by Jim Kerr
Auto manufacturers have many ways of getting more power from an engine. Bigger displacement was the easiest and cheapest method in the 60’s and 70’s. If you wanted more power from a smaller engine, high compression pistons, radical camshafts, and wild looking intake and exhaust manifolds were the methods of choice. This made power, but often at the loss of every-day driveability. Then, along came emissions standards and fuel economy guidelines. The horsepower race was over – or was it?
Today, auto manufacturers are making more power per litre than was dreamed possible a few decades ago, and achieving decent fuel economy while meeting emissions standards at the same time. Electronic fuel injection, multiple camshafts and valves, and computerised control of many engine operations make today’s vehicles perform better than ever before.
Honda still leads the race when measuring power per litre. Their S2000 sports car uses VTEC variable valve control and lots of rpm’s to produce 240 horsepower from a 2.0 litre engine. The 2003 Dodge Viper is coming with 500 horsepower from an 8.0 litre V10. There is no substitute for cubic inches! Corvette’s Z06 puts out 405 horsepower out of a 5.7 litre V8, achieved with careful attention to airflow in and out of the cylinders. That is the key to engine performance: airflow.
Airflow in most engines is only about 80% efficient. This means that at normal running speeds, the cylinders are only filled 4/5ths full. This called the engine’s volumetric efficiency. Increase volumetric efficiency and we can get a lot more power. There are lots of restrictions to airflow. The air cleaner, throttle plates, bends in the manifolds, intake and exhaust valves, and the exhaust system all slow airflow into the cylinder. Better designs and new technology have helped improve engine volumetric efficiency.
Interesting examples of new technology are found in BMW’s new 7 series sedan engine. Instead of throttle plates, engine speed is controlled by varying the height of the intake valve opening. No throttle plates means less air restriction. Several manufacturers are using “staged” variable length intake manifolds to optimise volumetric efficiency at certain rpm’s. BMW has gone one better by using a rotary core in the intake manifold that can be turned to provide the best length intake runner for a wide range of engine speeds. Technology has enabled us to sometimes improve volumetric efficiency even beyond 100%.
By using the inertia of the incoming air and the exiting exhaust gases, it is possible with the right design to create a “ram” effect of air into the cylinder. The cylinder actually contains air under a slight pressure. Unfortunately, this usually only can be designed into a narrow rpm range of engine operation. To get the cylinders full over a wide rpm range, we need to pump air in.
Turbochargers and superchargers are two methods of pumping extra air into the engine. Turbochargers are the most efficient air pumps, using exhaust gas flow to turn the compressor, but there are disadvantages. Exhaust system routing for turbochargers can be difficult in tight engine compartments, and then there is “turbo lag”. Turbo lag is the time it takes for the turbocharger to spin up and produce boost when the vehicle is accelerated. Volvo and Saab come to mind when thinking of turbochargers. Both manufacturers produce excellent vehicles using smaller, lightweight turbochargers to reduce lag, but slight lag still exists.
Superchargers are typically belt driven air pumps. A Roots-type blower manufactured by Eaton is commonly used. The Roots blower uses two counter-rotating rotors to force air into the engine. Several manufacturers have built supercharged vehicles. Ford plans to build a Mustang V8 with one. GM had used them on 3800 V6 engines. Jaguar is building several supercharged models, including the new 400 horsepower S-Type sedan.
While these superchargers may share some features with the “hot rod” versions, original equipment superchargers bypass the pressurized air back to the inlet under cruise conditions so there is no load on the supercharger or engine. Step on the throttle and a valve closes, directing pressurized air into the engine immediately. No lag, just performance.
How much performance? In the Jaguar S-Type 4.2 litre engine, horsepower jumps from 300 without a supercharger to 400 with one! And all this without radical camshafts or wild intake designs.
The horsepower race is far from over. Only now, it is civilized enough to drive every day.