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.