From an engine builder's perspective, looking at new OE engine designsalmost always leaves us with a trace of self satisfaction and maybe ahint of smugness. Looking over a newly introduced factory powerplant, wemay say, "That's a nice piece." Inside, however, the hot-rodder thatlives in our consciousness leaves us with a pleasant thought: Nice for astocker, but I can make it better.
Qualified that way, we feel an air ofhaughtiness, real or imagined, while reveling in what the OE left on thetable for us to exploit. Some porting in the heads, a little morecompression, trick rings and bore finishes, lightweight parts, and soon; our hot-rodder's bag of tricks runs deep. Unconstrained by theimpositions faced by mass production, our custom efforts allow us thefancy of outdoing monolithic corporations wielding unimaginableresources. Normally, this ego self-gratification is part and parcel ofany OE engine review. Then it happened: the LS7 small-block debut. Theonly reactions for someone truly in the know are deference and awe.
Thesize alone gives it credence: 427 cubic inches in its all-aluminumsplendor. Matching the displacement (coincidentally?) made famous by themost legendary of performance Corvettes from a generation long past,this is the largest "small-block" engine ever produced by GeneralMotors, while maintaining the external dimensions of previous Gen IVsmall-blocks. But it encompasses so much more than its internal girth.With a rated output of 500 SAE net horsepower at 6,200 rpm, the realmechanical energy emanating from this beast far outstrips any of thebig-blocks of yore.
Based on the groundbreaking Gen IV small-blockarchitecture introduced as the LS1 in 1997, the LS7 is much more thansimply a displacement increase. It's an exotic 7,000-rpm race-derivedpowerplant brought upon the public at a level of execution unrivaled inany production car. Dave Muscaro, assistant chief engineer for passengercar V-8s, put it succinctly, "In many ways, the LS7 is a racing enginein a street car. We've taken much of what we've learned over the yearsfrom the 7-liter C5-R racing program and instilled it here. The realityis, there has been nothing like it offered in a GM production vehicle."
It seems intuitive that larger displacement offers a greater potentialfor power. Detroit manifested this theorem in the big-block engines ofthe '60s. A keener appreciation for the need to increase displacementcomes with a more intimate knowledge of the relationships between torqueand horsepower, torque potential, and rpm. We'll illustrate the point aswe spell out a simple proof. Normally aspirated engines have a finitetorque potential based on volumetric efficiency and displacement.Horsepower is derived as a computation based on torque and rpm.
Thinkabout these two physical realities: Torque is limited by cubic inchesand efficiency. Horsepower is derived from torque and rpm.
If the goalis more horsepower, and torque production is at the limits of the sizeand efficiency available, there are only two avenues left to achievemore power: increase size or raise rpm. In the progression of the GMsmall-blocks, the torque potential of the original LS1's displacementwas significantly exploited, particularly in the LS6 configuration ofthe previous Z06 powerplant. Increasing output by a significant measure,such as the 25 percent gain achieved with the LS7, could only have beenaccomplished by employing much higher rpm, or the larger engine. Thelarger engine was the obvious choice. The larger displacement deliversglorious torque in greater abundance from lower in the rpm range thancan possibly be achieved with a smaller engine in normally aspiratedform. Maintain that torque as far up the power curve as practical, andyou've found horsepower nirvana. That's what was accomplished with theLS7 engine.
While it may seem like a simple requirement, maintainingtorque into the high-rpm ranges becomes increasingly difficult as thedisplacement of the engine is expanded. Torque peaks when airflow andvelocity through the intake ports and the limits of cam timing arereached. In the racing aftermarket, this point in the power potential ofan engine is referred to as "port saturation." Apply the same cylinderhead to a larger-displacement engine, and the torque produced will behigher as a function of displacement, but the limitations of port flowand velocity curb torque production earlier in the rpm range, whichhinders peak power output. To achieve their goals of abundant poweroutput into the upper rpm range with the larger- displacement engines,the LS7 required a spectacular cylinder head, and a close examination ofits layout makes it clear this requirement wasn't lost on the GMdevelopment team.
Cylinder Head And Valvetrain
We examined the LS7 cylinder head at the GMTech Center in Southern California, and were stunned by the execution.On paper, the specifications alone were impressive: 2.200-inch intakevalves, 1.61-inch exhaust valves, and CNC-porting of the intake andexhaust runners and chambers. These are healthy valve sizes, and theCNC-porting is a unique attribute. However, the port configuration andshape made an impression. A cylinder-head port is more than a hole in achunk of aluminum. Coaxing air through a passage at high rates is an arthard to appreciate without having lived with the flow bench anddie-grinder, carving shapes, testing, chasing the elusive motion ofinvisible gasses. It's a specialized world unto its own, mastered byfew, where the subtle hips of the port throat are as sexy as those of apop princess. Those passages become personal and identifiable, and asindividual as a signature. Those ports are the work of a mastercraftsman with a grasp of the nuances of airflow and port shape thatmade it clear these cylinder heads are truly something special. Sureenough, we found out the cylinder-head design was a collaborative effortbetween General Motors Engineering and renowned aftermarket airflowexpert Mike Chapman.
The intake ports are substantially enlarged,providing the critical cross-sectional area required for maintainingtorque production high into the rpm range. Allowing the greater crosssection are offset intake rocker arms, spreading the pushrods away fromthe port centerline and eliminating the restraint to the port width theyimpose. To gain a more advantageous geometry for airflow into thecylinder, the intake ports are significantly raised in comparison toLS1/LS6/LS2 configurations. The intake-valve inclination angle wasreduced to 12 degrees from the previous engines' 15-degree angle. Thehigher port approach and shallower valve angle are modificationsstraight from the realm of serious small-block race-engine building.
Thevalves are longer overall in stem length than those used in otherengines from this family. Longer valves ease the constraints to portheight, valvespring height, and valve lift, which can be applied toproducing a performance-minded cylinder head. The valve lift isincreased to .591 inch, a level unheard of in a production engine.Helping achieve that level of lift is an increase in rocker-arm ratio to1.8:1, in contrast to the 1.7:1 seen in other small-blocks. Theadvantageous use of the rocker ratio results in a fast action at thevalve, allowing the valvetrain to take advantage of the cylinder heads'abundant high-lift flow, without the requirements of extreme camshaftduration. This would be impractical from the standpoints of emissions,efficiency, and smoothness. The radical valve-opening rates can presenta control limitation at high rpm. The use of titanium valves on theintake and a lightweight, hollow-stem, sodium-filled exhaust valvelighten the reciprocating weight at the valve. A longer, highly refinedvalvespring with small titanium retainers offer control. Again, thesetechniques are derived from the race-engine building world.
Flow isgiven in engineering parlance of 203 grams per second at the intake and146 grams per second at the exhaust, measured at the peak valve lift of.591 inch. We were officially told the cylinder heads flow 43 percentbetter than the LS6 on the intake and 26 percent better on the exhaust.Leaning on GM sources for numbers without the strain of volume/massmetric/imperial conversions, we heard flow figures of 360 cfm on theintake and 214 on the exhaust at .591-inch lift, at the performanceindustry's standard measuring spec of 28-inch water depression. Flownumbers like these are so far beyond the realm of production enginesit's astounding. Put into perspective, a good aftermarket head for anold-style small-block flows 250 cfm. A good sportsman race head breaks300 cfm, while these numbers were seen on full-house NASCAR-style heads.
Although great things were achieved with the cylinder head,it is one link in the chain of flow into and out of the engine. The flowsystem begins with the Donaldson PowerCore airbox where attention isgiven to flow efficiency and flow is increased 20 percent as compared tothe LS2. To work with the revised cylinder-head intake-portconfiguration, an all-new intake manifold was developed to meet the highairflow demands. Like the other Gen IV small-block intake, this intakeis of friction-welded, three-piece construction, produced from acomposite material. The throttle body was enlarged to 90 mm, and theinjectors were uprated to meet the demands of the engine's requirements.The exhaust system features hydroformed individual tube headers leadingto a unique inline collector flange. A sweeping collector forms atapered transition from the header flange into the wide-mouth high-flowcatalysts mounted immediately aft.
The exhaust system features hydroformed individual tube headers leadingt a unique in-line connector flange. A sweeping collector forms taperedtransitionfrom the header flange into the wdie-mouth high-flow catalystsmounted immediately aft. The exhaust system features a pipe diameter of3 inches.
The bottom end of the engine comprisesthe short-block assembly, and the exotica found at the top of the LS7 iscontinued downstairs. The job was to accommodate the displacementincrease, and build in a level of strength and reliability suitable forhigh-rpm operation. The LS7 block is unique, partially to meet theincrease in displacement and partially to withstand the output theengine is designed to produce.
The engine block features a bore size of4.125 inches and, rather than the cast-in-place liners of the other GenIVs, receives thicker-walled pressed-in dry cylinder liners. The boresare finished with torque plates fastened to the decks to simulate thedistortion created by the cylinder-head fasteners. The main caps areinstalled and torqued during machining operations. This procedure,typical of custom-built racing engines, allows machining and finishsizing to be accomplished while the block is stressed analogous to afterfinal engine assembly. Truer tolerances in the assembled engine are theresult.
Adding to the strength of the bottom end is the use of six-boltdoweled forged-steel main caps in the place of the powdered-metalmaterial used on other Gen IVs. The main caps secure a 4.00-inch strokecrankshaft forged from 4140 chrome-moly steel, with rolled fillets wherethe journals meet the cheeks. This specification of crankshaft is moreakin to a race crankshaft than the undercut-fillet carbon-steel forgingsused in older production engines. While the high-grade steel in thecrank is notable, the real news in materials relates to the connectingrods. The use of titanium provides a uniquely high ratio of strength andfatigue life to weight. Titanium is exotic even in the racing world, andits use here is a first for a domestic auto manufacturer.
Other aspectsof the LS7 engine borrow from principles having more in common with theracetrack than production automobiles. The lubrication system of the LS7is unique, as it's a variation of the dry-sump arrangement normally seenonly on the racetrack. Rather than supplying oil from the bottom-mountedsump directly into the engine's pressure-fed lubrication system, the LS7employs a two-stage oil pump and reservoir tank. The scavenge stage ofthe pump continually evacuates the engine's lubricant, sending it to theremote reservoir tank. The tank allows high capacity without the penaltyof windage losses, aeration, and control difficulty associated with oilcollection in a conventional sump. The oil accumulates at the reservoirtank and is drawn back to the pressure stage of the system, providinguninterrupted and reliable pressurized oil under the most demandingdriving conditions.
Compression ratio has been moved up to 11:1, seriousterritory for a pump-gas powerplant, made possible through advancedengine management and the fast-burn technology built into thecylinder-head design. The pistons are full floating and are designedwith the pin supports moved in. This allows for a shorter pin, which isstiffer and reduces reciprocating weight. The piston ring lands areanodized for improved surface hardness and wear resistance, while theskirt portion is coated with a lubricious polymer to reduce borefriction.
You might notice the recurring themes of strength and weightreduction, and this isn't coincidental. Lighter-weight components induceless internal engine stress, which is exponentially increased with rpm.Larger displacement through longer strokes increases piston speed, andthese factors conspire to place a demanding load on a voluminoushigh-rpm engine. Reducing weight reduces component stresses. Figuringout how that can be accomplished while adding strength is a credit tothe engineers on the program, and the goal of any racing engine builder.
Build It Better
The LS7 is not a product of a standard mass-productionassembly line, but rather is created at the new GM Performance BuildCenter, where each engine is hand-assembled by an engine-buildingtechnician specialist. The facility is dedicated to crafting engines forspecialty applications such as the Z06 in an environment akin to customrace-engine assembly. Each engine is viewed as an individual unit, andeach is seen through the assembly process as the responsibility of adedicated builder.
The engines are rigorously inspected and hot-testedin a 20-minute run cycle upon completion. Timothy Schag, site manager ofthe Performance Build Center, explains, "This process brings a higherlevel of quality because each builder is personally involved in everyaspect of the assembly. It was important to step away from thehigh-volume world we all had lived in for so long and soak in thecadence of these specialized environments. We learned a lot andestablished a low-volume manufacturing system on par with the world'sbest niche builders, but we didn't lose sight of the quality already inplace at GM."
We haven't failed to recognize the GM team's achievementin making this new small-block a milestone in the performance world.While we appreciate the engineering execution, it leaves us eager forthe thrill of shaking down the new Z06 in the flesh. Indications are of0-60 performance in the 3s, and quarter-mile acceleration in the 11s.Missile-like acceleration on that level would be recommendation enoughto indulge our own automotive requirements. However, acceleration andspeed alone are only one dimension of the capabilities of this best-yetCorvette platform. We're anxiously awaiting a set of keys to experienceit firsthand.
Rocker-ratio intake: 1.80:1
Rocker-ratio exhaust: 1.80:1
At camshaft: Intake: 0.331 in. Exhaust: 0.328 in.
At valve: Intake: 0.593 in. Exhaust: 0.588 in.
LS7 General Specs
Engine type: Cam-in-block 90-degree V-8Block configuration: Cast-aluminum with pressed-in cylinder sleeves and 6-bolt, forged-steel main bearing caps
Bore x Stroke (mm/in): 104.8x01.6/4.125x4.00
Crankshaft: Forged steel
Connecting Rods: Forged titanium
Pistons: Cast aluminum
Compression Ratio: 11.0:1
Cylinder Heads: CNC-ported aluminum; 70cc chamber volume
Valve size, intake (mm/in): 56/2.20 (titanium)
Valve size, exhaust (mm/in): 41/1.61 (sodium-filled)
Camshaft: Hydraulic roller; 15 mm (.591 in) lift (intake and exhaust)
Rocker arms: 1.8:1; offset (intake only)
Air intake: Composite manifold with 90mm single-bore throttle body
Fuel: Premium required; 91-octane minimum
Horsepower: 500 (373 kW) @ 6,200 rpm
Torque (lb-ft): 475 (644 Nm) @ 4,800 rpm
Engine redline (rpm): 7,000