When we decide to build a new engine for one of our Mopars, we nearly always want to build an engine that is more powerful and more durable than the engine it will replace. And with the abundance of aftermarket parts available to choose from these days, making more power is easier than ever. Whether running on pump gas or race fuel, normally aspirated or with a power adder, there are plenty of ways to build your Mopar engine to make incredible power. One question we’re often asked is whether or not a supercharged engine is more durable or makes more power than a normally aspirated engine. The answer to this question is not so much which engine is better, since durable and powerful engines can be built either with or without a supercharger, but rather, which technique is best for you. This month we’ll describe the differences of engines with similar power levels, both normally aspirated and supercharged, and you can decide which is right for your Mopar.
We all know that in the Top Fuel and Funny Car racing classes, superchargers rule the day. That power, however, comes at a price as these engines only last one pass down the quarter mile (actually 1,000 feet) before needing to be rebuilt. The fact is that the Mopars we drive and race don’t need nearly the amount of power that a Top Fuel car does, and building an engine of moderate power for our street or race car can be accomplished in a number of ways. For the purposes of comparison, we’ll describe what it takes to build a Mopar street engine with approximately 600 horsepower, both normally aspirated and supercharged, and the advantages and disadvantages of each method.
01 Superchargers, like this centrifugal unit from ProCharger, compress air, and force it i
02a Since a supercharger forces air into the combustion chamber, cylinder head porting is
02b To achieve 600 horsepower without the benefit of a supercharger, aggressive head port
Basics and Terminology
Both normally aspirated and supercharged engines rely on something called manifold pressure (pressure inside the intake manifold) to get air and fuel into the engine’s cylinders. Manifold pressure is measured in inches of mercury (inHg), which is also the standard by which atmospheric pressure is measured. In a normally aspirated engine, manifold pressure is limited to the pressure of the atmosphere which is measured with a barometer (barometric pressure), while a supercharged engine can boost the manifold pressure by compressing the air that enters the manifold. Weather conditions and altitude affect barometric pressure, but as a universally recognized standard performance calculations are based on a barometric pressure of 29.92 inches of Mercury at sea level.
To convert inches of mercury to pounds per square inch (psi), inches of mercury is multiplied by the factor .49109778. So a standard atmospheric barometric pressure reading of 29.92 equals approximately 14.69 psi, which is what helps engines (and people) breath. To keep things simple, this atmospheric pressure is generally thought of as zero psi, and indicated as such on most automotive pressure gauges. Any pressure above this standard will read as a positive pressure and anything below will read as a vacuum (negative pressure) on most gauges.
03 Supercharged engines don’t necessarily need to utilize aggressive cam profiles since a
04a Since a supercharger forces air into the engine, more fuel must be added to achieve t
04b This means special tuning and bigger fuel injectors for an injected engine, or a spec
A normally aspirated (non-supercharged) engine’s manifold pressure is limited to the barometric pressure in the atmosphere. So at wide open throttle, the pressure in the intake manifold forcing air into the cylinders is the same as the air pressure in the environment, no more, no less. At less than wide open throttle settings an engine’s manifold pressure drops. This pressure drop is defined as manifold vacuum, and is caused by the pistons trying to suck more air into the engine than the throttle opening will allow. So if a certain part-throttle setting causes a manifold pressure of 19.92 inches of Mercury, and the barometric pressure is 29.92 inches of mercury, the engine is said to be making 10 inches of vacuum. This theory of vacuum applies to both normally aspirated and supercharged engines.
The difference in a supercharged engine is that the supercharger (no matter what style), compresses air and forces that air into the intake manifold, allowing manifold pressures greater than atmospheric pressure. In automotive applications, this additional pressurized air is referred to as boost, and the pressure is expressed in pounds per square inch (psi) instead of inches of mercury. Converting this extra five inches of Mercury to boost (psi) equates to 2.45 psi of boosted, or compressed air.
There are some obvious advantages of running a supercharger on your engine, and the biggest advantage is definitely the power potential. The performance of an engine is directly related to the amount of air and fuel the engine takes into its cylinders, so the ability to force pressurized air into the engine dramatically increases the amount of power the engine is capable of, and the bigger the supercharger, the bigger the power. Even a small supercharger can get the average pump gas street engine to 600 horsepower pretty easily, and more boost equals more power. Another advantage of adding a supercharger is that the engine and drivetrain components just need some simple upgrades but not wild modifications.
Because the supercharger is forcing air into the engine, aggressive cam profiles, cylinder head porting, or high compression ratios aren’t necessary to achieve big power levels. In fact, supercharged engines respond to camshafts with wider lobe separation angles, which are conducive to smooth idle and big midrange torque as well. And since the supercharged engine can make impressive low end and mid-range power, a high rpm stall converter or low gear ratio isn’t usually necessary for quick acceleration. In fact, we’ve installed superchargers on fairly stock engines and with moderate (4-6 psi) levels of boost factory driveline components can survive just fine. And since the supercharger doesn’t stress engine components during low rpm operation at no or low boost levels, supercharged engines tend to remain reliable and durable for extended periods if maintained properly.