One-thousand horsepower, four-cylinder engines are today’s reality in import drag racing. This reality includes the leading-edge forced induction and engine-management technologies that make power production the easy part of building a racecar. Today’s performance engines are running at higher boost pressure levels and higher compression ratios than ever before. Understanding how both compression ratios and boost pressures affect performance is a key to maximizing performance from your street or race vehicle.
By Michael Ferrara // Photos by DSPORT Staff
4-stroke Engine Basics
Without going into a lengthy explanation of internal combustion engine dynamics, your vehicle’s engine is a machine designed for energy conversion. Using a four-stroke cycle, a fuel-and-air mixing strategy and a spark for ignition, the internal combustion engine’s first task is to convert the chemical energy stored in the fuel into thermal energy (heat) through a process called combustion. The engine’s second task is to convert this thermal energy into kinetic energy in the form of horsepower at the flywheel. How well an engine can convert the heat (thermal energy) into power (kinetic energy) is quantified by an engine’s thermal efficiency. An engine’s thermal efficiency is highly influenced by the engines static compression ratio.
As the name indicates, the compression ratio of an engine indicates how much the air-fuel charge is compressed during the compression stroke of the four-stroke process. A 10-to-1 compression ratio means that the air- fuel mixture gets squeezed down from the full volume of the cylinder to a volume that is just roughly one tenth of the cylinder’s size. So how does an engine’s compression ratio affect performance? All other factors being equal, an engine with a higher compression ratio will deliver a higher thermal efficiency. This means that the engine is able is turn more of the heat generated from the combustion process into horsepower instead of wasted heat. In basic terms, higher thermal efficiencies translate into additional horsepower and better fuel economy.
How much additional power can be expected with a higher compression ratio? The old-school rule of thumb is that each additional point that the compression ratio is raised will deliver an additional 4 percent power. In fact, more accurate projections can be found in the accompanying DSPORT chart. These values were obtained using the thermodynamics equation to establish the thermal efficiency of an Otto cycle engine.
Plugging through this equation we find an increase in compression ratio from 8.0:1 to 11.0:1 should result in a 9.2-percent increase in power. Likewise a reduction in compression ratio from 11:1 to 7.0:1 should result in a 12.3-percent decrease in power.
Believe it or not, high-compression engines of the late ’60s, with compression ratios up to 12.5:1, had higher thermal efficiencies than many of today’s engines. For the same size engine, the older engine would have been more fuel efficient if they had the fuel, cylinder head and ignition technologies of today combined with the high-octane gas of the 60s.
In dealing with naturally-aspirated applications, high compression ratios are the key to serious power levels. In dealing with forced-induction applications, it’s well known that increasing boost pressure on a properly sized turbocharger will increase power production (at least, to a point when the capacity of the turbo or fuel system is exceeded). Of course, the big downside to higher boost pressures is that the likelihood of encountering engine- damaging detonation also increases.
The balance of boost versus compression ratio has been an engine builder’s and tuner’s challenge for years. Picking up a copy of one of the 60’s- technology forced-induction manuals will highlight their solution. The higher the boost pressure, the lower the compression ratio of the engine. For “serious” race forced-induction setups compression ratios of 7.0:1 were not uncommon.
Fortunately, poor manifold and fuel delivery designs, as well as low-efficiency “blowers,” are not found on too many of today’s popular performance vehicles. Today, the average high-performance street or strip turbocharged four-cylinder race engine sports a compression ratio of 9.5:1, with some even running compression ratios as high as 11.5:1 or more on alcohol or E85. Modern technology allows our racing generation to get the best of both worlds. High boost pressures with high compression ratios.