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The people working on powertrain at GM have been nothing if not busy (and we're not even talking about the new line of Ecotec engines that were announced in March 2014 and which has its first application in the China-market Chevrolet Cruze; this lineup consists of 11 engines. There are three- and four-cylinder models. The size range is from 1.0 to 1.5 liters. The horsepower range is from 75 to 165. The torque range is from 70 to 184 lb-ft. And there are normally aspirated and turbocharged variants.) The powertrain engineers have developed an all-new engine for the 2015 Corvette Z06 that combines features found on two other engines found under the hoods of 'Vettes: the Stingray's LT1 Gen 5 Small Block and the ZR1's LS9. The new engine is the LT4, a super- charged, 6.2-liter V8 that is SAE certified at 650 hp @ 6,400 rpm and 650 lb-ft of torque @ 3,600 rpm. The LT1 uses cylinder deactivation and continuously variable valve timing; so, too, does the LT4. The LS9 is supercharged; so, too, is the LT4. While the LT4 is based on the Gen 5 small block of the LT1, there are several differences. For example, the fuel pump for the LT4 operates at 2,900 psi; it is 2,175 psi for the LT1. The fuel injectors provide 25 cc/second in the LT4; those in the LT1 20 cc/second. The design of the combustion system is similar, but the LT4 uses Rotocast A356-T6 aluminum cylinder heads. The LT4 combustion chambers are 65.47 cc; they are 59.02 cc for the LT1. The LT4 has 54-mm solid titanium intake valves; the LT1 has 54-mm hollow intake valves. Both have the same 40.4-mm sodium-filled exhaust valves. While both the LT4 and the LS9 are supercharged, the Eaton superchargers in the engines are vastly different. Physically, the R1740 TVS 1.7-liter supercharger in the LT4 spins at 20,150 rpm. The R2300 TVS 2.3-liter supercharger in the LS9 spins at 15,180 rpm. The R1740's four-lobe rotors are shorter in length (7.9 in. vs. 8.3 in.) and smaller in diameter (3.9 in. vs. 4.4 in.), which helps account for the high rotational rate, and which help provide boost earlier in the rpm band, which helps contribute to low-end torque (the engine produces 457 lb-ft of torque just off idle and 625 lb-ft by 2,800 rpm). In addition to which, the R1740 is 20 lb. lighter. One interesting aspect of the super- charged LT4: it is only 1 in. higher than the LT1, which means that they're able to maintain a low q Elements of the 8L90: efforts were made to make the components light but capable. AD&P; > November 2014 > FEATURE > GM Powertrain Engineers: New Transmission; New Engine > Gary S. Vasilash > gsv@autofieldguide.com 24