When engineers at General Motors say they have created one of the lightest and most compact 650-hp engines in production, our interest is piqued and we're paying close attention. That's especially true when that engine, a supercharged 6.2-liter LT4 V-8, is destined for the all-new 2015 Chevrolet Corvette Z06.
Although the switch from the previous Z06′s high-revving 7.0-liter LS7 to forced induction may have upset some purists, the beating heart of Chevy's gnarliest car is also the most powerful production engine GM has ever built. And it keeps pretty exclusive company: The list of current production engines producing 650-plus horses is a short one: the recent Hellcat Hemi V-8 from Dodge, a trio of Ferrari V-12s (found in the F12berlinetta, the FF, and the LaFerrari), the Lamborghini Aventador's V-12, the Bugatti Veyron's insane W-16, and the McLaren P1's twin-turbo V-8. (The plug-in-hybrid Porsche 918 Spyder misses the cut by virtue of its V-8 producing 608 horsepower on its own.)
In addition to output, GM's small-block needn't worry about its size. Pushrod V-8s, such as the new LT4 and the LT1 on which it is based, make the most of a single camshaft mounted low in the block. Dual-overhead-cam V-8s have four camshafts mounted high in the engine, making those powerplants—all else being equal—taller and wider overall, not to mention heavier. We love the sound of 32 valves banging and four cams spinning as much as anyone else, but there is an elegant simplicity to a 16-valve pushrod engine that can't be ignored. It gets the job done.
If there's one drawback to the design of the LT4, it's that its single camshaft's sole phaser can't vary the timing of the intake and exhaust valves independent of one another. To date, the only pushrod engine with that feature is the Dodge Viper's V-10, which has a virtual cam within a cam.
Bolting a supercharger into the vee usually adds a good bit of height, but GM worked with supercharger specialist Eaton to develop a 1.74-liter four-lobe unit—code-named R1740—that adds less than an inch to the overall height of the LT4 when compared to the blower-less LT1. The supercharger forces air past titanium intake valves and into oversquare cylinders at a maximum pressure of 9.4 psi, down from 9.7 in the C6 ZR1's LS9. The LT4's compression ratio is 10.0:1, which is right between that of the LT1 (11.5:1) and the LS9 (9.1:1). The camshaft profile of the LT4 also delivers greater valve lift and duration than did the LT1's.
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Fully dressed, the LT4, with aluminum block and heads, weighs 529 pounds, which is a moderate gain in mass over that of the 454-pound LS7. Considering that the LT4 makes more power than the LS7 at any given engine speed, we're thinking the new supercharged engine will win over those who miss the previous, naturally aspirated one when keys finally land in customer hands next year. Even though we've reported the Z06 to be the heaviest Corvette yet, we're confident that it will also be the quickest Chevy two-seater in history, especially when mated to GM's new optional eight-speed automatic.
from Car and Driver Blog http://ift.tt/nSHy27
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