It's a done deal. It'll never happen (just don't rule out the aftermarket).
707 horsepower Vs 640 hp. That’s the comparison being made right now in regards to the 2015 Dodge Challenger SRT Hellcat and the Dodge SRT Viper. A supercharged V8 is now more powerful than a naturally aspirated V10. So wouldn’t it make sense to transplant the Hellcat V8 into the engine bay of Chrysler’s flagship supercar? Yes, it would. However, it’s not technically possible. For starters, the dimensions of a Hemi simply won’t allow it to fit under the Viper’s hood.
Blame tall cylinder heads for that. Modifying the Viper’s hoodline would also severely alter its aerodynamics. What’s more, the Hemi is too wide and it won’t clear the Viper’s chassis rails. There’s also a problem with the exhaust manifold and supercharger hardware, which add to the dimensional problem. The supercharger also runs on a separate cooling loop. That right there is a whole other issue to overcome. But SRT powertrain director Chris Cowland expects that "somebody will do it (the engine switch). It won’t be us." Fortunately, the Viper is expected to receive a slight power bump for 2015.
The 8.4-liter V10 will now reportedly produce 645 hp. Torque will remain the same at 600 lb-ft. Yes, this was done in order to better prepare the Viper for its upcoming fight against the new Corvette Z06, which has 650 ponies. So now we're just waiting to see which aftermarket tuner will be the first to successfully do that Hellcat-Viper engine transplant. It'll certainly be an interesting thing to see happen.