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This book is dedicated to a singular purpose—to investigate the opportunities that exist to build a large displacement small-block Chevy. Horsepower and torque depend on several situations that exist within an internal combustion engine, but one crucial component is displacement. Compute the volume of a cylinder based on the diameter of the bore and the length of the piston’s stroke and you have a displacement in cubic inches. Multiply that by its number of cylinders, and you have size of your engine (see the “Calculating Displacement” sidebar). We’re going to investigate how to stretch the conservative boundaries of that original 265ci small-block Chevy and see how many extra inches we can squeeze into that small-block case.
Let’s start with a few basic parameters that determine how big an engine we can build. Bore spacing is a term used to describe the distance between the centerline of each cylinder. A smallblock Chevy has a bore spacing of 4.40- inches, which means there is a finite amount of room to make the cylinders larger in diameter. The original 1955 265ci small block had a very small 3.75- inch bore that had grown to 4.125-inches by 1970. As you can guess, this leaves very little room to make the cylinders much larger. All original small blocks utilized cooling water jackets that surrounded each cylinder. The exception to this is the 400ci small-block case that employs siamesed cylinders that connect the common cylinder walls to enhance structural rigidity. This removes cooling water from between the cylinders, which created all kinds of early rumors of cooling problems with the original 400ci small block that history has proved unfounded.
As you can see, unless we stretch the bore spacing of our original small block, it will be difficult to increase the bore much beyond about 4.155-inches (0.030-inch overbore on a stock 4.125- inch bore) for most production blocks. The reason for this is the cylinder walls become much too thin to remain stable at high horsepower and torque output. When the cylinder walls begin to move around, we lose ring seal and power will fall off. However, both GM and the aftermarket has come up with several subsequent versions of the small-block Chevy that allow slightly larger bores to increase displacement while retaining the small-block’s standard 4.40-inch bore spacing. We will get into those details in the chapter on cylinder blocks, but it’s possible to expand the bore size out to an amazing 4.250-inches, while still using the small block 4.40- inch bore spacing.
If we are limited in bore diameter, then the next place to look to increase displacement is with stroke. Here, we have a little more room to move around. The original 265ci small block used a very short 3.00-inch stroke and the block was designed with plenty of clearance in the crankcase area for a larger stroke. Again by 1970, GM engineers had stretched that stroke out to 3.75- inches to create the 400. Increasing stroke means you must also increase the size of the crankshaft counterweights in order to offset the weight of the piston and rod swinging through a larger arc. This means there must be room inside the crankcase to accommodate these larger counterweights. Where this clearance usually becomes critical is at the bottom of the cylinder bores and the oil pan mounting rails. Larger strokes will require additional clearance to prevent interference as the engine spins over.
Another area often overlooked when increasing stroke is the clearance between one or two connecting rods and the camshaft. The increased stroke pushes the large end of the rods closer to the camshaft and often requires custom stroker connecting rods and/or a camshaft with a small base circle to create the required clearance. Of course, you can also opt for a tall-deck block with taller cylinders that also raises the camshaft centerline relative to the crankshaft to allow more clearance. This move also requires other custom components that drive up the price of the engine, but that’s something you must factor in when considering which displacement is right for your wallet and your power requirements. We’ll get into the specifics of how big we can build a small block in later chapters, but it is possible to build a small-block Chevy up to a 454ci, and even larger, if you are willing to spend the big bucks.
The reason for building a larger displacement engine is to make more power. But it’s important to understand where this power will be produced within the power curve. It is an accepted fact of internal combustion engines that power per cubic inch is not a linear function. In other words, horsepower per cubic inch (hp/ci) decreases as displacement increases. All other factors being equal, small displacement engines generally make more hp/ci than larger displacement engines. This is due to several factors, including increased friction created by greater piston travel due to longer stroke, larger bearing diameters that create more friction, and the inherent power absorption from larger, heavier components that require power to accelerate their greater mass. Despite these limitations, the larger displacement still makes serious power, which is why everyone is so intrigued with increasingly larger engines.
Let’s take a look at each of the two functions of power—torque and horsepower. When cylinder pressure is created on top of the piston, this forces the piston down. Pushing against the crankshaft.The length of the crankshaft arm, or throw, from the crankshaft centerline is leverage that multiplies the piston force and spins the crankshaft. Torque is defined as the twisting motion created by the crankshaft.Without getting into a complex discussion of how dynamometers work, let’s shorten the learning curve by describing the typical dyno as using a water brake with a strain gauge to measure the amount of torque the engine creates at any given RPM. This torque can be described as a given amount of twist measured in pounds of force delivered over a given lever length — usually a foot. This creates a force measured in pounds per foot. For example, if we create a force equivalent to placing a one-pound weight on the end of a one-foot long lever, then we’ve created one foot-pound of torque. Since engineers like to apply a shorthand to their descriptions to save time, this has been shortened to torque in ft-lbs. So if a given engine generates 400 ft-lb of torque, this could be described as 400 pounds of force exerted over a lever measuring 1 foot in length or in various other equal configurations. This twisting motion tends to be a static function and does not describe work being done since time is not included in this measurement. Work over time is the definition of horsepower.
James Watt (1736-1819) was a Scottish engineer who needed to come up with a way to equate power from his newly developed steam engine to a power level that people could easily understand. Through observation and measurement, Watt developed an estimate that the average draft horse could produce 33,000 ft-lb of work in one minute. This can be shortened in the following equation to the now-standard horsepower equation:
HP = (Torque x 2 x Pi x RPM) / 33,000
HP = TQ x RPM / 5252
As an example, let’s say that our 400ci small block makes 450 ft-lb of torque a 4,400 rpm. Given this power output, what is the horsepower?
HP = 450 ft-lb x 4400 rpm/ 5252
HP = 376.99
This formula can also be inverted and used to calculate torque based on horsepower:
TQ = HP x 5252 / RPM
For example, let’s say our small block is making 500 hp at 5,000 rpm:
TQ = 500 x 5252/5,000
TQ = 525.2 ft-lb of torque at 5,000 rpm
Now that we have a basic understanding of how torque and horsepower are calculated, we need to know a little more about these power measurements. As we stated earlier, small displacement engines tend to be more efficient due to their shorter stroke, which reduces the piston travel distance, and frictional losses from ring drag. The friction created by cylinder pressure that pushes the rings against the cylinder walls helps to seal the rings, but it also creates massivefrictional loads. This friction is power that is lost to heat and represents a little less than 50 percent of the total power lost to heat, friction, and pumping losses. Given this, increasing stroke causes the piston and ring package to travel a greater distance for the same RPM, which means the frictional losses from a long-stroke, large-displacement engine will be far greater than an engine with a shorter stroke and smaller displacement. This is the trade-off to building a larger displacement engine.
So if larger displacement engines suffer from all this lost power, why build them? Despite these inherent inefficiencies, adding displacement is still the easiest way to make outstanding power. A good rule of thumb for street type performance engines is 1.1 hp/ci. This means that if you have a 406ci small block, then 406 x 1.1 = 446 hp. Not all street engines will achieve this level of performance. Some may dip down as low a 0.8 hp/ci, while others can push the envelope up as high as 1.25 to 1.30 hp/ci and higher.
While horsepower has always been the most popular measurement, torque is also a big-time player and should not be overlooked. The problem with placing too much emphasis on horsepower is falling into the trap of creating an engine that relies on RPM to create the horsepower while sacrificing low and mid-range torque. For a street engine, this can result in a sluggish engine that requires deep gears and lots of RPM to perform well. Let’s take a look at the relationship between torque and horsepower and where the power curve falls.
THE TORQUE CURVE
The advantage that large displacement engines offer is that wonderful kick in the seat of your pants that torque delivers. That long stroke delivered by a larger engine creates displacement that makes torque at extremely low RPM levels. It’s this torque that accelerates the vehicle forward. Frictional losses that long-stroke engines must sacrifice occur at all engine speeds. However, at higher RPM the piston must travel that greater distance at the same RPM as a shorter stroke engine and the friction generated begins to really take its toll on horsepower. However, the added leverage combined with a large piston area really generates some serious torque. It’s not unusual for 400ci street engines to create 480 to 530 ft-lb torque output at relatively low engine speeds of 3800 to 4500 rpm. These are power levels more often reserved for big-block engines displacing 454 cubic inches or more. Add in the weight advantage of an aluminum-headed small block weighing as much as 100 to 200 pounds less than an iron-headed big block making the same torque, and it doesn’t take much to realize that you have built a much quicker and faster car. This is the advantage of a large displacement small-block Chevy.
When looking at power numbers from an engine dyno, most enthusiasts head straight for the peak horsepower number. The more astute readers will also look at the peak torque and the RPM where each of these power levels are attained. But the truly sharp power connoisseur will closely evaluate not only the peak power levels but also the entire power curve from the lowest RPM test point to how radically the engine falls off its horsepower peak. All of these functions determine how well this engine will perform in a particular vehicle. One of the most important considerations when building a performance engine is to spend time deciding exactly how the engine will be used and in what vehicle it will be used in. These parameters help determine the kind of engine that should be built. Too often, enthusiasts will build an engine based on unrealistic goals or power levels and then place it in a car or other vehicle that is entirely unsuited for it. The result is a car that never performs up to expectations because it was built all wrong in the first place. This is an important point worth emphasizing. Engine dynos are great tools for measuring engine power. But ultimately — we race cars, not dynos.
Let’s look at a 377ci small block that initially looks impressive. Using a 4.155- inch bore 400 block and a steel 3.48-inch stroke crank, this small block used a set of Edelbrock Victor Jr. aluminum heads combined with a rather long duration Isky mechanical roller cam that’s spec’d out at 264/272 degrees of duration at 0.050-inch tappet lift. The induction system consists of a Victor Jr. intake and a Barry Grant 750 cfm annular discharge carburetor. The engine created impressive peak numbers with 589 horsepower at 6,500 rpm while peak torque came in at 516 ft-lb of torque at 5,200-5,400 rpm.
The first thing we want to look at is the horsepower per cubic inch. This short-stroke, big-bore engine makes an outstanding 1.56 hp/ci. Let’s take a look at where this occurs. Peak horsepower comes in at 6,400 rpm and falls off only slightly, losing only 13 off the peak all the way up at 7,000 rpm. This makes the shift point for this engine right at 7,000 rpm, which is serious RPM for a street engine. The torque is also excellent with a 1.37 ft-lb/ci rating but it occurs at a rather high 5,200 rpm. Looking at the low end of the curve, the engine builder decided not to test the engine below 4,200 rpm although the engine did make 434 ft-lb of torque at that point. It’s possible that if he had tested the engine at a lower RPM, then engine still would have made at least 400 ft-lb of torque at around 3,800 rpm. Another interesting point is the RPM spread between peak horsepower and peak torque. In this application, there is a relatively narrow 1,200 rpm spread between the two peaks, which means that this engine would work best with a 4 or 5-speed manual transmission to keep it within its power band either in a drag race or road race application.
First, let’s acknowledge that this is a serious small block making almost 600 hp! But for a street engine, the long duration camshaft would offer poor idle quality and would be sluggish below 3,000 rpm due to the excessive camshaft overlap. This engine would be better suited with a manual transmission but if an automatic were chosen, it would require at least a 4,000 rpm stall speed converter in order to put the engine closer to the strong part of its torque curve. Ideally, this engine would demand almost a 5,000 rpm converter in order to put it at its torque peak for a drag race application. Again, this is easily accomplished, but it seriously detracts from its street manners.
Now let’s investigate a second engine, a 420ci small block using a 4.155-inch bore, a 3.875-inch stroke and a 236/244 degree at 0.050-inch tappet lift mechanical roller Comp Cams camshaft. The induction system consists of mildly ported 195cc Airflow Research heads, a Victor Jr. intake, and a Holley 750-cfm HP series carburetor. The headers used in this test were a set of 1 7/8-inch street headers complete with a 2 1/2-inch exhaust system and Flow master mufflers. This engine was intended to be driven on the street with a goal of at least 500 hp in a relatively heavy street car with an automatic.
Peak horsepower occurs at a reasonable 6,000 rpm while the torque peaks at a much lower 4,500 rpm giving us a power band of 1,500 rpm from 4,500 to 6,000 rpm. Let’s compare these two engines’ strong and weak points. Unquestionably, the 377ci engine makes a ton more horsepower, with 589 versus the larger engine’s 524, an advantage for the 377 of 65 hp. But looking at the torque curves, the larger 420 makes much more torque, out producing the 377 by 45 ft-lb at 4,500 rpm and perhaps as much as 100 ft-lb at 4000 rpm. By 5,000 rpm, the 377 has assumed command, delivering more horsepower from this point on up. If each engine were optimized for drag racing, the more powerful 377 would easily out-accelerate the larger 420. But compromise the gear ratio and transmission selection for street car use and the stronger torque curve of the larger 420 would create a much better acceleration rate given a less-than optimal rear gear ratio.
It’s this stronger torque curve created by the larger displacement engine that helps acceleration and makes horsepower — without resorting to long duration camshafts and higher compression ratios. If you look at the basic horsepower equation, maintaining the torque curve at high-RPM levels will deliver very impressive horsepower numbers. Unfortunately, this comes at the price of a very peaky engine that only runs strong above 5,000 rpm. That’s why it is so important when evaluating horsepower numbers to always ask to see the power curve or at least ask at what RPM the engine achieved its peak horsepower number. It’s not that difficult to build a naturally aspirated 700 hp 302ci small block if you use big heads, a big cam, a large carburetor, headers, and a large plenum intake manifold. Of course, you’ll have to spin this engine to over 8,000 rpm to make this kind of power; the power band will be narrow, perhaps as confined between 6,800 to 7,800 rpm. Even worse for a street engine, it will be a stone below 4,000 rpm.
This is why big cubic-inch street engines are becoming increasingly popular. Using our previous example, the 420 makes a respectable 1.24 hp/ci with a similar 1.23 ft-lb /ci at a very low 4,500 rpm. The beauty of all this torque is that you can put a very streetable 3.50 gear behind this engine and the car will still run high 11’s in a properly set up 3,500-pound street car. Combine that with an overdrive transmission like a TH700-R4 automatic or the popular late model T-56 6-speed that’s used in the fourth generation Camaros and Fire birds and you’d have a strong street engine that would easily handle loafing down the freeway in overdrive at 70 mph at just above an idle.
THE SIREN SONG OF RPM
Another good reason to build a large displacement small block that builds torque rather than horsepower is the evil stuff that can and does occur at high RPM. There is something to be said for an ultra-powerful 400-plus cubic inch small block that spins to 7,000 rpm and makes gobs of horsepower. But the price you pay for that luxury is expensive valvetrain parts. The first item to consider when contemplating a high-RPM engine is valvetrain durability. To control a mechanical roller cam (which is the only real cam to use in this situation), requires high spring pressure, which tends to abuse the valvetrain even when things are going they way they’re supposed to. Even mechanical roller cams designed for the street will abuse roller tappets. Also, spring pressures of over 500 pounds on the nose will require a dedicated inspection plan to prevent internal damage. This could mean inspection of the roller tappets as often as every 5,000 miles if you intend to put lots of street miles on an engine of this kind.
As you can see, there’s a ton of information to know if you are going to build a big-inch small block, more than we can cover in this short overview. Much of this information involves engine building common sense and attention to detail. Constructing a 600 hp naturally aspirated small block that makes all its power below 6,500 rpm is easily accomplished and can be used as a specialty street engine with very little concern for durability. The key to this is the proper selection of parts combined to create the proper torque and horsepower curve to suit your application. After careful parts selection and assembly, all that’s left to do is go out and enjoy all your new found power. Don’t be surprised if traction becomes a serious issue. But that’s a good problem to have.
Written by Graham Hansen and Posted with Permission of CarTechBooks