Stroking Options for Big Inch LS Engines

Displacement is king. Whether it’s in NASCAR Sprint Cup, NHRA Pro Stock, or Formula One, professional race teams always build the biggest motors that their respective rule books allow. This universal quest for maximizing displacement is hardly a coincidence. Internal-combustion engines are nothing more than glorified air pumps, and, as such, the engine that moves the most air in and out of its cylinders and combustion chambers will make the most power. Obviously, there’s far more to optimizing an engine combination than displacement alone. Cylinder head airflow, intake manifold design, compression ratio, camshaft selection, valvetrain setup, cylinder wall finish, and the weight of the rotating assembly are just a few of the myriad factors that ultimately determine an engine’s horsepower output. Nonetheless, having a greater volume of space inside the cylinder bores to cram full of air and fuel yields a competitive advantage few engine builders and hot rodders are willing to sacrifice. Increasing displacement by building a stroker motor is one of the easiest ways to boost performance, and thanks to the recent influx of affordable aftermarket crankshafts and connecting rods, it’s now cheaper than ever.

 

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Understanding why big cubic inches reign supreme requires examining the relationship between horsepower, torque, and displacement. All other factors being equal, bigger engines generate more torque than their smaller counterparts, as they can ingest more air and fuel into their larger cylinders at any given RPM. Torque is the rotational force an engine’s crankshaft produces, measured in foot-pounds (ft-lbs). This force is the product of the expanding air/fuel mixture pushing down on the pistons during the combustion process. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotating motion. If a 1-pound weight is placed at the end of a 1-footlong lever, the twisting force it exerts would be equivalent to 1 ft-lb. Expanding upon this example, an engine that generates 500 ft-lbs of torque produces the same amount of force as a 500- pound weight hanging off the end of a 1-foot lever.

 

 

 

However, torque is a static measurement that doesn’t accurately express the total amount of work an engine can accomplish over a given duration of time. Recognizing this problem, eighteenth-century Scottish engineer James Watt developed a formula for horsepower to calculate the amount of work his newly invented steam engine could accomplish. He concluded that the average horse was capable of performing 33,000 ft-lbs of work per minute, and he coined his new unit of measure “horsepower.” To covert the linear value of 33,000 ft-lbs to accurately represent the rotational motion of a crankshaft, Watt divided that figure by 6.28 to establish a mathematical constant of 5,252. That’s because the circumference of a circle with a 1-foot radius is 6.28 feet. In other words, since the end of a 1-foot lever attached to a crankshaft would travel 6.28 feet per each revolution of the crankshaft, dividing 33,000 ft-lbs by 6.28 yields a constant of 5,252. This figure, which converts linear motion to rotating motion, is a key component in Watt’s horsepower formula:

 

 

HP = Torque x RPM x 33,000 / 2π

HP = Torque x RPM / 5,252

Using this formula, we can calculate that an engine that produces 500 ft-lbs of torque at 6,000 rpm will make 571 hp (500 x 6,000 / 5,252). Furthermore, Watt’s formula offers several important revelations in terms of how torque and horsepower are interrelated. Torque is multiplied with every revolution of the crankshaft, and horsepower is simply the total cumulative torque an engine produces in one minute. In other words, horsepower is nothing more than the rate at which torque is produced. Consequently, the only way to boost horsepower is to increase torque output or RPM. Both are viable options, but there is a practical limit to how many RPM a motor can turn. Even the healthiest of small-blocks will rarely make usable power beyond 7,500 rpm, and most street/strip motors live in the 1,000- to 6,500-rpm range. Given this small window of RPM in which a typical street/strip motor operates, the most practical way to bulk up horsepower curves is by maximizing torque, which is what building a big-inch stroker motor is all about.

 

 

 

Granted, long-duration camshafts and big-port heads can shift the bulk of the torque curve higher up in the RPM band, which can yield tremendous gains in horsepower, but it comes at the expense of compromised idle quality and decreased low-speed torque. Likewise, taking full advantage of a large cam requires turning more RPM. This not only mandates the use of exotic valvetrain hardware that’s expensive and more prone to failure, it often results in compromised reliability and poor drivability that even the most hardcore of enthusiasts couldn’t bear to endure. Furthermore, a high-winding engine combo needs shorter rear-end gearing, which increases cruising RPM and adversely affects gas mileage. Ultimately, it’s all about personal preference, and an engine combination that one person deems radical and unstreetable is perceived as docile and well-mannered to someone else. The beauty of a big-inch stroker is that it can make more power than a smaller motor without having to turn as many RPM. And regardless of your personal tastes or tolerance level for cam lope and lowRPM surge, cubic inches are your friend.

 

Efficiency

Chevrolet reached a huge milestone in 1957 when it managed to wring out 1 hp per cubic inch (hp/ci) from its 283-ci fuelie small-block. The horsepower-percubic-inch metric has been used to gauge efficiency for decades, and the fuelie was one of the first mass-produced engines to reach that mark. Even today, making 1 hp/ci is nothing to balk at. When the 346-ci LS1 first made its debut in the 1997 Corvette, its 345-hp rating was right at 1 hp/ci. However, since hot rodders aren’t burdened with the same design constraints as factory engineers, the efficiency of a motor can be dramatically improved with the addition of highflow cylinder heads, more aggressive camshafts, and a big helping of cubic inches. Due to the staggering airflow capabilities of LS-series cylinder heads, it’s not uncommon for a stroker Gen III/IV engine combo to effortlessly reach the 1.5- hp/ci mark while retaining acceptable street manners. In the wake of LS small-blocks, 408-ci stroker motors belting out 600 hp are a dime a dozen. The fact that they produce peak power at a perfectly streetable 6,000- to 6,200-rpm range is even more impressive. More radical combinations can easily surpass 1.5 hp/ci, with all-out race engines approaching and sometimes exceeding 2 hp/ci.

 

 

Surely, enthusiasts who grew up building Gen I small-block Chevys may have a hard time believing these outstanding efficiency numbers. However, such skepticism is merely a testament to how good the LS1 platform is from the factory. With nothing more than a larger camshaft, with roughly 220 to 230 degrees of intake duration at .050-inch lift, a stock 346-ci LS1 easily makes 475 hp. That works out to 1.37 hp/ci, which is still considered very respectable for a stroker Gen I motor. Nonetheless, efficiency alone only gets you so far. It takes a motor with both big-time efficiency and bigtime displacement to make the most of the LS small-block’s impressive hp-percube capabilities. You don’t need to be a professional engine builder to realize that for any given level of efficiency—1.5 hp/ci, for instance—a bigger motor makes more power than a smaller one.

That’s not to say that small motors can’t make serious power. NASCAR Sprint Cup engines measure just 358 ci, yet churn out 850 hp. The 500 ft-lbs of torque a Cup motor produces at its 9,000- rpm horsepower peak isn’t any more than that of a healthy stroker LS motor, but the fact that it turns so many RPM is the reason it can make so much power. However, not only are the astronomical engine speeds of a Cup motor impractical for a street motor, due to cost, reliability, and drivability issues, NASCAR engines are a perfect example of why RPM is the single biggest limiting factor of how much power small motors can produce. The current limit of steel valvesprings is 83 to 85 cycles per second, which translates to roughly 10,000 rpm in a fourstroke engine. For motors that run for any appreciable length of time, 9,000 rpm is as high as you want to go.

Since horsepower is nothing more than torque multiplied by RPM, and smaller motors produce less torque, they must turn more RPM to match the power output of larger motors. Consequently, considering the limits of current valvespring technology, it’s impossible for even an all-out race engine to exceed the 9,000- to 10,000-rpm threshold. If engine builders at NASCAR shops could wring another 1,000 rpm out of their motors, horsepower output would skyrocket accordingly. Unfortunately, this isn’t feasible.

 

   
 

Similarly, whether it’s due to budget, reliability, maintenance concerns, or how large of a camshaft you can tolerate, there are only so many RPM an engine can turn. Additional cubic inches helps combat this problem, since bigger motors produce more torque, and increasing displacement reduces the RPM at which peak power is produced. For example, if you unbolted the cylinder heads and camshaft from a stroker motor and installed it on a stock-displacement short-block, the smaller motor would produce less torque and similar peak horsepower, but at a higher RPM. Putting this theory to practice, let’s examine two very similar real-world LS engine combinations built by HK Racing Engines in Houston, Texas.

 

 

 

 

 

The first is a 346-ci motor with Air Flow Research 205-cc heads, a FAST intake manifold, a 238/242-at-050 cam, and an 11.9:1 compression ratio. This stout mill puts out 590 hp at 6,600 rpm and 524 ft-lbs of torque at 4,800 rpm. The second motor is a 408-ci combo with Air Flow Research 205-cc heads, a FAST intake manifold, a slightly larger 244/250-at-.050 cam, and 11.5:1 compression. On the dyno, the 408 kicks out 604 hp at 6,100 rpm and 568 ft-lbs at 4,500 rpm.

The differences in dyno numbers between these two engine combos are quite revealing. With the exception of the 408’s slightly larger cam, these motors are nearly identical on paper. As expected, the larger motor produced an additional 44 ft-lbs of torque at peak, which is 300 rpm lower than that of the 346 and 14 more hp while turning 500 fewer rpm. So while the 346 posts a very impressive 1.70 hp/ci compared to the 1.48-hp/ci mark of the 408, the stroker motor boasts more streetable torque and power curves. Not only is this easier on valvetrain parts, which improves reliability, it also yields a combo that requires less rear-end gearing for more relaxed freeway cruising and better gas mileage. Furthermore, the extra cubic inches of the 408 effectively increases its appetite for air, which means that it can swallow up a substantially larger camshaft while still retaining drivability characteristics similar to that of the 346. The difference is that if the 408 had a camshaft large enough to extend its peak power to 6,600 rpm, the difference in horsepower would blow the 346 into the weeds.

Perhaps the greatest advantage of a stroker motor has less to do with science and more to do with parts availability. Small motors must turn lots of RPM to make big power, but they are ultimately limited by the maximum cycles per second that modern steel valvesprings can handle. On the other hand, by nature, larger motors have a greater demand for airflow at any given RPM compared to their smaller-displacement counterparts. Although attempts to extend the RPM limit of valvesprings are moving forward at a deliberate pace, there’s no shortage of monstrous cylinder heads to feed the voracious appetite of big-inch stroker motors. As displacement figures continue to grow with more commodious aftermarket blocks and longer-stroke crankshafts, the aftermarket is more than happy to keep up with the demand with bigger and better-flowing cylinder heads. AFR released the first aftermarket LS1 cylinder heads in 2004, and within five years, intake port volumes have already grown from 205 cc to 265 cc. Should the need arise for more cavernous ports, rest assured that the aftermarket will deliver.

 

 

 

Displacement Defined

Even soccer moms know that engines come in different shapes and sizes, but since the subject at hand is how to increase the displacement of a motor, it’s prudent to define exactly what displacement is and how to calculate it. The two dimensions that determine an engine’s displacement are the diameter and height of its cylinders, which are referred to as bore and stroke, respectively. As we learned in high school geometry class, the volume of a cylinder is calculated using this formula: V = πr2h. In other words, a cylinder’s volume is equivalent to its radius squared, multiplied by pi (3.1415), multiplied by its height. The final step is multiplying that product by eight, since the GM LS-series small-block distributes its total displacement over eight cylinders. Putting this formula into practice, the displacement of a GM LS7 small-block— which features a 4.125-inch bore and a 4.000-inch stroke—can be easily calculated as follows:

 

 

 

This formula can be simplified even further to bore x bore x stroke x .7854 x 8. For example, plugging in the LS3’s 4.065-inch bore and 3.622-inch stroke dimensions yields: 4.065 x 4.065 x 3.622 x .7854 x 8 = 376 ci.

Now that we’ve established how to calculate displacement, it’s easy to conceptualize how it can be increased by enlarging the bore and increasing the stroke. In geometric terms, this means that both the diameter and height of the cylinders are being increased. What makes this possible is the fact that every motor has some “spare capacity” in its architecture to accommodate additional cubic inches. An engine block is merely the envelope in which its cylinders are housed. In fact, every different iteration of the Gen III and Gen IV family of engines, and their varying displacements, is simply the product of combining different bore-and-stroke dimensions. For instance, the original LS1 utilizes a 3.900- inch bore and a 3.622-inch stroke to achieve a displacement of 346 ci. Its LS2 successor uses the same 3.622-inch stroke, but combines it with a larger 4.000-inch bore, for a total of 364 ci. Likewise, the massive LS7 matches up a 4.125- inch bore with a 4.000-inch stroke, which equates to 427 ci.

That said, it’s important to clarify that when increasing the stroke of a motor, the actual height of the cylinder is not being changed. Instead, a longer-stroke crankshaft merely increases the distance the pistons travel between top dead center (TDC) and bottom dead center (BDC). Consequently, displacement, in automotive terms, refers to the piston-swept volume of the cylinders. In other words, although the height of the actual cylinder in an LS1 is greater than its stroke of 3.622 inches, the volume of air it can draw into each cylinder is limited by how far the pistons travel down the bores. Therefore, the effective displacement—or piston-swept displacement—of a four-stroke internalcombustion engine is determined by bore diameter and stroke length, not the actual height of the cylinder itself. Furthermore, unlike the bore diameter, the length of the cylinder sleeves is built into the block and can’t be altered without casting a brandnew block.

 

Pushing the Envelope

The maximum displacement attainable in an engine block is dependent on a multitude of factors, including bore spacing, deck height, cylinder wall thickness, camshaft location, piston design, connecting rod length, and the distance between the oil pan rails. Of these, a block’s bore spacing and deck height are the most important factors in determining displacement potential.

Bore spacing is simply the distance from the center of one cylinder to the center of an adjacent cylinder. Larger bore spacing can accommodate larger bore diameters inside the block. Like its legendary Mouse motor forebear, the LS engine family features a 4.400-inch bore spacing. This offers an engine package with relatively compact external dimensions, from front to back, that still has plenty of internal real estate to support lots of cubic inches. An aluminum LS-series block with aftermarket cylinder sleeves can handle up to a 4.200-inch bore. However, as the diameter of the bore increases, the thickness of the cylinder walls decreases. This can potentially weaken the block, which is why it’s imperative to sonic check the cylinder walls prior to machining to ensure that there’s adequate wall thickness for your desired bore diameter. Ultimately, both bore spacing and cylinder wall thickness determine the maximum bore that a block can handle.

Deck height, on the other hand, is the distance between the centerline of the crankshaft and the deck of the block. All production Gen III and IV smallblocks have a deck height of 9.240 inches, and since the block must have sufficient space inside to house the crankshaft and connecting rods, only a portion of the total deck height can be dedicated to cylinder height. A stock LS-series block can swallow up a 4.000-inch-stroke crankshaft without much fuss. In fact, the stock cylinder sleeves are long enough to accommodate a 4.125-inch stroke, but at that point, careful attention must be paid to the piston and connecting rod selection to prevent the pistons from hanging too far down the bore at BDC. Several aftermarket block offerings—from companies such as RHS, GM Performance Parts, ERL, and World Products—incorporate taller deck heights. This allows stuffing in a 4.500-inch or longer crank for engine combinations of nearly 500 ci.

 

 

 

Clearancing

Since the basic tenant of stroking an engine involves taking advantage of unused space inside the block, it’s imperative to make sure that there’s adequate clearance between the various moving components. This isn’t difficult, but it does require paying close attention to parts selection and carefully clearancing any potential problem areas when necessary.

Lengthening a crankshaft’s stroke involves increasing the distance between the crank centerline and connecting rod journals. Doing so moves the crankshaft counterweights farther outward, which means that the clearance between the counterweights and the oil pan rails become much tighter. Most aftermarket stroker cranks have profiled counterweights in order to prevent them from coming in contact with the block. However, since different blocks within the LS engine lineup have different internal dimensions, checking for clearance is a mandatory step in any stroker buildup. Using a die grinder to remove metal from the problem areas of the block is a very common procedure. Furthermore, the crank counterweights can potentially come in contact with the oil pan, which requires massaging the inside of the pan with a mallet for extra clearance, or grinding it out if the pan is built from cast aluminum. One of the primary advantages of aftermarket blocks is that most have larger crankcases and additional space between the oil pan rails to more easily fit longer-stroke cranks.

 

   
 

Another consequence of longerstroke crankshafts is that they increase the angularity of the connecting rods as they swing from side to side in the block. This is simply due to the fact that a longer stroke pulls the pistons farther down the cylinder bores. The drawback is that this causes the connecting rods to come into much closer proximity of both the bottom of the cylinder sleeves and the crankcase. Again, the easy fix is to grind metal off areas where the connecting rods or bolts contact the block, a procedure that has already been performed with most aftermarket blocks.

 

 

 

 

Increases in stroke and rod angularity also push the connecting rods closer to the camshaft as their respective pistons approach TDC. The most effective solution is to grind down the connecting rod shoulders where necessary, and using a camshaft with small base circle lobes. In fact, many aftermarket rods feature profiled shoulders for this purpose, and countless off-the-shelf camshafts are offered with small base circle lobes. Perhaps the most effective method of increasing rod-to-camshaft clearance is to raise the cam location, a feature offered in many aftermarket blocks.

 

The Power of Cubic Inches

Nothing illustrates the benefits of building a big-inch stroker motor more than real-world dyno and dragstrip testing. To make the point resoundingly clear, let’s take a look at a pair of LS engine combos built by the School of Automotive Machinists (SAM) in Houston, Texas. As part of a class project, SAM’s students and instructors pulled the stock 346-ci LS1 out of their 1998 Camaro shop car and increased its displacement to 375 ci. This was accomplished by re-sleeving the stock LS1 block, boring it out to 4.060 inches, retaining the original 3.622-inch crankshaft, and matching it up with a set of Eagle steel rods and Wiseco 10.8:1 forged pistons. Working in concert with a Competition Cams 252/264-at-.050 solid roller camshaft, factory LS1 cylinder heads ported to flow 290 cfm, and a FAST intake manifold, the 375-ci small-block produced an impressive 570 hp and 496 ft-lbs of torque. Despite a chunky race weight of 3,700 pounds, the motor propelled SAM’s Camaro down the quartermile in 10.58 seconds at 127 mph.

Although the 375 is no slouch, the crew at SAM decided to step it way up. By utilizing an ERL Performance talldeck LS2 block, SAM combined a 4.202- inch bore with a massive 4.500-inch stroke crank for a total of 500 ci. The crank was matched with Carrillo rods and 10.8:1 Wiseco pistons. Since big displacement calls for big airflow, a set of lightly massaged GM LS7 cylinder heads and a stock LS7 intake were bolted atop the short-block. A very conservative Comp Cams 248/254-at-.050 hydraulic camshaft was installed to actuate the valvetrain. On the dyno, the 500-ci mill churned out 717 hp and 630 ft-lbs of torque. After pulling out the 375 and dropping in the 500, SAM’s 1998 Camaro ran 9.96 seconds at 135 mph in the quarter-mile. That’s an improvement of more than six tenths of a second and 8 mph, despite a smaller camshaft!

 

 

 

 

 

 

 

 

 

These results are quite revealing. Even more impressive than the 500’s additional horsepower and torque output over the 375 is how it goes about producing it. Not only is the 500’s 248/254-at- .050 camshaft less substantial than the 375’s 252/264-at-050 unit, its hydraulic roller design means that its lobes aren’t nearly as steep as the solid roller lobes found on the smaller motor. As you’d expect, the 500 doesn’t need to turn as many RPM to reach peak power. It hits its peak of 717 hp by 6,500 rpm, while the 375 doesn’t hit its peak of 570 hp until 6,800 rpm. Thanks to an additional 125 ci, the 500 stomps its smaller counterpart by 147 hp and 134 ft-lbs, and does so while producing more street-friendly power and torque curves. Since it doesn’t need to turn as many RPM, the 500 can get away with using lighter valvespring pressure and hydraulic lifters, which reduces maintenance and extends both valvetrain longevity and reliability.

 

 

The biggest highlight when comparing the two motors, however, is how much extra power potential the biggerinch motors offer. The greater the displacement of a motor, the greater its airflow demands. Match that appetite for air with some high-flow cylinder heads, and the result is serious, yet streetable, horsepower. Although there’s no debating that bolting the 375’s heads on the 500 would only yield a slight gain in horsepower, combining a 500-ci shortblock with LS7 heads that flow an astounding 400 cfm yields tremendous gains in horsepower and torque while actually improving streetability. Furthermore, with slightly more cam duration, the 500 could easily produce another 50 to 60 hp while still retaining acceptable street manners.

 

 

 

Written by Barry Kluczyk and Posted with Permission of CarTechBooks

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