Chevrolet Corvette 427 Blocks - The Great Cylinder-Block Debate

Race-Designed C5-R Or Economical LS7: Two Choices For Building A 427 Street Engine

Barry Kluczyk Apr 6, 2007 0 Comment(s)
Vemp_0704_01_z Corvette_cylinder_block_comparison C5R_and_LS7_blocks 2/19

Between 1999 and 2005, the Corvette C5-R racing program annihilated the competition in the American Le Mans Series. The team collected 45 wins in 66 starts-a win ratio of about 70 percent. It will certainly go down in history as the most auspicious racing program in the Corvette's history.

Much of the credit for those continuous wins can be ascribed to the fortitude of the powerful C5-R 7.0L racing engine. The C5-R was based on GM's Gen III small-block V-8 but featured numerous enhancements designed to provide the strength needed for daylong endurance races. It was also designed as a structural member of the Corvette C5-R race car's chassis.

Vemp_0704_02_z Corvette_cylinder_block_comparison LS7_block 3/19

The LS7 block is based on the same casting as other Gen IV engines, including those with the cylinder-deactivating Active Fuel Management (AFM) feature. The cast towers of the LS7 are carryovers from Gen IV engines with AFM, but because the engine isn't equipped with it, the towers aren't machined.

After some internal prodding by far-thinking employees and external begging from some deep-pocketed enthusiasts, GM Racing released the C5-R cylinder block for civilian use through the GM Performance Parts (GMPP) network. Weekend racers and street enthusiasts rejoiced-and winced. The retail price for a C5-R was (and is) more than $6,000. That's just the cylinder case, mind you-not a whole engine. Granted, for the money, one receives a race-prepped block with some serious hardware. To date, about 500 C5-R blocks have been purchased outside of the Corvette racing program.

A paradigm shift in affordable big-inch LS engines has taken place with GMPP's release of the LS7 cylinder block (PN 17802854). Like the C5-R, the LS7 block is an aluminum block that enables the construction of a 427ci engine. The LS7's production manufacturing, however, means it costs substantially less than the C5-R block. In fact, at around $3,200, the LS7 block costs about half as much.

This is huge. It opens up a new world of engine-building possibilities for those who have trouble rationalizing a $6,000 charge on the American Express card.

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The C5-R block doesn't have AFM provisions.

The High Cost of Racing
Invariably, the question is begged ... if the LS7 is half the price of the C5-R block, is it only half as strong or half as good? Not at all, according to GM Racing's Bob Cross. "The C5-R block was originally designed only for competition use, so it features a lot of exotic production treatments and racing hardware," he says. "This block was never intended to be offered to the public, and its cost reflects the processes involved in its manufacture."

In other words, the cost disparity between the C5-R and LS7 has less to do with what the LS7 lacks than what goes into manufacturing the C5-R.

Vemp_0704_09_z Corvette_cylinder_block_comparison LS7_bulkhead 5/19

The LS7 block's rear bulkhead has a cast-in reminder of its intended displacement.

"Keep in mind, the LS7 was designed to support up to 650 hp," says Cross. "The C5-R was designed for 750 normally aspirated horsepower, but it also had to integrate within the chassis as a structural member and last for 24 hours of all-out competition."

Admittedly, those are performance parameters most private enthusiasts won't push-making the LS7 look even more like a value. One of the caveats of jumping onto the LS7 bandwagon is the current absence of an over-the-counter controller from GM for the engine. But just as engine builders have adapted stand-alone systems for their C5-R-based engines, so too will they find a workaround for LS7-based buildups. FAST, for example, recently introduced an adapter that allows the factory LS7 MAF to clip onto the LS2 wiring harness, making the swap that much easier.

Vemp_0704_04_z Corvette_cylinder_block_comparison C5R_cam_position_sensor_boss 6/19

The large hole at the rear of the Gen III-based C5-R block is a boss for the camshaft-position sensor. The LS7 block is based on the Gen IV design, which also includes the LS2. Gen IV engines have a front sensor mounted in the timing cover.

Side By Side
There are obvious differences between the LS7 and C5-R blocks. Production differences include details such as varied external ribbing-the C5-R has increased ribbing because the engine was designed as a structural member of the race-car chassis-and conventional (LS7) versus screw-in (C5-R) galley plugs. The major differences are the ones that account for much of the price disparity between the two blocks.

To begin, the C5-R block is cast with a very specific recipe of 356M aluminum. Once cast, it is heat-treated and hipped. Hipping, a reference to hot isostatic pressure, is a procedure in which the block undergoes a multistep treating process that pressurizes, heats, and cools it to ensure strength and all but eliminate porosity. Approximately 20 percent of the C5-R block castings that undergo this expensive (several hundred dollars per block) and time-consuming process are rejected. The production LS7 block doesn't receive this treatment, but it also wasn't designed for professional endurance.

Vemp_0704_08_z Corvette_cylinder_block_comparison LS7_front_view 7/19

The fronts of the blocks are similar...

Both the LS7 and C5-R blocks receive precise machining and leak testing, but the C5-R block features align-honed camshaft and crankshaft journals. The align-honed cam journals ensure the camshaft and crankshaft are squared up, which is essential to engine life under extreme racing conditions.

The main caps are also significantly different between the blocks. The LS7 uses strong, powdered-metal caps and bolts, while the C5-R uses billet-steel caps with racing-spec A1 Technologies stud fasteners. Also, when ordered from GM Performance Parts, the C5-R block includes a hardware kit with premium fasteners-and it ain't a bag of cheap nuts and bolts.

Vemp_0704_07_z Corvette_cylinder_block_comparison C5R_front_view 8/19

...but the C5-R has more-pronounced oil galleries and water jackets.

The C5-R block obviously carries the street cred of the haloed C5-R racing program, but for street enthusiasts, the LS7 block will likely prove to be the cylinder case of choice. It's strong, well designed, and comparatively affordable.

"There's no denying the strength and capability of the C5-R. It exceeds at the performance it was designed to deliver," says Cross. "But the LS7 is an exceptionally strong foundation that will serve most enthusiasts well. It offers tremendous value for the money."

When building a big-inch street engine on a budget, value is just as important as surviving 24 hours of racing at Le Mans.

Vemp_0704_17_z Corvette_cylinder_block_comparison LS7_carburetor_intake_manifold 18/19

A new carburetor-type intake manifold is available for the LS7, allowing the factory port-injection system to be replaced with a simple carb setup. An ignition driver is all that's needed to complete the swap. Versions are available for both the LS7 and L92 heads.

Retro-Carb Capability
OK, here's the scenario: You've just decided to "restify" that old C2 or C3 in your garage, and the Gen IV LS7 seems like the hot ticket to put a post-modern 427 in the car. Trouble is, the production engine doesn't look at home under a Stinger or L-88 hood.

Solution: Ditch the port fuel injection for a carburetor. The swap is much easier than you might think, and GM Performance Parts has the replacement intake manifold to do it, too. You'll need an ignition driver fter that, but they're readily available.

The carb manifold is based on the Gen IV's square-port cylinder-head design. GMPP already offers a carb intake for Gen III (LS1, LS2, and LS6) engines with "cathedral" port heads (PN 88958675).

Vemp_0704_18_z Corvette_cylinder_block_comparison LS7_carburetor_intake_manifold 19/19

Cast-in bosses on the intake manifold are machined for EFI on some versions. Nitrous can also be plumbed through the bosses.

The Gen IV intake features an open plenum and a 4150-type square-bore mounting pad. Bosses are cast at the base of each runner, with some versions machined with injector bosses for EFI applications (yes, the bosses can be used for nitrous).

Four versions of the Gen IV intake are available-two for the LS7 and its higher, squarer ports, and two for L92 heads used in the Cadillac Escalade/GMC Yukon Denali. The two versions for each engine include a nonmachined carb version and the machined-for-EFI version. Here are the part numbers for each Gen IV intake.

Sources

Chevrolet Performance Parts
Detroit, MI 48232
800-577-6888
www.gmperformanceparts.com

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A comparison of the LS7 and C5-R Corvette cylinder blocks to examine which is the better choice for building a big-inch street ...
Barry Kluczyk Apr 6, 2007

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