By Jim Kerr

In case you think higher fuel prices have slowed the horsepower race, think again. Cadillac has a new engine for their high performance STS-V sedan and XLR-V sports car that will challenge some of the most powerful cars on the road. The big difference with this latest generation of high performance engines is that they can get good fuel economy too.

This new engine is based on Cadillac’s Northstar V8 and shares some design characteristics and parts. In fact, about half the engine parts are new or heavily revised. There is one big difference however between the 4.6 litre Northstar engine and this new 4.4 litre V8: it is supercharged.

A supercharger is a mechanically driven air pump that can be driven from the engine’s crankshaft by a gear set or by a belt. In this 4.4 litre V8, the supercharger is bolted to the valley area of the engine between the cylinder heads and it is driven by the same belt that turns other engine accessories.

Inside the supercharger, two helical rotors pump air from the air cleaner into the engine intake manifold. This improves volumetric efficiency of the engine by making sure the cylinders are completely full of pressurized air when the driver demands performance. Normally aspirated engines, those without superchargers or turbochargers, operate in the 75% to 85% volumetric efficiency range because restrictions in the air intake ducting and engine prevent the cylinder from filling completely. Compressing the air charge overcomes this, but not without side effects.

2006 Northstar SC 4.4L V-8
2006 Northstar SC 4.4L V-8. Click image to enlarge

It takes a lot of horsepower to operate a supercharger. The supercharger used on Top Fuel Drag Race cars can use more horsepower than a powerful passenger car V8 engine would put out. Fortunately, the supercharger enables the engine to produce more power than it consumes. For a passenger vehicle however, we don’t need that high horsepower all the time, so an air intake bypass valve is added on the STS-V supercharger so that air can bypass the compressor for most operating conditions. The supercharger rotors spin freely with no load on them until the driver puts the gas pedal down. Then the bypass valve closes and the supercharger instantly pressurizes the air to over 10 PSI. The computer adds extra fuel at the same time and hold onto your hats – the STS-V can accelerate from 0 to 60 mph in less than 5 seconds!

Another effect of supercharging is increased heat in the cylinders. The Northstar’s pistons have a polymer coating on the skirts to resist additional load and heat. Also, oil spray nozzles in the crankcase direct engine oil to the bottom side of the pistons to help cool the piston top. This oil cooling method has been used by other manufacturers such as BMW, but it is much more common in diesel engines than gasoline engines.

2006 Northstar SC 4.4L V-8
2006 Northstar SC 4.4L V-8. Click image to enlarge

One major disadvantage of supercharging is that the compressed air gets hot. Hot air expands so there is not as much oxygen getting into the cylinders. This can be overcome by intercooling, or cooling the air after it is pressurized. Many engines with turbos or superchargers use an air-to-air intercooler in front of the radiator to reduce intake air
temperatures. Cadillac does it with an air-to-liquid intercooler mounted integral with the supercharger. It’s the first thing you see on top of the engine.

The intercooler has it’s own cooling system. Coolant is passed through tubes in the intercooler, absorbing heat from the air passing around the tubes. This heat is then dissipated through a smaller radiator in front of the regular engine radiator. An electric pump keeps the coolant flowing. This cooling system is completely separate from the regular engine coolant. Its whole purpose is to keep the air cool, maximizing performance.

While the 32-valve cylinder heads (four valves per cylinder) remain largely unchanged from the Northstar V8, they have used Extrude Honing for the exhaust ports. This patented process forces abrasive grit in a paste material through the exhaust passages in the cylinder head to polish the ports and improve exhaust flow. Normally only used in racing engines, this process improves engine horsepower by increasing airflow through the engine. In simple terms, if you can’t get the exhaust gases out, you can’t get fresh air and fuel in.

This supercharged 4.4 litre Cadillac engine offers amazing performance. It puts out 469 horsepower @ 6400 rpm and 439 lb.ft torque @3900 rpm but that high torque output starts right at idle, thanks to the supercharger. Add variable camshaft timing and you have an engine that puts out power that we only had in race engines just a few short years
ago. And the icing on the cake – its fuel economy is rated at 10.7 L/100 km on the highway. Now that’s performance.

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