Sneak Peek: GM’s ALMS GT Challenger, The Corvette C6.R

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The engine is based on an LS7 but is destroked and optimized for running through restrictors.  We think this means super high compression.  The engine also runs on E85.

The engine in the GT race car is based loosely on the production LS7 found in the Z06 Corvette.  The LS7 is an all aluminum compact overhead valve engine that displaces 7 liters or 427 cubic inches with a 4.180” bore and a 4.000” stroke.  The 2 valve heads have straight tunnel type intake ports with titanium valves and are CNC ported.  The lubrication system is dry sump and the intake manifold has a large single plenum and single throttle body.  The stock LS7 produces 505 hp @ 6300 rpm and 470 lb ft of torque @ 4800 rpm. Although it is a single cam pushrod engine the LS7 is quite a sophisticated world class engine with excellent power density and superior packaging.

The engine use ITB’s with relatively long runners and one plenum for each cylinder bank vs one large plenum like the production engine.  The ITB’s are for good throttle response despite very limited airflow from the restrictors. This it probably done to make the best use of E85’s higher latent heat of evaporation, using the E85 to cool the intake charge by introducing it further back in the intake tract.  All of this carbon cost a pretty penny we bet.

The racing version of the engine found in the C6.R is based on the LS7.  Although Pratt and Miller were reluctant to discuss any sorts of specs with us we can glean a little bit about the engine from press release materials and a bit of guessing.  The engine is first destroked to 3.32” for a displacement of 6 liters or 366 cubic inches.  We also think the engine runs a pretty high compression ratio due to the small twin 28.6 mm rules required inlet restrictors limiting massflow into the engine.  These restrictors are smaller than your typical 2 barrel carburetor!
The engine is thus limited to 470 hp and 535 lb ft of torque.  The relatively high torque output with the very oversquare bore and stroke indicates a very high compression ratio.  The power delivery of this engine is probably pretty flat and broad within limits.

The engine breathes through these tiny restrictors.  The fact that the engine makes so much torque is a telltale sign of super high compression.  Tiny restrictors mean that some tricky engine management needs to happen.  When the airflow goes sonic in the restrictors, airflow stalls and the engine will choke.  This stops the engine deader than a rev limiter and it takes a bit before the flow can normalize again.  Thus it is imperative that the engine will never reach choke flow conditions despite differences in air density and the speed of sound as ambient conditions fluctuate.

This device pre heats the engine’s coolant so the engine has a super short warm up cycle.  This reduces engine wear.

The other major difference between the GT engine and the production LS7 is the intake manifold.  The GT engine has an all carbon individual runner intake system with twin plenums.  The car runs on an E85 blend of unleaded gasoline and cellulostic ethanol.  GM’s engineering department calculated the car’s total carbon footprint and found the C6.R to be the world’s greenest race car for all that’s worth.

Stainless headers are optimized for power production with plenty of room for a close to optimized design.  Note the O2 sensors for closed loop operation and probably for air fuel ratio data logging.

The side exit exhausts help aerodynamics by reducing underbody air flow intrusions form the car’s sides.  This helps improve the effectiveness of the rear diffuser and splitter.