GM 3.6 Liter Twin Turbo V6 LF4 Engine
The LF4 is a six-cylinder, 3.6-liter twin-turbocharged engine produced by General Motors for use in high-performance vehicles. It is a more powerful version of GM’s 3.6-liter V6 LF3.
Rated at an estimated 455 horsepower (339 kW) and 445 pound-feet of torque (603 Nm), the LF4 has several unique features that expand its performance range and support track capability, including:
- Low-inertia titanium-aluminide turbines
- Lightweight titanium connecting rods
- An efficient and patented low-volume charge-air cooler
These features contribute to optimal boost production and more immediate power delivery.
Approximately 90 percent of the LF4’s peak torque is available from 2,400 rpm to 6,000 rpm, meaning that the engine has broad torque curve that is conveyed in a feeling of responsive, sustained power at all rpm levels.
In addition, a high-performance lubrication system is designed to maintain optimal oil pressure and ventilation during high-lateral driving maneuvers typically encountered on a track. Even the turbochargers are designed for track performance, with the compressors matched for peak efficiency at the engine’s peak power levels, which means they demand less heat rejection (intercooling) for higher sustained power during track driving.
The twin, low-inertia turbochargers’ featherweight titanium-aluminide turbines are used with vacuum-actuated wastegate control for precise, responsive torque production. In fact, the titanium-aluminide turbines reduce rotating inertial load by 51 percent, compared to conventional Inconel turbine wheels. That means less exhaust energy – which spins the turbines – is wasted in stored inertial loads.
In practical terms, that means the relatively small size of the turbochargers and their lightweight turbines foster more immediate “spooling,” which virtually eliminates lag, for an immediate feeling of power delivery. They produce up to 18 pounds of boost (125 kPa).
A single, centrally located throttle body atop the engine controls the air charge from both turbochargers after the temperature is reduced in the intercooler. This efficient design also contributes to more immediate torque response, while reducing complexity by eliminating the need for a pair of throttle bodies.
Unique vacuum-actuated wastegates – one per turbocharger – are used with the Twin Turbo for better management of the engine’s boost pressure and subsequent torque response for smoother, more consistent performance. They are independently controlled on each engine bank to balance the compressors’ output to achieve more precise boost pressure response.
The wastegates also work in concert with vacuum-actuated recirculation valves to eliminate co-surge from the turbos – a condition that can result in dynamic flow reversal, such as the moment immediately after the throttle closes. This overall system integration contributes to the engine’s smoother, more consistent feeling of performance.
Patented Low-Volume Charge-Air Cooling
The Cadillac Twin Turbo’s patented, manifold-integrated water-to-air charge cooling system also contributes to more immediate torque response, because the compressors blow through very short pipes up to the intercooler.
With no circuitous heat-exchanger tubing, there is essentially no lag with the response of the turbochargers. In fact, airflow routing volume is reduced by 60 percent when compared with a conventional design that features a remotely mounted heat exchanger.
“It is a very short path from the turbos to the throttle body,” said Bartlett. “The compressors draw their air directly from the inlet box and send their pressurized air through the intercooler basically immediately, for a tremendous feeling of power on demand.”
The charge-cooling heat exchangers lower the air charge temperature by more than 130 degrees F (74 C), packing the combustion chambers with cooler, denser air for greater power. Also, the air cooler system achieves more than 80 percent cooling efficiency with only about 1 psi (7 kPa) flow restriction at peak power, which contributes to fast torque production.
Because the Cadillac Twin Turbo is based on the same architecture as the existing naturally aspirated 3.6L V6 LFX, it benefits from many proven technologies including dual-overhead camshafts, variable valve timing and direct injection.
It also has unique features that strengthen the foundation to support the greater cylinder pressure that comes with turbocharging, including:
- Cylinder block and cylinder head castings that are specific to Cadillac Twin Turbo variants
- Lightweight, low-inertia titanium connecting rods are unique to the ATS-V’s engine and contribute to faster engine speed acceleration to match the quick spool time of the turbochargers, along with lowering the reciprocating loads on the rod and main bearings
- Machined, domed aluminum pistons with top steel ring carrier for greater strength
- An oil pan optimized for track performance
- 10.2:1 compression ratio
- Optimized direct injection fuel system for improved combustion efficiency and lower emissions
- Tuned air inlet and outlet resonators, aluminum cam covers and other features that contribute to exceptional quietness and smoothness
The cylinder block casting has cast-in provisions for turbocharger coolant and oil connections, as well as positive crankcase ventilation passages. It uses nodular iron main bearing caps for greater strength to manage the higher cylinder pressures that come with turbocharging. It also features re-targeted piston oil squirters for improved temperature control
The unique cylinder heads feature a high-tumble intake port design that enhances the motion of the air charge for a more-efficient burn when it is mixed with the direct-injected fuel and ignited in the combustion chamber. The topology of the pistons, which feature centrally located dishes to direct the fuel spray from the injectors, is an integral design element of the chamber design, as the piston heads become part of the combustion chamber with direct injection.
“The high-tumble heads were developed with advanced modeling programs that helped us determine the optimal design in less time and with less trial and error,” said Richard Bartlett, assistant chief engineer. “Literally hundreds of simulations were performed to optimize the port flow, injector spray angle and pattern, and piston topology to produce a highly efficient yet powerful combustion chamber.”
Large, 38.3-mm intake valves and 30.6-mm sodium-filled exhaust valves enable the engine to process tremendous airflow. In some conditions, the continuously variable valve timing system enables overlap conditions — when the intake and exhaust valves in a combustion chamber are briefly open at the same time – to promote airflow scavenging that helps spool the turbochargers quicker for faster boost production.
Hardened AR20 valve seat material on the exhaust side is used for its temperature robustness, while the heads are sealed to the block with multilayer-steel gaskets designed for the pressure of the turbocharging system.
The heads also feature integral exhaust manifolds with upper and lower water jackets that provide uniform temperature distribution and optimal heat rejection.
A modified version of the production-spec LF4 engine, dubbed LF4.R, powers the Cadillac ATS-V.R race car.
More information on the LF4.R.
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