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Watch this 650-hp Hennessey Jeep Grand Cherokee go from 0-60 in 3.9 seconds

Hennessey Performance Engineering has created an HPE650 package for the Jeep Grand Cherokee SRT fitted with the 6.4-liter Hemi engine. When it leaves the showroom floor it has 470 horsepower and gets from 0-60 miles per hour in 4.8 seconds. When it leaves Hennessey's garage floor after the upgrade it has 650 horsepower - 530 hp at the wheels, according to Hennessey - and gets to 60 mph in 3.9 seconds on its way to running the quarter-mile in 12.6 seconds.
There's more than just a Roots-type supercharger bolt-on, with ported cylinder heads and high-flow headers among the many changes made to the engine. You can watch this pre-2014 facelift silver model prove its spec sheet on the drag strip in the video below.

Here's what it'll take to build a Jeep Grand Cherokee Hellcat

Let's get one thing straight: We want a 707-horsepower Grand Cherokee Hellcat to happen. Badly. The latest report from Motor Authority is encouraging; the bonkers SUV supposedly has a codename, Project K, and has been given the green light for production. Fingers crossed. You might be wondering why the Trackhawk isn't already a thing. Hellcat engines exist, SRT Grand Cherokees exist, so just combine the two, right? It's not quite that easy. Here, we outline what needs to happen, why it should be the quickest Hellcat vehicle out there, and why it won't come anywhere near 200 miles per hour. How To Build A Hellcat Jeep The first engineering problem is feeding the air-intensive beast that is the 6.2-liter supercharged V8. The first engineering problem is feeding the air-intensive beast that is the 6.2-liter supercharged Hellcat V8. Breathing is important on two counts: pulling in enough air for the combustion to put out 707 hp, and then cooling the various heat exchangers once the engine is up to temperature. Dodge did it with the Charger and Challenger, it can do it with the Jeep. This is one place where the Grand Cherokee's larger frontal area might be a boon, as it gives the engineers more surfaces through which to suck air. Once you generate the 707 horsepower and 650 pound-feet of torque, it has to get to the wheels somehow. Jeep's current SRT all-wheel-drive system will at least need some beefing up to handle the torque. It could require a more complete re-engineering. We at least know the ZF-supplied eight-speed auto, used in the Dodge Hellcat models, is up to the task. The Hellcat engine should fit in the Grand Cherokee, as it's about the same size as the 6.4-liter currently in SRT Jeeps, but the Hellcat is taller because of its supercharger. The hood may need to be raised or at least resculpted for clearance, as well as to address those cooling needs. Quicker Than Everything, But Not Faster 200 mph? We're skeptical, from both a physics standpoint and a legal one. A reminder of the quick/fast distinction: quick is acceleration, fast is road speed. The Jeep's all-wheel drive will help put the Hellcat engine's power to the ground in a more manageable way than the Charger and Challenger do through just the rear wheels. That means better acceleration times than the Dodges (11.0 seconds in the quarter-mile for the Charger Hellcat, 11.2 for its Challenger sibling).

Jeep still working to improve Cherokee's 9-speed auto

Fiat Chrysler is hoping an upcoming software update will stem the tide of consumer complaints surrounding its nine-speed automatic transmission. Owners of the 2014 Jeep Cherokee have reported a number of problems on the National Highway Traffic Safety Administration's SaferCar.gov website, since the new model and its troubled gearbox arrived way back in October 2013. The software update is "intended to keep the vehicle performing as intended, and to prevent durability issues from occurring in the future," an FCA spokesperson told Automotive News, and will be available to owners of both the 2014 to 2015 Jeep Cherokee and the 2015 Chrysler 200, which also uses the 9AT. While FCA will be notifying consumers of the update, owners can also request the software reflash if they happen into their dealer before then. Despite the widely documented problems with the transmission, the only complaints on NHTSA's website relate to the 2014 Cherokee – neither the 2015 Jeep nor the 200 have received any complaints. That bodes well as FCA prepares to begin deliveries of the 2015 Jeep Renegade and launch the Fiat 500X, both of which pair the 9AT with the 2.4-liter Tigershark four-cylinder. "We have had to do an inordinate amount of intervention on that transmission, surely beyond what any of us had forecast," FCA CEO Sergio Marchionne told Automotive News. "There are things that we have done – that we continue to do. Our proactive customer care intervention has actually increased in intensity on these vehicles in 2014, especially in the second half." What's fascinating about the 9AT's problems are that they haven't been the fault of manufacturer ZF, but have related to software that wasn't "mature" and had "teething problems," Marchionne has said previously, AN reports. With the lack of criticism for the 9AT in 2015 models and this pending software update, though, here's hoping that FCA has finally figured out its fuel-sipping gearbox. Related Video: