Every once in a while something really special comes out of the Big Three, something well beyond the standard production powerplants, something clearly made to fulfill a mission. Going back in time, low-production race powerplants like the original '64 race Hemi, Ford's NASCAR 429, or Chevy's ZL1 are all examples of max-effort mills--engines brought into being by the OE guys getting serious, throwing out constraints, focusing only their mission to make power. We are talking about efforts more than a cut above the norm; the stuff legends are made of. After a long drought, recent years have brought several noteworthy special powerplants--Mopar's Viper V-10, dual-cammer Fords, Chevy's LT5 and LS6, among them. Unlike the afore noted '60s race engines, these were all conceived with public consumption in mind. With the all-new 427-cube LS7 destined for Chevrolet's flagship Z06 Corvette, it appears Chevrolet has raised the bar, blurring the lines between production engines and all-out race hardware. Stats of 500 SAE net hp, and a redline of 7,000 rpm provide a glimpse into its capabilities.
"When we started to look at upgrading the LS6, the first thing we did was sit down with the (C5-R) race group and talk about what they had done to build a 7-liter small-block," explains Dave Muscaro, GM Powertrain assistant chief engineer for small-block engines. "What did they do to the block to make the cylinder bores bigger, what did they do to the heads to increase the airflow? That was the key to creating the LS7."
The standard that had been set with the production 405 hp LS6 in the C-5 Z06 meant it would require more than just a good engine to get noticed when the next generation of Z06 makes the scene. GM wanted the effort to be world class, intoxicatingly powerful, and the engine package was critical to achieving those goals.
"The racing experience enabled us to visualize how we could get more power out of the production engine," Muscaro continues. "How much you get depends on where the bar is set, and racing helped us set realistic goals. A street engine has the constraints of emissions, noise and durability standards, but the race engine really challenged us to produce maximum power from a given package."
With a displacement of 7-liters or 427 cubic inches, shared with the Le Mans winning C5-R powerplant, the new LS7 is the largest "small-block" powerplant ever offered by GM, mirroring the capacity of the famed 427 cid big-blocks so fondly remembered. The all-aluminum LS7 arrives at its displacement with a bore size of 4.125 inches and a stroke of 4.000 inches, a substantial increase in comparison to the 3.90 x 3.62-inch specifications of the LS1 and LS6 that preceded it. Why the voluminous displacement? The simple answer is as old as the hot-rodder's credo of "There's no replacement for displacement." The high level of execution in the previous generation's LS6 made cubic inches the obvious choice for a substantial gain in usable output. Upping the output of that powerplant by the magnitude accomplished with the LS7 could not have been done in normally-aspirated form within production realities.
Race engine builders and factory engineers alike understand that horsepower is derived from torque and rpm. More horsepower can come about in two ways, make more torque, or make torque at a higher rpm. With the LS6 engine's peak output coming in at 6,000 rpm, potential power gains from rpm alone were clearly limited, leaving an increase in torque as the most viable avenue to pursue. Torque is a product of displacement and efficiency. The LS6 set a high mark for efficiency, making displacement the most direct path to big power. The LS7 represents a 23 percent increase in capacity compared to the Gen III small-blocks, while the external dimensions remained unchanged. By upping the cubes, torque production is increased from the very bottom of the engine's operating range, supplying power "right-now." While big cubes and hefty torque production were enough to deliver satisfaction with some of the old big-blocks of yesterday, as we shall see, GM clearly had much more in mind with this one.
To accommodate the larger bore, the LS7 utilizes its own block casting, designed to encompass large pressed-fit dry liners, as opposed to the cast-in-place sleeves of related small-blocks. Digging deeper, we find that the powdered metal main caps employed in other Gen III/IV engines have been supplanted by doweled forged steel mains, fastened to the block with six bolts per cap. Hmmm, it's getting interesting. Retained by the caps is the new longer-stroke crank, a unit forged from 4140 chromium molybdenum steel with rolled fillet-radius journals. This is a serious piece, having more in common with a custom race crank than a traditional carbon steel factory forging. The parts list becomes even more exotic with the connecting rods, which are machined from lightweight forged titanium, a first for a domestic production engine, and uncommon even in race engines.
Special cast pistons were developed for the LS7, which feature an anti-friction skirt coating, anodized ring lands, improving hardness and wear resistance. Larger-bore pistons are by nature heavier than their smaller-bore counterparts, and here the LS7 design team countered this tendency by moving the pin bosses inward and employing a shorter high-strength full-floating piston pin. This approach yields a piston assembly that is lighter, while the shorter pin represents a mechanically stiffer arrangement. With a flat-top design, the pistons deliver a compression ratio of 11:1. The mechanical line-up downstairs makes it abundantly clear that this mill was designed from the ground up to turn a number.
ENTER THE AIRFLOW
A substantial increase in torque, particularly lower in the rpm range, is endemic when considering large displacement. Carry that torque curve hard up the rpm range, and the result is horsepower in magnanimous excess. That's easier said than done, requiring hedonistic cylinder-filling efficiency to provide for the displacement-draw of large cubes at high rpm. It takes an extraordinary set of heads. The cylinder heads of the LS7 are nothing short of magnificent.
Taking the racing cues of a higher port entry and a flatter valve angle, the intake ports have been substantially raised at the manifold face, while the valve angle has been changed from the 15-degree angle of other engines in this family, to a more advantageous 12-degrees. The ports themselves have been significantly enlarged, aided by the use of offset intake rockers, moving the pushrods adjacent to the intake runners outward. The pushrod pinch, commonly a point of cross sectional constriction in the intake ports has been eliminated. Overall, the port cross section has been enlarged, in keeping with the requirements of high-rpm torque production with a large displacement engine.
The intake valve diameter has been increased to a big-block-like 2.200-inches, while the exhaust measures 1.610-inchs, and the valve stems have been lengthened. The longer valves facilitate port design, provide the room for deep ports in the bowl and short turn, as well as allowing for greater valve lift and more valvespring installed height. The finishing touch is in the ports' shape, masterfully CNC-carved into the cast metal, reportedly a design collaboration of GM engineers and distinguished race cylinder head man Mike Chapman of Chapman Racing Heads. CNC sculpting is also visible in the combustion chamber, with a smooth gradual contour to the deck further enhancing flow.
The list of features is impressive, and so is the airflow, reportedly pegging the manometer at 360 cfm on the intake and 210 cfm on the exhaust, as measured at the LS7's maximum valve lift. Intake airflow is up some 43 percent over the LS6 head, while the exhaust shows a 26 percent gain. In terms of flow capacity, that's serious territory, far beyond any factory Chevy smallblock head of the past, and well past any production big-block heads. As a measure of comparison, a decent high-performance aftermarket head for the standard smallblock Chevy will typically flow around 100 cfm less than the production LS7 intake, and only the most exotic race two-valve heads compare. The LS7 heads just cook the books on anything we've seen in the past.
To supply the enormous flow capacity of the heads, as well as to mate up with the revised raised-port location, a dedicated intake manifold was developed for the LS7. The intake configuration is the familiar cross ram plenum design initially introduced on the LS1, but the ports and plenum have been redesigned to match the requirements of the new engine. Manifold construction is of a low thermal transfer polymer, molded in three pieces and friction welded. At the front of the intake is a single-butterfly 90mm throttle body, a substantial increase in size from the previous 78mm production piece, another mod in the name of high airflow.
CAM, VALVETRAIN AND MORE
All of that flow capacity would be sitting idle if the cam and valvetrain were not designed to take advantage of it. To that end, the valve lift has been increased from 0.525-inch to a lofty 0.591-inch, again breaking new ground for a production powerplant. Providing the valve action for that lift is a camshaft measuring 211 degrees of duration on the intake, and 230 on the exhaust. The camshaft is aided in achieving valve lift by rockers with a ratio of 1.8:1; up from the 1.7:1 ratio of GM's other current small-blocks. The relatively short duration and dramatically high lift help the LS7 achieve very high power levels while timing valve events for an acceptable level of refinement and emissions.
The valve action is necessarily very quick, and here measures were taken to ensure valvetrain control at high rpm. Of primary importance in this regard is weight on the valve side of the rocker, a key element of valvetrain stability. To lighten the valves for maximum rpm potential, the intake valves are machined from titanium, while the exhaust valves feature hollow sodium-filled stems, which reduces weight while aiding cooling. As a further step towards valve heat transfer, the valve seats are copper infiltrated, improving their thermal transfer characteristics over standard seats. Other impressive features of the valvetrain include the use of beehive-shaped single valvesprings, which reduce spring weight and inertia, and are less subject to harmonic disturbance, a leading cause of lost valvetrain control. The beehive springs provide for even more weight savings with their small upper diameter, allowing for a small lightweight retainer.
The innovations in the bottom end, cylinder heads, and valvetrain are remarkable on their own, but it doesn't stop there. A two-stage dry sump system is used to ensure minimal windage losses, while providing a reliable source of pressurized lubricant. With the new Z06's designed capabilities in handling, braking and cornering, a wet sump system would be hard pressed to manage oil control. The tandem pump's scavenging stage draws the sump dry of spent oil, routing it to a remote dry-sump tank for accumulation and separation. From there, the pressure stage draws from the large reservoir for an uninterrupted supply of lube. An engine oil cooler is incorporated into the system.
To say we were impressed by this new powerplant would be a serious understatement. The technology and execution is a bullseye in terms of our hot-rodding, wrench-turning, engine-building sensibilities. Perhaps GM's engineer Dave Muscaro sums it up best by saying, "In many ways, the LS7 is a racing engine in a street car, We've taken much of what we've learned over the years from the 7.0-liter C5R racing program and instilled it here. There really has been nothing else like it offered in a GM production vehicle." We'd have to agree.
BUILD IT BETTER
It's not hard to conclude that the new LS7 is anything but ordinary, and that notion follows through in how it's built - the process is anything but ordinary. GM has invested heavily in the facilities and protocols required to build exceptional high performance engines in an environment removed from normal high-production assembly lines. The result is the new GM Performance Build Center in Wixom, Michigan, a 100,000 square-foot testimony to The General's seriousness in building serious performance engines. The entire facility is dedicated to hand crafting low-volume extreme performance engines like the LS7 for specialty vehicles such as the upcoming Z06.
More akin to a specialty race engine shop, the traditional assembly line model is dispensed with in favor of assigning a single skilled builder to see through the assembly of each specific engine from start to finish. The intentions are summed up by Timothy Schag, the Center's site manager, "It's a premium manufacturing technique for premium products. This process brings a higher level of quality, because each builder is personally involved in every aspect of the assembly." In researching what it takes to successfully employ such an approach to specialty engine building, Schag, along with other GM Powertrain Team members, sought out specialty race engine builders in the US and low-volume niche engine manufacturers in Europe for insight. Schag relates, "It was important to step away from the high-volume world we all had lived in for so long and soak in the cadence of these specialized environments. We learned a lot and established a low-volume manufacturing system on par with the world's best niche builders, but we didn't lose sight of the quality practices already in place at GM."
The assembly technicians weren't just picked up from the local builder's warehouse parking lot; they were selected from GM's experimental engine lab in Pontiac, Mich., for their engine-building experience. Each engine starts with a block mounted to an assembly stand, which the builder runs through approximately 15 stations containing the specialized tools required to perform a specific assembly procedure. The builders rotate from one station to the next as a task is completed until the engine is fully assembled. It's a highly orchestrated procedure that allows the builder to address any assembly or component problem that might arise.
Inspection and verification is built into the process every step of the way, and upon completion, a final series of quality checks are made, and the builder signs off on that particular unit and documents who assembled it. Each engine is then sent to a hot test area, then subjected to a 20-minute running "hot" test. With a stamp of approval from the final inspection, the engine is shipped to the vehicle assembly facility.
Special Thanks to Tom Read of General Motors