What is Forced Induction

by | Jul 21, 2017 | Forced Induction 101, Series | 0 comments


The ­displacement and efficiency of a naturally aspirated ­engine limit how ­much power it can make. The engine can only inhale so much air because the atmospheric force is only 14.7 lbs. per square inch, at sea level mind you.  Atmospheric pressure decreases with elevation. Air density also decreases with temperature because hot air is thinner than cold air.  On top of all of this, most stock naturally aspirated engines can only achieve a peak efficiency ­of 75% to 85%.

Small block or big block Chevy, Ford or Chrysler engines are ­usually ­limited to two valves per cylinder and fixed valve timing, but if you’re working on a late-model engine, multiple valves per cylinder and variable valve timing can help improve breathing ­efficiency, and do so at extreme levels compared to just 10-15 years ago.

Common ways to improve airflow on naturally aspirated engines

• Installing a higher lift, longer duration camshaft.

• Modifying the stock heads or replacing them with aftermarket performance heads that have larger valves and better ports.

• Installing an intake manifold with taller and longer runners to help ram more air into the cylinders.

• Installing a larger throttle body or carburetor (or multiple carburetors) that can flow more CFM (cubic feet per minute).

• Adding an air scoop or cold air intake ­system to help route more and cooler denser air into the engine.

• Improving exhaust scavenging with headers and crossover pipes that help improve air flow out of the cylinders.

With such improvements, it is possible to boost an engine’s volumetric efficiency into the 90% range or even higher. But achieving 100% or higher volumetric efficiency (especially at higher rpms) usually requires some type of forced ­induction system such as a turbocharger or ­supercharger…and we call this Forced Induction.

Forced Induction

A forced induction system overcomes the limitations of atmospheric pressure by pushing more air into the cylinders. Consequently, the engine’s power output becomes a function of how much boost it gets. What’s more, dialing up the boost pressure overcomes a lot of deficiencies in the induction system and cylinder heads that would otherwise limit air flow and the engine’s volumetric efficiency.

After all, it is much easier to push air into an engine with a turbo or blower than to suck it in with intake vacuum alone.

Even with a relatively moderate amount of boost like 6 to 8 psi, a forced induction system can easily increase the power output of a typical street engine 150 or more horsepower.

Turn up the boost pressure to 14 to 16 psi and you can usually double the power output of most engines. Crank it up even more and you’re off to the races.

We’ll address the the differences of the various air induction systems and technologies in the rest of the series, so stay tuned!


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