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.

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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.




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