Launched in the 2014 model-year, the 6.4 liter “truck Hemi” V8 was specially created for heavy duty Ram trucks. Despite having the same displacement, this engine was quite unlike the 6.4 liter SRT Hemis (also used in certain non-SRT cars), which was rated 470-485 hp and 470-475 lb-ft of torque.
The truck engine was redesigned for durability, a broad torque curve, and economy with heavy loads; it ran on 89 octane gas, and was matched to a six-speed automatic transmission only.
Taking cues from Mopar engines of the past, the designers used sodium-filled exhaust valves, tougher valve seat materials, aircraft-grade stainless steel exhaust gaskets and fasteners, and oil jets for cooling the pistons. They installed a high-volume oil cooler and optimized the cooling system by using computational fluid dynamics. In the end, Ram was confident enough to warranty the engines for five years or 100,000 miles.
* Manual transmission is limited to 410 lb-ft torque.
** On chassis cabs over 10,000 lb GVWR, 367 hp @ 4,600 rpm
An active dual-runner-length intake manifold boosts both low-end and high-end power, by maximizing the natural “supercharger effect” of the manifold. The PCV system was integrated into the intake manifold to prevent oil loss.
The engine has cooled EGR and cylinder deactivation for efficiency. It was a deep-skirt design with a cast iron block and forged steel crank; main bearing caps are cross-bolted (the block design is similar to the 5.7 Hemi). Heads are 356 aluminum.
At launch, the engine was hooked up to the 66RFE automatic transmission, with the six-speed Aisin optional. On Ram chassis cabs with a gross vehicle weight rating over 10,000 pounds, the 6.4 was rated at 367 hp at 4,600 rpm at launch, a lower rating than the 5.7 in light-duty trucks. In lighter-duty vehicles, the 6.4 Hemi was rated at 410 hp at 5,600 rpm, with 429 lb-ft of torque at 4,000 rpm.
Variable valve timing uses a hydraulic cam phasing system under computer control (developed by Chrysler). The camshaft has oil passages in front to drive the cam phaser sprocket.
Eight coils fire 16 spark plugs; the coil fires a plug directly and has a wire going to the opposite cylinder bank. The extra plug fires during the power stroke to more fully burn the fuel; it provides more power in the down stroke while lowering the number of catalyst plates needed in the catalytic converter. This also allows the firing to take place closer to top dead center, cutting NOx emissions and ozone.
cylinder deactivation system, dubbed “Multi Displacement System” (MDS), turns off the fuel in
four cylinders when power is not needed. Bob Lee, Powertrain Product Team vice president, said, “The MDS was part of the engine's original
design. This resulted in a cylinder-deactivation system that is
elegantly simple and completely integrated into the engine design. The
benefits are fewer parts, maximum reliability and lower cost.” Transitions take place in under 0.04 seconds, by cutting out the valve lifters; energy is not lost by pumping air
through those cylinders, though some power lost through unnecessary compression.
As of today, there were no known problems with this engine.
5.7 Hemis • SRT Hemi V8s • HellCat Supercharged Hemi • 426 Hemi • Hemi History
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