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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, the carburetor and low-efficiency "blowers" are only use on nostalgic performance applications. 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 11.0:1.Modern technology allows our racing generation to get the best of both worlds. High boost pressures with high compression ratios. Who could ask for anything more?

Compression Ratio-Friend or Foe?
Without going into a lengthy explanation of internal combustion engine dynamics, let's review what function a car's engine serves. Simply, the 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 the 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.

High thermal efficiencies result in more power per pound of fuel burned in the engine. In basic terms, this means more horsepower and better mileage.

So what does thermal efficiency have to do with compression ratio? Plenty. In fact, an engine's static compression ratio is the primary factor in determining the engine's thermal efficiency. The old-school rule of thumb is that each additional point that the compression ratio is raised will deliver an additional 4% in power. In fact, more accurate projections can be found in the accompanying DRAG Sport chart. These values were obtained using the thermodynamics equation to establish the thermal efficiency of an Otto cycle engine.

POWER EQUATION
Power increase or decrease (%) = [ (1 - 1/ rnew(.4)) / (1 - 1/ rorig(.4)) ] -1 x 100

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 that 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 yesteryear.

17:1 Compression Ratio and 45psi Boost Pressure
No! Don't go out there and try to build a 17:1 compression ratio race engine with the boost pressure cranked up to 45psi. As the late Gene Humrich of Centerforce Clutches used to always tell me, "For every action, there's going to be a reaction. And if the repercussions of the reaction are worse than the benefits of the action, you are going to get screwed." So what is the reaction to the action of raising your compression ratio on a forced induction application? A combination of too much boost or too much compression will increase the likelihood of detonation.

So how much compression ratio should you run for a specific amount of boost pressure? It depends primarily on three factors. Fuel quality, intercooler efficiency and the tuning state (how well the fuel curve and ignition curves are set) of the engine. Methanol will allow higher compression ratios than racing gasoline. Better intercooler systems will also allow higher compression ratios. Some tuners can optimize the engine despite having the narrower tuning window of a higher-compression/high-boost application. In the end, engine development is the only way to get the answer to the question of the perfect compression ratio and boost pressure.

Looking back forty years ago, Chevrolet reigned supreme when its ultra-high-performance, 283-cubic-inch small block generated an unprecedented 283 horsepower-one horsepower per cubic inch. High compression pistons, a racing-profile solid-lifter camshaft and a pair of four-barrel carburetors made the impossible possible. Today, a Japanese-spec Type-R Integra engine generates almost twice that figure with an output of nearly 1.9 horsepower per cubic inch. Double-overhead camshafts, four-valves-per-cylinder, computer-controlled valve timing and electronic fuel injection take credit for the advances in naturally-aspirated power production.

Although the average power output per cubic inch has been doubled during the last 40 years, today's four cylinder powerplants feature displacements that average one-third of their ancestors. Since the historic powerplants were generally installed in cars that were 600 to 1000 pounds heavier than today's sports cars, the majority of yesteryear's muscle cars would run neck and neck with one of today's technological marvels, like a Type-R.

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