After promising at least 625hp since it was shown to media in December of last year, Chevrolet has been tight-lipped regarding the actual power output of the supercharged 6.2L LT4 that lives under the carbon fiber hood of the 2015 Corvette Z06. Today, Corvette Chief Engineer Tadge Jeuchter announced at an SAE conference that the engine produces an SAE-certified 650hp at 6,400rpm and a peak of 650lb-ft of torque at 3,600rpm, a significant step up from the 460hp LT1 engine in the Stingray. In case you've forgotten, that's more powerful than the SAE-certified 638hp from the LS9 in the previous range-topping C6 Corvette ZR1.
650 is an Awfully Round Number
The 650hp certification makes the all-new 2015 Corvette Z06 the most powerful production car in Chevrolet's history, and puts it into rare territory among high-powered supercars. Chevrolet points out that unlike other supercar engines, the LT4 has power available practically everywhere in the engine's range, producing 457 lb-ft just off idle and 625 lb-ft by only 2,800 rpm. Like the C7 Stingray's LT1 engine, the LT4 is direct injected which improves both efficiency and emissions. Thanks to an efficient 1.7L supercharger that is able to spin 33% faster than the 2.3L supercharger on the C6 ZR1's LS9, the LT4 is able to build boost sooner and still maintain an incredibly broad power band. What we found interesting is that the engine doesn't seem to stop making power even at its 6,400rpm peak. We learned that, again like LT1, the LT4's fuel cutoff is at 6,600rpm, so while it has been certified to make 650hp at 6,400rpm, that's not to say it doesn't make a few more before the supercharger runs out of steam and the fuel stops flowing.
In the Company of Exotics
The 2015 Corvette Z06 and its 650hp puts it among the top 10 most powerful cars for sale in the US for 2015, and only 1hp down on the Ferrari FF. There's a lot of European carbon fiber on that list, and surely Chevrolet would have loved to move one up one more spot among the exotics, so we spoke with Tom Read at GM Powertrain Communications. It turns out that GM wasn't too worried about the Ferrari ranking because, like many manufacturers, Ferrari doesn't get its engines SAE certified
. Tom didn't technically say that Ferrari is a bunch of dirty rotten liars, actually he didn't even imply it in the slightest. But we will.
Also worth noting is that the Viper is about to get some company on that list when the Hellcat's horsepower
is announced, adding one more V8 with a supercharger and leaving only one Lamborghini on the chart. Before anyone asks, the Bugatti Veyron and Shelby GT500 are no longer in production.
Power:Weight Trumps HP/L
Because it caries its 1.7L supercharger low in the lifter valley, the LT4 in the 2015 Corvette Z06 is only about an inch taller than the compact LT1 engine in the C7 Stingray. That allows the Z06 to make a lot of power from a small total volume, which has been a hallmark of Chevrolet small-block V8s from the 265 through the LS engines that debuted in the C5 Corvette. Jordan Lee, chief engineer for Small Block engines, notes that, “The power density of the LT4 makes it one of the smallest and lightest 650-hp engines in the industry.” Long live pushrods!
To cope with the added cylinder pressure and power output, the LT4 was equipped with the following features that build on the already robust LT1 engine:
• Rotocast A356T6 aluminum cylinder heads that are stronger and handle heat better than conventional aluminum heads
• Lightweight titanium intake valves (The LS7 also used titanium valves)
• Machined, forged powder metal steel connecting rods for reduced reciprocating mass
• High 10.0:1 compression ratio – for a forced-induction engine – for enhanced performance and efficiency
• Forged aluminum pistons with unique, stronger structure to ensure strength under high cylinder pressures
• Stainless steel exhaust manifolds and an aluminum balancer that are lighter than their LT1 counterparts
• Standard dry-sump oiling system with a dual-pressure-control oil pump.
Bring on the Crate Engines
If you're anything like us, you're already seeing the engine bays where an LT4 could be bolted in. Although they haven't been swapped into anything yet, we all know that LT engines are very similar to LS engines, and those can go just about anywhere. So how long will it be before we see LT4 crate engines? Chevrolet has already announced that the LT1 engine from the C7 Corvette Stingray will be available as crate engines from Chevrolet Performance, expect to see an update at SEMA 2014. Because LS9s from the C6 ZR1 have been available as crate engines for years, it can't be long before the 2015 Corvette Z06 follows suit and LT4 crate engines are providing muscle cars with supercar levels of power.