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SRT V8 Engines: 6.1 and 6.4 (392) V8s; Supercharged 6.2 Hemi

2011+ 6.4 liter production V8: 392 Hemi engines

The 6.4 Hemi (392 Hemi) is used in the 2011 and newer SRT8 cars including the Jeep Grand Cherokee SRT8. The engine is different from the one in the Challenger Drag Pack (the Mopar crate engine); for one, the standard 392 shuts off four cylinders when they are not needed, to increase gas mileage. The crate and production engines have a different bore, stroke, cam, heads, compression, and intake.

For 2015, power in the Charger and Challenger versions of the 392 was boosted from 470 to 485 horsepower, with torque climbing from 470 to 475 pound-feet (in the Grand Cherokee, power was still 470 hp, 465 pound-feet).

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oh2o told Allpar readers about the engine in 2005, with horsepower estimated between 450 and 500; in 2008 we firmed our estimate at 480, and in 2010 it was launched with 475 horsepower and 460 lb-ft of torque (on Challenger; on the 2012 Charger SRT8, it was 465 hp and 465 lb-ft; in 2015 Challenger, it is 485 hp, 475 lb-ft). To limit emissions, both spark plugs were used at top dead center and one was used before top dead center to burn off the hydrocarbons, allowing the use of less restrictive catalytic converters.

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The 392 Hemi (6.4 liter) will not have MDS (four cylinder mode) when paired with a manual transmission.

A new, expensive but sturdy eight-speed automatic transmission will eventually go along with the 6.4 Hemi, along with the V6 and standard Hemi.

An "Apache 392" has reportedly been under development, a supercharged version pushing out over 600 hp. Rumor has it going into the Challenger and Grand Cherokee. Allpar member "autojunkie" wrote:

I can say that the 392 supercharging is a done deal. It was spotted on some Chrysler engineering documentation and slated for 2014. It specifically mentioned the Grand Cherokee for production intent.

I've seen many engineering documents that had projects that were eventually cancelled. The document detailing the supercharged 392 did point to production-intent, but it does not mean that it couldn't fizzle off the plan still.
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Whither the 500 horsepower Hemi?

The 392 was originally rumored to hit 500 horsepower (though Allpar was always more conservative and predicted 450-480 hp). Tom Langston noted that Kraig Courtney, SRT engine design supervisor, had addressed these rumors in Mopar magazine (March/April 2011), saying that they had improved the area under the torque curve at the expense of peak numbers:

We were pretty confident that we could get 500 hp, but ... the prototype 392 was strong up high, but as you accelerated, it didn't give you that kick in the butt we expected- a kick in the butt approaching that of the muscle cars of old. ... The first 6.4 just wasn't as strong feeling as the numbers said it should be. So we took a look at the intake that's being used in our trucks right now, it's an SRV dual-runner intake that we can run short runner or long runner. Obviously, you run short runner for power and long runner for torque. So we made up some manifolds and ran them in a car and it felt a lot better. Not as much horsepower, but the area under the torque curve was far bigger. ... We can go out and, quite honestly, take on some of the competition's cars that are more powerful according to their numbers.
6.1 SRT8 6.4 (392)
As used in...ChallengerChallenger
Horsepower425 @ 6,000470-485
Torque420 @ 4,800*460-475
EPA mpg13/1814-15/23
* Manual transmission is limited to 410 lb-ft torque.

Dual spark plugs

Michael E. Gemmel wrote: "Each cylinder has an ignition coil pack over one spark plug, and a
regular plug wire connected to the other spark plug. Further, the coil
pack also has a plug wire attached to it that extends to the opposite
cylinder bank. Each cylinder shares a coil pack with
another cylinder. Each of the two plugs on a given cylinder is fired
by a separate coil. One plug has a coil directly attached, and the
other is fired via an ignition wire connected to a coil located on
another cylinder on the opposite bank. The benefits would be one-half
the number of coils (8 vs. 16) compared to each plug having its own
coil, and of course less weight."

"Cryptojoe" wrote: "The extra plug fires during the power stroke to more fully burn the hydrocarbons. ... the second ignition allows additional power in the down stroke while lowering the need for restrictive catalyst plates in the converter.

"In the 1980s Japanese manufacturers reduced unburned hydrocarbons by placing spark plugs either in the exhaust pipe (which fired with every piston ignition) or in the exhaust manifold (which fired each time their corresponding cylinder fired). Chrysler morphed this idea to include dual fired plugs on each cylinder, which allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke."

Patrick added: "This [also reduces] NOx and ozone. Full combustion results in heat, water, and carbon dioxide. NOx emissions are only significant during incomplete or partial combustion, due to the lack of available oxygen, high temperatures, and various chemical reactions. That's why catalytic converters have been standard on cars for the past 3 decades. The extra set of spark plugs on the HEMI and on previous engines are designed to reduce emissions before a catalyst is needed. They add some horsepower, but not very much."

Specific advantages

The source for most of this information is Popular Hot Rodding.

The cam was placed high up in the block to keep the pushroads as
short as possible. The hollow cam has oversized journals and lobes to
minimize side loading on the roller-style lifters. The valve springs
are beehive types, more effective than standard springs so they can be
lighter, with less lifter collapse. Rockers have much less inertial
mass than usual, with the form and size carefully designed for a
conservative .500 inches of lift; but the valves flow well enough to
make this more than enough.

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The engine has been designed for lighter weight. The new
Hemi is precision cast, which allows it to be lighter than a typical
5.7 liter engine, even with a taller deck height than Chevy's; and,
partly to counter the inertia of its relatively long stroke, the
pistons were made light as well, using cast eutectic alloy. The
slipper-style piston has much in common with racing pistons, with a
weight of 413 grams. For longevity, the Hemi pistons use a hard anodize
on the top ring land, to act as a heat barrier and anti-micro weld mix,
and to allow the top ring to be only 3 mm from the top of the piston,
cutting emissions while bringing more power. As with the old 426 Hemi,
the rings are also relatively thin. Also in common with racing engines
is a reservoir groove underneath the top ring, to reduce the pressure
between the top and second ring.

The skirt is coated to allow for variance in production piston
sizes, increase the fit for ring seal, and reduce piston noise. The
lightweight wrist pin is also high-set.

The crank has larger inner counterweights than equivalent Chevy
engines; but their weight is offset by the lighter pistons and rods. A
windage tray sits underneath the crank, while the serpentine belt
pulley also acts as a torsional vibration damper. The connecting rods
are also designed for strength and low weight, negating the need for a
balance pad. A cap bolt is used instead of a through bolt.

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A speed density system is used for measuring air into the engine rather than air mass.

The 2 inch ports flow 270 cfm at .600 inches of lift, with peak flow at .375 inches. At .250, the stock head also had excellent flow. The exhaust port hits 161 cfm at .600 lift, with a 1.55 inch valve. Both have unusually good velocity and distribution throughout their range - even compared with a Chevy LS6 engine. The engine apparently takes well to simple porting.

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The original Hemis and the current one both have dual rocker shafts,
large (similarly sized) valves, and two valves per cylinder arranged in
a similar pattern for efficiency. The engine's unique two-valve hemispherical combustion chamber
provides impressive air flow, torque, and power.
Fifty-six pounds lighter than the 5.9-liter V-8, the 5.7-liter Hemi
produced 41% more power and 12% more peak torque in its original form.

An electronic throttle control system compensates for changes in the engine load needed
by the air conditioning system, compressor, alternator, power steering
pump and automatic speed control.

Repair notes

Hesitation when shifting (up to 2005).
The engine computer lowers torque
when shifting to preserve transmission life, but a service bulletin covers shift hesitation during
wide-open throttle on early (2005) SRT-8 cars. When the
gas pedal is floored, the engine RPM may go beyond the redline before
the shift from first to second is made, so there's a "short
hard bump" as the shift begins, especially when
the pedal is floored from about 15 mph. The problem has
been solved with new software, and only applied to cars built before July 25, 2005. There is also reportedly a computer upgrade that fixes intermittent engine RPM limiting and ineffectual A/C in early (pre-2006?) Hemis.

What made the 6.1 SRT version different

The SRT engine includes numerous modifications from the standard Hemi, such as:

  • Reinforced bulkheads.
  • Larger cylinder bores, by 3.5
    millimeters, honed with torque plates to ensure a truer
  • Oil squirters, aimed at the underside of each piston, aided by a special oil pump
    pressure relief valve.
  • Modified oil pan and windage tray.
  • Larger-diameter, flat-top pistons with high-load capability.
  • Revised connecting rods using higher-strength powder metal material. New
    floating piston pins.
  • Crankshaft forged from
    micro-alloy steel, in tri-metal main bearings, with retuned damper.
  • Head ports designed
    with larger cross-sectional area. This allows 11 percent higher flow in
    the intake ports, and 13 percent higher flow in the exhaust ports.
  • A billet steel, high-strength camshaft features more overlap and lift for better performance.
  • Intake valves feature hollow stems and 2 mm larger heads. The hollow exhaust
    valve stems are filled with sodium to dissipate heat.
    Premium valve springs with external dampers enable higher engine speed operation to
    6,400 rpm and allow the peak output
    engine speed to increase to 6,000 rpm from 5,000 rpm.
  • Specially designed intake and exhaust manifolds.
  • Cast aluminum intake manifold with shorter,
    larger-diameter and tapered runners for high-speed tuning. Internal
    runners are core-dipped to smooth the runner finish and improve air
  • 14% higher fuel injector flow capacity; higher-flow air
    cleaner box with a tuned resonator delivering a deep
    performance sound character (and good for an extra eight horsepower).
  • Exhaust headers are individual
    tubes encased in a stainless steel shell. Exhaust runners allow
    increased gas flow while maintaining fast catalyst light-off, while
    adding 12 horsepower over the 5.7-liter engine's cast manifolds.
  • Dual knock sensors with premium


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