by Hib Halverson
November 26, 2014
imagery by GM Communications.
Page 3 of 4
Heads for 3.6L V6tt engines are made using the Rotocast™ process which provides the higher material strength needed in a high specific-output engine. The intake port is a unique design which imparts a tumbling motion to air flow as it spills into the combustion chamber through the 38.3-mm intake valve. The motion from this “high-tumble intake port” contributes to more efficient combustion after the fuel is injected and the spark ignites the mixture. The exhaust valve is 30.6-mm and has a sodium-filled, hollow-stem for adequate valve head cooling under high-rpm/high-load duty cycles and it closes on a specific, AR-20 seat needed for durability at power levels these engines are achieving. The heads’ integral exhaust manifolds have both upper and lower coolant jackets for better heat rejection in a high-performance, turbocharged application. Because of the high combustion pressure typical of turbo engines, multi-layer-steel head gaskets are used
The turbocharger housings are liquid-cooled. LF4 turbos have titanium-aluminide exhaust turbines which, because of their lower mass, allow the turbo accelerate or “spool-up” more quickly. Titanium-aluminide turbines reduce rotating inertia by 51%, compared to conventional, Inconel turbines. That means less of the exhaust energy, which spins the turbines, is wasted in stored inertial loads. On LF4, the turbos’ compressor wheels and their inlet adapters are optimized for best efficiency at the higher engine speeds and power output attainted by ultra-high performance version of the 3.6L V6. The relatively small turbocharger size and their lightweight turbines reduce lag and provide a more immediate feeling of power.
Both the LF3 and LF4 have vacuum-actuated wastegates to better manage the engine’s boost and throttle response. The waste gates are independently controlled which balances the compressors’ output and achieves more precise boost response. Rich Bartlett, Assistant Chief Engineer (ACE) for the LF3 and LF4 told the Cadillac V-Net, “The wastegates work in conjunction with vacuum-actuated recirculation valves to eliminate turbocharger “co-surge”, a condition typical of twin-turbo architecture which can result in flow reversal under certain conditions, such as right after the throttle closes. This turbocharger system integration contributes to the engine’s smoother, more consistent feeling of performance.”
The turbos blow into a “low-volume” charge air cooler and induction system, “low-volume” meaning the displacement of the induction system is reduced by to shorter pipes between the turbo outlets and the cooler inlet and between the cooler outlet and the engine’s centrally-located throttle body. ACE Bartlett tells the V-Net that he estimates total pressurized volume of ATS-V’s turbocharger system is reduced by more than 60% compared to typical system designs of the past. That lower volume is another contributor to the engine’s rapid throttle response. The LF4’s turbocharger system can produce up to 18 pounds boost which makes use of at least 91-octane premium fuel required with 93-octane recommended for best performance.
The GM Powertrain folks tell us all this Cadillac engine magic will be worth 455-horses at 5750-rpm and 445-pound/feet torque at 3500 rpm. What will undoubtedly make the LF4–said by Cadillac to be the most powerful V6 in the compact luxury segment–a blast to drive is it can provide 90% of that 445-lb/ft torque between 2400 and 6000-rpm.
Those buying the littlest V-Series will have two transmission choices, the 6L95M, six-speed manual, previously used in the CTS-V, or a specific version of the, new Hydra-matic 8L90, eight-speed automatic.
The manual has the “Active Rev Matching” feature GM introduced on the 2014 Corvette. It can anticipate downshifts and blip the throttle to match engine speed to transmission speed for a quicker and easier change to a lower gear. The six-speed is also part of PTM and, when the system is enabled, the driver can achieve “no-lift” upshifts and wide-open throttle standing starts using “launch control.”