Can You Swap A 5.7 HEMI With A 6.4 HEMI?

With nearly 50 years of turning wrenches on just about everything with tires and an engine, I've been involved in more than a few engine swaps. Those swaps range from lawn mowers and mini bikes, to a "Dentside" Ford F-150 pickup truck and classic Chevy muscle cars like Chevelles, Camaros, and a 1969 El Camino. The most radical engine swap I've ever had the pleasure of working on involved replacing the 1969 Chevrolet El Camino's stock inline six-cylinder with a well-built 327-cubic-inch Chevy small block V8.

While the straight-six-to-V8 engine swap required the extra step of changing engine mounts, swaps in vintage cars didn't require the same degree of consideration associated with the electronics that control modern engines. If you're looking at replacing your 5.7 HEMI with a 6.4 HEMI (or 392 HEMI if you prefer), there are some electrical controls and ECU tuning required to get the best results. However, the good news is that it's not only possible, other than the 6.4 HEMI intake manifold, nearly every other required component transfers directly from your existing 5.7 to a 6.4 long block.

While there are some minute details that require your attention when swapping HEMI engines between trucks and cars, it's a fairly simple process otherwise. First, you're going to need some tools in addition to a good selection of wrenches, sockets, and ratchets, such as pry bars, an engine hoist, and an engine stand. Fortunately, Harbor Freight carries a good selection of tools that will come in handy.

You'll also need a 6.4 HEMI to replace the 5.7 in your current ride. This can be a remanufactured long block, a complete engine either in new-in-the-crate form or in preowned condition from a donor car or a salvage, or some combination of the above. While the complete new crate engine option is attractive, some aren't legal for use on public roads and could prove cost prohibitive depending upon your budget. 

For reference, Mopar Genuine Parts sells a complete Performance 392 HEMI SRT crate engine for $8,045 (plus an installation kit and jumping through some other hoops to maintain the warranty) whereas the long block option starts at $5,238. Keep in mind you'll have to source the 6.4 HEMI intake separately if you go the long block route. However, either way, you'll need to add an SRV (Short Runner Valve) controller to your existing wiring to make use of the 6.4 intake's active runner system, and you'll need to have the system tuned.

Swapping a 6.4 (392) HEMI in place of a 5.7 (345) provides more power. The basis for that power gain comes from the additional displacement of 0.7L (47-cubic-inches) thanks to the 6.4's larger diameter cylinders (4.090 vs 3.917 inches) and longer crankshaft stroke (3.720 vs 3.578 inches).

The 6.4 HEMI produces up to 485 horsepower and 475 lb-ft of torque straight from the factory. For comparison the stock 5.7 HEMI in a 2023 Dodge Charger is rated at 370 horsepower and 395 lb-ft of torque. While the 115 horsepower and 90 lb-ft of torque gains are a factor of the larger displacement to some degree, the 6.4 HEMI intake manifold's active runner system plays a role as well.

The science behind choosing the best length for the intake manifold's runners is complicated. However, in the end all we need to know is that shorter runners often make the most horsepower while longer runners are typically best for torque. The stock 5.7 HEMI intake is designed to provide the best compromise on runner length while maintaining acceptable performance on both fronts. The 6.4 HEMI uses an Active Runner system that switches between short and long runners as warranted by the load being placed on the engine.

Is it worth swapping a 6.4 HEMI into a 5.7 HEMI car? Maybe, if the 5.7 in question is in need of a replacement. Or if you happen to have access to a solid 6.4 HEMI donor engine on the cheap. While you can sell your old 5.7 after you pull it, this isn't a low-budget swap if you have to buy a new 6.4 HEMI.

If the existing 5.7 HEMI is otherwise healthy and the only reason for the swap is the power gain, it might make more sense to either swap out the 5.7 intake manifold for one designed for a 6.4 with active runners. You'll get some power gains that way, but of course, can't take advantage of the increased displacement.

Another option that could make more power than the swap and potentially cost less, is adding a supercharger to your 5.7 HEMI. I know what you're thinking: OR I could swap in a 6.4 HEMI AND add a supercharger to that. The sky's the limit with the only things holding any of us back are time, money, and imagination.