Remember the Ferrari 348? Its 3.4-litre V8 produced less than 300bhp at the model’s launch in 1989. Yet just 20 years later the turbocharged 2.5-litre five-cylinder motor in the Mk2 Focus RS developed similar power, while today there are several hot hatches with even smaller engines that have outputs around the 300bhp mark.
So how have engineers made more power from smaller engines? One of the key considerations for engine designers is the direct relationship between the mass-flow rate of air entering an engine and brake horsepower. In other words, more air means more power. This gives a number of areas where engineers can focus to increase power, but the most important are volumetric efficiency, charge density and engine speed.
Volumetric efficiency is how well the car breathes, and specifically a measure of how much fresh air is entering a cylinder per cycle compared to the swept volume of the cylinder. Engineers strive to improve the inlet and exhaust systems, head design, compression ratio and the cooling system to improve performance and economy. This area is one of the most studied by engine-development engineers.
The most commonly used method to increase an engine’s charge density is through forced induction. Superchargers or turbochargers – or both – can be used to compress the air before it enters the engine. This also increases volumetric efficiency levels above 100 per cent. The effectiveness of turbo/supercharging is why so many manufacturers are downsizing and ‘boosting’ their power units.
Quite simply, if the engine speed is increased there is more ‘suck, squeeze, bang, blow’ occurring during any given period of time. Unfortunately, we can’t run road-car engines at F1-engine speeds (approximately 15,000rpm) due to reliability and NVH (noise, vibration and harshness) concerns. It’s fine to run race engines at high revs, as the increased stress is dealt with by utilising very expensive, lightweight and strong materials. A road car needs to last a long time and be cheap to produce, so this isn’t an option.
As you can see, it’s simple engineering that explains why manufacturers are downsizing and using forced induction: it’s the most effective way of meeting today’s strict emissions targets while maintaining competitive performance. Not great news for naturally aspirated purists, but good for drivers who want cleaner and/or more powerful engines.