A new era of IMSA Prototype racing commences with January’s Daytona 24 Hour race, which will be fought out between four brand new LMDh cars. Those are from defending champion Acura, new challengers Porsche and BMW and established rival Cadillac.

Over the past 6 months Acura, Porsche and BMW have revealed information on their LMDh engines as noted in previous instalments of this Log. However, at the time of writing, less than a month before the 24 hour event, Cadillac was remaining tight lipped about its power unit.

LMDh marries an internal combustion engine to a spec transmission, which incorporates a 50 kW/67 bhp MGU. The Bosch MGU is housed within an Xtrac spec transmission case which carries Xtrac gearbox internals. Fitted within the chassis, the associated hybrid system battery is from Williams Advanced Engineering.

The total output of this rear wheel only power supply must conform to a mandated power curve, with a potential maximum engine output (according to balance of performance adjustment) of 520 kW/ 700 bhp at 9500 rpm. The existing Cadillac DPi engine is a naturally aspirated 5.5 litre V8 that produces 572 bhp at 6800 rpm - air restricted.

The bmep requirement is such that the existing engine could in theory be developed for LMDh but in practice such an increase in operating speed would amount to a lot of work so GM has started over. The existing engine is a pushrod, 16 valve Small Block derivative whereas the new LMDh engine is understood to be derived from the double overhead camshaft LT6 32 valve all-aluminium production engine.

The LT6 retains the 4.4 inch bore spacing of the classic Chevrolet Small Block. It was the basis of the Corvette GTE engine using a special block casting, which is likely to be the case with the LMDh engine. In fact all aspects of the LMDh engine might be bespoke. As with the current LT6-based, close to stock Corvette GT3 engine (as covered in the November Log) this is an in house project at GM’s Powertrain Performance & Racing Center in Pontiac, Michigan.