Hellcat technical overview and highlights
The Hellcat is a 6.2-liter supercharged HEMI® pushing 707 horsepower to the rear wheels.
The SRT Hellcat is named after the Grumman F6F Hellcat, an aircraft carrier-based U.S. Navy fighter plane used during WWII.
The Hellcat was a rugged, high-performance fighter. It proved to be a very high quality aircraft with a straightforward design. Because of those qualities, the Hellcat was the least modified fighter of the war, with 12,200 built in just over two years.
Unique Engine Features
To create an engine to honor this legacy, the engineers in the SRT division started with a 6.4-liter Ram cylinder block. To stiffen and strengthen the forged crankshaft, they shortened the stroke. Forged pistons, diamond-like carbon (DLC) coated wrist pins, and forged connecting rods round out the changes to the bottom end. The engine uses a new oil pan with extra baffles and a windage tray to keep the oil where it belongs under hard acceleration .
As a testament to the naturally great design of the HEMI head, the only significant changes are the use of unique exhaust valves, which use a different material for the valve head, and an advanced alloy for the cylinder head casting.
Adding a supercharger dictated some new hardware and several changes to the fuel management system.
A bypass valve, controlled by the powertrain control module (PCM), is mounted to the supercharger to vary boost, as required.
The gaskets, drive pulley, and coolant manifold are serviceable.
The only replaceable components inside the supercharger are two charge-air coolers (CACs). The supercharger weighs over 40 kg (90 lbs).
Auxiliary Supercharger Cooling
The system uses an auxiliary cooling loop to reduce the temperature of the compressed air charge by transferring the heat from two water-to-air CACs, inside the supercharger, to two radiators.
The auxiliary cooling system is completely separate from the engine cooling system, with a separate reservoir, coolant pump, radiators, and hoses. Also, the supercharger auxiliary cooling system operates at a lower pressure than the engine cooling system. Do not mix up the pressure caps.
An electric coolant pump, mounted to the left of the engine radiator, draws coolant from the CACs. The coolant pump receives battery and ignition voltage from the rear power distribution centre (PDC). The PCM controls the pump over a local interconnect network (LIN) line.
The coolant pump pushes the coolant through the auxiliary low temperature radiator (LTR) before passing through the primary LTR.
Servicing the auxiliary cooling system is straightforward. The CACs, coolant pump, radiators, and plumbing are replaceable. The auxiliary cooling system uses the same organic additive technology (OAT) coolant as the engine cooling system.
The PCM reads coolant temperature from a sensor mounted in the auxiliary coolant manifold to the right and rear of the supercharger .
Do not activate the auxiliary coolant pump any longer than required to verify proper operation. Coolant acts as lubricant for the pump. So, it will be damaged if it is operated without coolant. Always refer to service information for specific instructions.
While both cooling systems include a bleed port that is designed to release trapped air, always use a vacuum fill device. if the system has been opened. Manually bleeding the cooling system may not remove all of the air, causing the system to fail.
Oil Cooler with Drain Plug
With horsepower comes heat. So the Hellcat uses an external oil cooler. The cooler has a drain plug. So, be sure to drain the oil cooler during oil changes.
A mass airflow sensor (MAF) is attached to the inlet of the huge (92 mm) throttle body.
Oiled air filters should never be used with a MAF sensor. The high volume of air passing through the air filter can pull oil from the filter element and coat the sensor wire, which will cause false airflow readings (Figure 10).
Four manifold absolute pressure (MAP) sensors are installed on the supercharger.
The Hellcat engine uses a GPEC 2a PCM. The Hellcat GPEC 2a is unique, because it has an on-board barometric pressure sensor.
Two Different Keys
The vehicle is delivered with two keys : A red and a black key. The red key unleashes the full 707 horsepower and allows the driver complete control over the stability and traction control systems. The black key allows the engine to produce about 500 horsepower, with lower RPM shift points, full stability control, and other performance modifications for a less exhilarating driving experience.
Mass Airflow Sensor Operation
The MAF is a heated-wire type sensor that allows the PCM to calculate the volume of air entering the engine. Intake air, flowing over the wire, cools the wire. This reduces the resistance of the wire, which increases current flow through the wire. The PCM monitors the current to calculate airflow.
The airflow is displayed on the scan tool in grams per second. The MAF, which includes a temperature sensor, is used to verify the rationality of the MAP sensors and provide inlet air temperature information to the PCM.
Four MAP Sensors
Four MAP sensors, which also include temperature sensors, are used on the Hellcat engine. One is installed at the supercharger air inlet. A second MAP sensor is installed on the pressure side of the supercharger.
In addition, two additional MAP sensors are installed on the supercharger, one on each bank near the CACs. All MAP sensors are interchangeable.
Barometric Pressure Sensor in PCM
While most engine management systems read the MAP sensor at Key ON to determine atmospheric pressure (baro), the GPEC 2a PCM uses its onboard pressure sensor for this function. If the sensor goes bad, the PCM must be replaced.
Variable Speed Fuel Pump
The Hellcat engine uses a fuel system similar to the High Fuel Efficiency (HFE) Ram truck. The system includes a fuel pressure sensor, fuel pump control module (FPCM), and variable speed fuel pump.
The PCM controls fuel pressure by varying the speed of the pump by using pulse-width modulation, through the fuel pump control module.
The fuel pressure sensor is mounted in the fuel rail crossover tube, at the right, rear of the supercharger.
The fuel pump control module is mounted inside the trunk, behind a trim panel on the right side.
As mentioned earlier, there is a bypass valve between the inlet and pressure sides of the supercharger. The PCM controls the valve, when required, to reduce manifold pressure. The bypass valve is a modified version of the Tigershark throttle body.
Instead of being normally closed at rest, like a throttle body, the bypass valve is modified to be normally open.
The PCM can set the bypass valve to any angle between fully open and fully closed to control boost.
More Fuel Control Sensors
A crankcase pressure sensor is located in front of the throttle body, in the make-up air tube.
Four oxygen sensors monitor the fuel control and catalyst systems. The upstream sensors are wide-band and the downstream sensors are narrow-band.
Wide-band sensors allow the PCM to control the air to fuel ratio more precisely under a wide range of operating conditions-from idle to full throttle.
Any car with this much power and torque needs a tough transmission or two available. The manual transmission is a Tremec TR-6060, similar to the SRT Viper’s gearbox.
A larger flywheel, stronger clutch, and gerotor oil pump round out the high performance manual package.
The oil pump sends transmission fluid to an oil cooler located in front of the radiator. The oil cooler is the same part number for both the automatic and manual transmissions.
The automatic, an 8HP90, is a Hellcat exclusive. The transmission features additional clutch packs and a larger torque convertor, and uses only “8&9-speed ATF.”
The 8HP90 features a new poly-stable electronic shifter that incorporates an electronic shift module (ESM). The shifter has no mechanical connection to the transmission.
The shifter is unique, because it provides tactile feedback by using detents for each position. It feels more like a conventional shifter and less like a toggle switch.
A manual mode can be activated by moving the selector to the left, then forward and backward to change gears.
A manual park release mechanism is provided in case electrical power is lost.
Front lights and air intake
The front lighting is unique to the SRT.
Instead of a projector beam, the left, inboard “lamp” is actually an air catcher inlet port that pushes air directly into the air box.
Since there is no screen at the air inlet, you never know what you are going to find in the air box!
When servicing the air cleaner, be sure to disconnect the air inlet hose from the throttle body to avoid breaking a locking tab.
Uconnect Performance Pages
There are a number of unique features available on this model through Uconnect’s Performance Pages. So many features, in fact, that there is a separate owners’ manual for the system!
Performance Pages are accessed via the Uconnect Touchscreen. Performance Pages includes several configurable drive modes that allow you to enter and save performance adjustments to the engine, transmission, steering response, and traction control.
Valet Mode and Paddle Shifter Enable
If an SRT Challenger comes in and feels low on power, the car may be in valet mode. If so, you will see an indicator in Electronic Vehicle Information Centre (EVIC) letting you know. You will need the customer’s password to take it out of valet mode.
Also, you will want to keep in mind that the paddle shifters can be turned on and off in the Performance Pages. If a customer brings in a Hellcat with a complaint of inoperative paddle shifters, quickly check the Performance Pages to make sure the paddle shifters are enabled.
2015 Challengers use the PowerNet architecture. This means that functions that were handled by the cabin compartment node (CCN) on previous models are now handled by the body control module (BCM).
The four controller area network (CAN) bus star connectors are in the same locations as current Chargers and 300s.
The rear axle has been refined for the entire Challenger line, including a 230 mm ring gear on all V8-equipped cars. The ring gear is welded to the carrier for improved strength and efficiency. Currently, only seals, mounting bushings, and fluid can be replaced. Defective gears and bearings require replacing the entire axle assembly.
2015 Challengers now use low-viscosity gear oil. Base models use 75W-85, and all SRT models use 75W-90. Always refer to service information for the latest lubricant and additive specifications.
Other 2015 Challengers have electronic power steering (EPS), but the Hellcat has a conventional, hydraulic power steering system.
Active Exhaust Tuning
PCM-controlled active exhaust valves provide exhaust tuning on all V-8 Challengers. An active valve is located on each bank, under the rear cradle. The actuators share the LIN bus with the auxiliary coolant pump.
The actuators and spring are serviceable, but the valves are part of the exhaust pipes.
Left and right side actuators are different, but can accidentally be installed in the wrong position. Check the part numbers.
The actuators run through their full range of motion each time the ignition is cycled ON. If there is an obstruction, or if an actuator is disconnected, the PCM will turn on the malfunction indicator light (MIL).
The PCM actuates the active exhaust valves based upon the selected drive mode; adjusting the position of the valve constantly, tailoring the sound to suit driving conditions.
The Sport setting is more aggressive, and the Track mode locks the valves wide open, which is a place where the Hellcat is very comfortable.
Hellcat Ship Mode
Like other PowerNet vehicles, the 2015 Dodge Challenger Hellcat arrives at the dealership in Ship Mode. Ship Mode deactivates all non-essential equipment in the car to control battery discharge.
Unlike other PowerNet vehicles, the Hellcat Ship Mode includes a special low power mode. In the “Ship,” low power mode, the engine operates at reduced power, regardless of which key is used. This allows pre-delivery technicians and other dealership personnel to operate the vehicle safely until it can be properly checked in.
Also, it allows transport drivers to drive the car onto the truck instead of through it!