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New for 2014 was a specially designed 6.4 liter Hemi V8 specially created for heavy duty Ram trucks. Despite the identical size, they are very unlike the 6.4 liter SRT Hemis, rated 470 hp and 470 lb-ft.
The engine was redesigned for durability, torque, and economy with heavy loads; it runs on 89 octane gas and is matched to a six-speed automatic transmission only.
For trucks, durability and a broad torque curve take priority over horsepower, so (taking cues from Mopar engines of the distant past) the designers used sodium-filled exhaust valves and tougher valve seat materials, with aircraft-grade stainless steel exhaust gaskets and fasteners, and oil jets for cooling the pistons. A high-volume oil cooler and revised PCV system increase oil efficiency, while computational fluid dynamics optimized the cooling circuit.
The extra durability steps prompted Ram to give the engine a five-year, 100,000-mile warranty, the same as with the Cummins diesel.
* 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 helps boost both low-end and high-end power, an old Mopar trick based on maximizing the natural “supercharger effect” of the manifold. The PCV system is integrated into the intake manifold to prevent oil loss.
The engine has cooled EGR for efficiency, cylinder deactivation for economy, and PVC integrated into the intake, with a deep-skirt, 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 on 2014 Rams, the engine was hooked up to the 66RFE automatic transmission, with the 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 cars or light-duty trucks — the emphasis, again, is on durability and heavy-load driveability.
Like the 5.7 Hemi, the fuel requirement is 87 octane (U.S. regular) — with midgrade (89 octane) recommended.
In lighter-duty vehicles, the 6.4 Hemi is 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 in the converter. This also allows the firing to take place closer to top dead center, and cuts NOx emissions and ozone.
cylinder deactivation or “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, keeping the
valves in four cylinders closed; energy is not lost by pumping air
through them, though some is lost through unnecessary compression.
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