Muscle Mustangs & Fast Fords
Forced Induction - Why Boost It?
Everything you've ever wanted to know about forced induction on your fast Ford.
In the simplest of terms, an engine is nothing more than an air pump. Air from the atmosphere is drawn through the throttle body (or carb), into the intake manifold, where it is mixed with fuel (this is known as the air/fuel ratio), before entering the combustion chamber. When the mixture is compressed and ignited, the energy released from the air/fuel mixture burning (and expanding rapidly) causes force to be applied to the pistons. The pistons apply force to the crankshaft, and torque is produced.
When relying on natural aspiration, air is drawn into the engine due to the pressure differential between the low pressure in the cylinder (created by the rapid down sweep of the pistons) and the higher (atmospheric) pressure in the manifold. Our atmosphere is roughly 14.7 psi, give or take, depending on the actual barometric pressure and the altitude where you happen to be standing.
So, basic theory tells us the more air an engine can consume, the more fuel it can burn (of course you have to maintain the efficient a/f ratio), and the more horsepower and torque it will ultimately produce. More air equals more fuel equals more power.
Because we rely on air pressure we are at the mercy of our location and Mother Nature to supply air, but in contrast, we can totally control fuel supply as deemed necessary. So, the tricky part is adding additional air.
Any time we modify an engine, adding air (improving airflow) is part of the goal. A new cold-air intake, for example, will generally flow more air and, in turn, pick up power. A bigger intake manifold? More air. Those new 325-cfm heads? Well, cfm is cubic feet per minute of airflow, which is exactly what we're after. Those headers you just installed? They help scavenge spent gasses from the exhaust ports and suck a little bit more air into the chamber through the intake valve, which creates more power!
But what happens if we've already optimized an engine with all of these parts? What happens when your all-motor combination is maxed out and you're flowing as much air as possible? Or you want a simple bolt-on worth big power? The answer? Force more air in with a power adder.
But how do we do that? Well, you add a supercharger or a turbocharger. As these devices acquire air at atmospheric pressure, compress it, and transfer that additional air into the engine, where it can be combined with extra fuel, and more power is made.
For starters, additional air forced into an engine is usually referred to as boost. Actually though, boost is the measure of additional air pressure (normally measured in pounds per square inch or psi) found within the intake tract (before it makes it to the combustion process). You can do this with a centrifugal supercharger, a positive displacement blower, a turbocharger, a pair of turbochargers, or any combination of the three. But to find the right one for you, it's important to first clearly define your goals (and budget) and then understand how each particular system works to benefit you and your engine.
Let's take a look.
Typically mounted on front of the engine in-line with the front accessory drive, these blowers are driven by a belt attached to the crankshaft. As the crankshaft rotates, the belt turns a pulley on the supercharger, which turns a set of internal "step-up" gears. These gears multiply the speed at which the impeller rotates (often to well over 100,000 rpm). Once the impeller is moving, it draws air in from the atmosphere and sends it into the supercharger's volute, where it's compressed before it's passed over a diffuser and delivered to the engine for consumption. Simply put, a centrifugal supercharger pulls air into itself and compresses it through centrifugal force before sending it through a series of pipes (and often an intercooler) to the throttle body (or carb). Centrifugal superchargers rely on centrifugal force to create high-pressure air within the intake tract.
Thermal efficiency: Centrifugal superchargers are inherently simple and efficient in both power consumption (how much power they take from the engine in order to operate) and thermal output (how hot the air is after it is compressed). The less heat they generate during compression, the lower the air intake temperatures, which results in increased power production and increased resistance to detonation. Plus, because the blower is not directly bolted to the intake, less heat is transferred to the engine from the blower itself.
Compact design: Modern centrifugal superchargers can be incredibly compact for their given output, and can fit within any modern engine bay rather easily. Because they mount in line with the front accessory drive instead of atop the engine, there is no need for an aftermarket hood, and many systems don't require the relocation or replacement of any OEM equipment.
Linear boost production: Driven by the engine's crankshaft, centrifugal superchargers often produce very linear boost curves, which makes them easy to drive on the street and simple to manage on the track. At low rpm, boost is minimal with big gains typically occurring above 3,000-3,500 rpm on a modular Mustang motor. This makes traction off the line easier to manage, with a big charge of power up top for large mph gains and a thrilling run to redline.
Linear boost production: Driven by the engine's crankshaft, centrifugal superchargers often produce very linear boost curves with very little airflow production at low- to mid-engine speeds. This results in a minimal amount of additional torque production during low and mid-rpm operation, which can feel "laggy" to drivers looking for a large dose of torque right off the line.
Belt slip: On high-horsepower applications where small-diameter supercharger pulleys are used, it is possible to run into belt-slip issues, in which the supercharger drive belt slips on the upper pulley, causing a drop in impeller rpm and a loss of boost. Modern belts, as well as sturdier brackets, increased tension, and thicker pulleys, have helped combat belt slip.
The rpm of the impeller is ultimately determined by engine rpm, the internal step-up gearing of the supercharger, and by the pulleys used on both the crankshaft and the supercharger itself. Adjustments are made by swapping pulleys of varying diameters, either on the crankshaft or on the supercharger itself, until maximum desired boost is reached. It's important to note that it's possible to exceed a centrifugal superchargers maximum rpm and doing so can potentially damage the unit.