hot spots, which may lead to detonation.
While three-angle valve jobs are certainly the most common, there are those who prefer five-angle and radius type valve jobs. A five-angle valve job simply adds an additional cut to each side of the seat allowing for a less pronounced, smoother overall shape. Radius valve jobs are actually quite similar to five-angle valve jobs. In this case, the angles next to the valve seat are blended to either the port or the combustion chamber for the ultimate in smoothness and flow.

Deshrouding Explained

Deshrouding the valve area is a common method employed for increasing air turbulence. Shrouding basically describes the obstruction of airflow created by the cylinder walls, which is a bad thing. A number of factors can increase valve shrouding and sacrifice performance, one of which is oversized valves. But this isn’t to imply that one shouldn’t opt for oversized valves, which clearly have their benefits. The solution is to simply deshroud the valve area within the combustion chamber. Deshrouding helps keep the cone-shaped airflow intact and undisturbed. Manufacturers have already tackled this problem for us in some ways. Both the hemi and pent-roof combustion chambers, which open their valves toward the center of the cylinder, allow plenty of room between the valves and the cylinder for flow. Another solution, though often impractical, is to increase the engine’s bore size. This, along with deshrouding the area within the combustion chamber, can significantly increase flow. Be careful when doing this without overboring as the result could expose the headgasket to combustion as well as pose a serious restriction. As you’d expect, different engines will respond in different ways in regards to deshrouding.

The Benefits of Squish

Just prior to the piston reaching TDC during the compression stroke, some of the unburnt air/fuel mixture undergoes a squish process. As the piston makes its way upward, some of the air/fuel mixture migrates toward the edge of the piston. As the piston gets closer to the cylinder head, this leftover air/fuel mixture is squished out and forced into the main mixture within the combustion chamber, the easiest, yet most beneficial, place for it to get to. This process creates a turbulent effect that aids in mixing the entire air/fuel mixture together, effectively allowing for more complete combustion.

This all occurs in the quench area, or quench zone, of the cylinder head, which is the flat area in which the piston comes in extremely close contact with at TDC. Despite similar combustion chamber sizes, some heads have very little quench area while some have very generous quench areas. Quench areas are implemented to squish the air/fuel mixture away from the edges of the combustion chamber and toward the spark plug. This increased turbulence and redirection of the air/fuel mixture results in a more complete burn and reduces the chances of detonation. Without an ample quench area, the air/fuel mixture is liable to move toward the outer edges of the combustion chamber where it is otherwise wasted since it never ignites with the spark plug. As is the case with volumetric efficiency and airflow itself, more is better and a generous quench area will result in a more efficient and usually more powerful engine.

Swirl Explained

Similar to quench, the premise behind promoting swirl within the cylinder is to achieve a more complete combustion and improved power output. Swirl may be achieved through careful port-shaping techniques that allow the air to flow past one side of the valve and not the other. This may be achieved either by implementing a curve into the port or even a bump or deflector of sorts. The results allow the incoming air to enter the cylinder from a different angle thus bumping into the cylinder walls or combustion chamber walls creating a spiral or swirl effect.  A proper swirl will tend to hold its motion all the way down into the cylinder. Swirl is definitely tricky to achieve and ever more difficult to do right. When implemented improperly, as the piston travels upward, the air/fuel mixture can actually become ever more turbulent and more unstable than without swirl. Additionally, swirl is also often achieved at the expense of flow. Since airflow is only aimed at one side of the valve during this process, volumetric efficiency may drop. This is not always the case however and swirl has certainly proven itself to be a powerful tool.  


Written by Aaron Bonk and posted with permission of CarTech Books

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