Big cubic inches are synonymous with serious power output. The musclecar years and the early glory days of the Corvette are testimony to that notion-and it has its basis in sound engineering. We've come full circle, and again there is 427 ci of displacement available under the hood of the top dog Corvette. The big-inch formula is just the ticket to delivering torque in abundance. Let it breath, and high-rpm power will come in spades. Effortless, seamless power that just keeps coming-that's the promise.
Big-inch LS-family engines are normally reserved for an exclusive club. The LS7, or the thoroughbred C5-R, are the usual invitations into the club. However, for the willing performance hound, the Darton MID (Modular Integrated Deck) system can represent an attractive alternative. We covered the Darton system in detail in the past, and have waited for an opportunity to put its capabilities to the test. Just the right opportunity came in the notion of building a serious track-day LS1. The idea was to build a powerplant for race power and endurance, rather than short squirts down the dragstrip. This would be a test, both of our engine building savvy and the pumped-up Darton LS1 block that would serve as our foundation. (See: Making Room)
Filling It OutThe Darton MID system expanded our diminutive 3.893-inch LS1 bores to a man-size 4.125 inch. With the bores suitably puffed, it was time to consider the rest of the rotating assembly. Here, rather than putting a package together piecemeal, the best course of action is to secure a complete assembly. Lunati has developed an enviable reputation for high-end rotating assemblies for LS-series builds. In their catalog we located precisely the parts to suit our requirements: the 427 LS1 package. Here was a complete kit, waiting to flesh out our engine's internals with some of the highest quality components to grace the insides of a crankcase. Filling a stroker combination can be a complicated proposition, but with the Lunati kit, all of the specifics, such as compression height, rod length, and clearances are already scienced-out. The assembly is balanced and ready to bolt-in, saving time and potential problems getting mismatched components to work together in harmony.
Included in the kit is Lunati's masterfully crafted, forged 4340 steel crank. Shaped, profiled, gun-drilled, and lightened, this 4.00-inch stroke brute is just the right backbone for a serious 427. Lunati's top-quality Pro Billet 6.00-inch connecting rods are part of the package, in addition to the lightweight forged pistons, rings, pins, locks, and bearings. With the Lunati kit, we had on hand the best of what the high-performance and race aftermarket has to offer.
Compression Ratio ConsiderationsDue to our race intentions, there was one change of spec to the basic Lunati assembly. The pistons in the standard 427 kit feature a -17cc reverse-dome design, right on the money for a street effort, providing 10:1-10.6:1 compression ratio, depending upon the cylinder-head chamber volume. We intended to run a set of Air Flow Research's new 225cc cylinder heads, which feature a standard 72cc combustion chamber and can be ordered factory milled to 66 cc. Since our application was geared towards track time, our engine would benefit from the added compression ratio of a flat-top piston. With this configuration, an 11.5:1 ratio can be achieved with the standard AFR 72cc chamber, and we would retain the ability to raise the ratio a full point further by simply milling the heads. These factors fit into our program, giving a greater compression ratio at the heads' full-chamber volume, and the ability to go to a race-fuel ratio of 12.5:1 with simple machining of the heads.
We went to Wiseco for the flat-top pistons to meet these requirements, and ordered them machined to accept their premium steel 1.2mm compression rings and 3mm oil rings. This ring package would offer a minimum of friction and the last word in toughness and durability. The pistons were also ordered with extra-deep valve notches to provide clearance for the aggressive valve action of a race roller cam.
Camshaft ConfigurationTwo characteristics we were after from this 427 were high output and unwavering reliability. There are seemingly limitless choices in racing cam lobes, but an endurance engine must run continuously at high rpm, lap after lap, which differs from a drag-race application, where the effort is full-kill, but over in a flash. We sought to have a custom solid roller cam ground for the 427 to peak at no higher than 6,500 rpm, while producing as much torque and horsepower as possible up to that point. Further, the engine would be required to live at these extended rpm's for prolonged periods.
After a thorough search of the available profiles, we selected the Competition Cams' RZ Low-Lift series of endurance lobes. These profiles are designed to maximize valvespring and valvetrain life by employing a more gentile nose profile on the cam lobe, thus limiting lift. With the available spring installed height of our cylinder heads in the neighborhood of 1.850 inch, there is a definite limit on the amount of lift that can be employed.
Making use of Comp's lobe number 4286, we had a single pattern cam custom ground with a lobe separation angle of 108 degrees. The relatively tight angle would create a strong torque surge in the midrange, and help reign the power production to fall within our intended peak rpm range. This lobe specs out at a healthy, but not outrageous, 262 degress of duration at 0.050-inch tappet rise, and with 0.400-inch lobe lift, theoretically, delivering 0.680 inch of valve lift with a 1.7:1 ratio rocker (neglecting the loss from valve lash). That's quite a bit of lift, despite the "Low-Lift" nomenclature, but in the world of racing cams, the idea of "Low-Lift" is relative. This lift level represents about the maximum that can be achieved with the valvespring installed height available from our choice in cylinder heads. To complement the solid roller cam, a set of matched Comp roller lifters were ordered, along with a set of Comp's No. 953 valvesprings. To drive the camshaft under high loads, we went with a Rollmaster two-row timing set, rather than an OEM-style single-row chain.
AssemblyBefore we were ready to wrench, the block and pistons were delivered to Jim Grubbs Motorsports (JGM) for final finishing of the cylinder bores. The honing process must be highly controlled to provide the proper operating clearance for the pistons, while establishing a surface finish that will promote low friction, oil control, and ring life. JGM is a shop that we would confidently entrust with the task. The MID system absolutely requires that a torque plate be installed while the cylinder bore is machined, and JGM was equipped to handle our big-bore LS. With the block cleaned and bagged from JGM, all that was left for us to do was mount it onto a stand and line up the parts for assembly.
Since we were starting from a bare block, there were many miscellaneous parts and pieces necessary to complete the build. On that list were the front, rear, and top covers, and associated hardware, as well as the block's gallery and core plugs. We tapped Scoggin Dickey Performance Center, a GM dealership familiar with the needs of the performance buyer, for those parts. With those items secured, and a set of main studs from ARP, we were prepared to assemble the short-block.
The assembly sequence should always begin with a trial assembly of the major components, checking clearances, and identifying any fitment issues that may need correction before final assembly can proceed. We were pleasantly surprised to find the bottom-end assembly was remarkably straightforward, with all of the components bolting in and falling to place, much like a stock rebuild. Credit much of that success to the advanced planning that went into the build and the quality of the components from the aftermarket manufacturers involved.
The accompanying photos detail the short-block assembly procedure and the sequence in which they were performed. Next month, we will complete the engine with serious top-end hardware, making the most of the expanded capacity below, and we'll put our efforts to the test with a full dyno evaluation.
Making RoomThere is no denying that Chevrolet's LS1 and its derivatives have set new highs in terms of unbridled output. The latest factory 427-cube LS7 variant has pushed production performance to levels unheard of since the legendary packages of the '60s, with Chevrolet taking its time-tested approach of building it bigger. When the limits of practical power versus rpm are arrived at, a greater displacement offers the opportunity to break out from boundaries imposed by displacement. While stroke increases alone will render the possibilities for greater displacement and significant increases in torque, the exclusion of bore size gains presents an inherent disadvantage. Greater bore carries twofold advantages, stepping up the sheer size, while opening the bore window area for greater access to airflow.
Getting bore size from an LS1 block is no simple matter. The factory dimension of 3.893 inch (stroke is 3.625 inch) is actually rather on the small side. The cast-in iron liner aluminum block that makes the LS1 a lightweight darling curbs the potential for bore-increases via the conventional boring-bar technique. A substantial increase in bore size requires a substantial rethinking of how such a goal can be accomplished, and in the end, a substantial level of engineering. Darton stepped up with both the thinking and engineering to produce a viable and proven big-bore solution for the LS1 blocks.
Darton's program for the LS1 is based on their exclusive Modular Integrated Deck technology, wherein the original cylinder barrels are machined away in favor of an interlocking arrangement of wet cylinder liners. The sleeves are centrifugal cast from a superior material to tolerances unobtainable by production techniques, resulting in a block that maintains far more favorable cylinder wall strength, wall thickness, and accuracy. The MID system requires specific and precise machining, an operation which Darton encourages customers to leave to specialty machine shops versed in the procedures as specified by Darton, and with the equipment to handle the task.
Our own big-bore mission is to build an LS1-based small-block that carries the thunder of a big-block within. To that end, we sought to achieve 427 ci from the diminutive and lightweight LS1 package. This displacement consists of a bore size increase to 4.125 inch, and a stroke gain to 4.000 inch, specifications readily accommodated by a rotating assembly from Lunati. We enlisted the services of Steve Demirjian at Race Engine Development to prepare the foundation for our muscle-bound monster by merging the Darton MID kit with our production LS1 block. Demirjian worked closely with Darton in the development of the LS1 MID system and installation techniques, making him eminently qualified for the precision task. Considering the proven potential those big cubes can offer to the already incredible LS1 package. Darton can deliver it.
Street Versus RaceWe had just two things in mind when assembling this combination: make as much power and torque as possible, and make it live. Our endurance engine may seem pretty extreme, and it really is, what with the relatively high-compression ratio, solid roller cam, and the aftermarket dry-sump system to come. How removed is it from a more "mild" street combination? Probably not as far as you might think. In fact, essentially there are just three components we would change if building a sledgehammer street bruiser, rather than a race performer: pistons, cam, and oiling.
We detailed our piston choice already, and there was good reason for making that change. The higher compression goes hand-in-hand with the stout cam timing of our solid race roller camshaft. In a purely street application with a milder cam, the basic Lunati rotating assembly and reverse-dome dished pistons would be right on the money for pump-gas performance, at about a 10.5:1 ratio, depending upon which cylinder head cc was used. With the 11.5:1 ratio our flat-top pistons provide, this hot-cammed engine will easily operate on 100-octane fuel and may even run on pump gas in a pinch. The AFR cylinder heads we will be running can be flat-milled to 65 cc without a loss in flow, enough to boost the ratio to a full 12.5:1 with our flat-tops. We decided to hold that card for now, knowing the extra ratio is available in a future freshen-up if we were to go for a full race-fuel combination.
The dry sump is another story. Rather than a play for more outright power, we elected to make this move for the assurance of an uninterrupted supply of precious lube-under all manner of vehicle forces. GM followed the same logic in the LS7 Z06 engine, though a simpler wet sump would serve normal street duty. Take this same Darton/Lunati 427, use the lower-compression reverse dome pistons, substitute a milder hydraulic roller stick, and the wet-sump underwear down below, and you can have your own 427-powered street Corvette. The power and torque will be suitably brutal.