Every vehicle comes to life with three simple things fuel, air and a spark. Take one out of the picture and your car is dead, but pump in a little extra and you might get out a bit more fun, hence the idea behind forced induction. Two systems can make this happen, superchargers and turbochargers.
While it’s true you can extract a large amount of power from them, people often incorrectly assume that any aftermarket supercharger or turbocharger can be tossed on to an engine for instant heaps of horsepower and torque. In fact, the addition of a turbocharger or supercharger needs to be carefully coordinated with your engine’s design and your desired performance characteristics or you’re going to have a really bad time.
A normally aspirated engine has a maximum manifold pressure equal to atmospheric pressure, 14.7 psi at sea level. In a forced-induction engine, the turbocharger or supercharger allows one to increase manifold pressure to a level above 14.7 psi, resulting in a denser air/fuel mixture in the combustion chamber. From there we get more power.
In principle, turbocharger and supercharger are very similar. Both compress air before it enters the combustion chamber. This results in a denser more compressed air/fuel mixture which generates a more powerful piston stroke. Because of this, forced induction allows you to create a smaller-displacement engine with the same energy output that a much larger-displacement normally aspirated engine would have.
How They Work
A supercharger is powered by a pulley that attaches to the crankshaft. As engine turns over and rpms increase, outside air is compressed and discharged into the intake system. There are three common types of superchargers: impeller or centrifugal, twin rotating screws and counter-rotating rotors. As the engine rpms increase the boost from the supercharger increases accordingly. Peak revolutions hit about 15,000 rpm for the screw-type, and up to about 40,000 rpm for centrifugal-type superchargers.
A turbocharger drives its compressor from the excess exhaust given off by the engine. While back pressure on the exhaust does rob a small amount of power from the engine, the boost from the turbo is generally thought of as free boost as compared to a supercharger. Typical peak revolutions of turbos can range anywhere from 75,000 all the way up to 150,000 rpm.
Factory-designed forced-induction engines are designed specifically to accommodate the additional stresses from the added boost. Forced induction engines are designed from the ground up and usually have very low compression ratios to compensate for the added pressures. Unsurprisingly, bolting a turbocharger or supercharger to a stock engine will result in more wear and tear.
Superchargers vs. Turbochargers
Power and efficiency
As we just learned a turbo runs off of the exhaust system, while a supercharger takes power directly from the engine crankshaft to run the blower. Because of this superchargers sap more power (up to 40–50 horsepower to spin the blower at full boost) from the engine to run the compressor than turbochargers. Turbos, however, also have their cost due to back pressure from the turbo and increased restriction due to the convoluted and twisting exhaust piping, overall these losses are minimal when compared to a supercharger. If you’re looking to squeeze the maximum amount of power out of a specific displacement turbochargers win hands down.
Power lag is the number one reason why a supercharger may be preferred over a turbocharger. Because the turbocharger is spooled up by the exhaust gases from the engine, it doesn’t achieve boost until the engine’s rpm reaches a certain level, so you get little or no boost in the lower rpm range. This is why a turbocharged car requires a pretty hefty and steady foot to build boost for a good drag launch.
When turbo boost does finally kick in, it comes on hard. The power surge also places additional stresses on a stock drivetrain and suspension components. Because a supercharger connects directly to the crankshaft it spins and creates boost at all times. Screw-type superchargers are able to create significant boost levels at low rpm, whereas a turbocharger has has almost no boost until it nearly instantly hits around 3,000 or –4,000 rpm. If your goal is speed off the line a supercharger is for you.
Turbocharger systems quite complex, and are generally considered less reliable than superchargers. All of the turbo system components work with exhaust gases, which creates additional heat stress and wear. For example, when you first shut off a car with a turbo system, temperatures can spike inside the turbo and the impeller bearings can quite literally be cooked by the high heat. Turbo timers are often installed to keep the car running at idle for a preset amount of time to allow proper cooling. Turbochargers also spin at much higher rpm than superchargers, which increases wear on bearings and the system as a whole.
The turbocharger system is powered by hot exhaust gases, which tends to also heat the intake mixture. Hot air expands and becomes less dense, something we don’t want. To solve this problem, most turbos require an intercooler, which increases the complexity and cost. Hot air is cycled through a large intercooler which cools the air before it goes into the intake manifold.
Installation and Tuning
Superchargers are usually exceedingly easy to install. Bolt-on kits can be found readily and installed over a weekend and usually there is plenty of room in the engine compartment to house them. Supercharger kits usually require only a few modifications to the fuel system or new fuel injection mapping to get the engine up and running. Most turbo installations involve complex routing of exhaust pipes, oil lines, and intercoolers are typically difficult to fit in the engine compartment. Turbochargers also require a fair bit of delicate tuning.
Forced induction is generally pretty expensive. Costs range from around $3,000 for a basic kit up to $10,000 for a complete setup and install. With their increased complicity, turbochargers tend to be slightly more expensive, particularly when you add the cost of an intercooler. Most supercharger installations are relatively straightforward so beyond the kit cost installation costs won’t sneak up on you.
More About Superchargers
There are two types of superchargers that are commonly installed: centrifugal and screw-type. The centrifugal supercharger is most like a turbocharger in that it compresses air using a spinning impeller. With centrifugal superchargers, you can often swap out impeller sizes and change the drive pulley to customize the boost curve for your particular needs. They are typically set to generate their peak boost at or near engine redline. Similar to a turbocharger they develop more of their boost at higher rpm and offer less boost on the low end of the rpm range.
Screw-type superchargers use a twin-screw mechanism geared off the front pulley to compresses air as it moves between the two screw blades. These are also often called positive-displacement units because they move a fixed amount of air per revolution. Compared to the centrifugal design this creates good compression at lower rpm, resulting in a significant increase in power from idle all the way through the rest of the power range. Drag racers looking for instant boost off the line typically go with the screw-type supercharger. This biggest issue here is that a screw-type supercharger takes up significantly more space than a centrifugal supercharger.
More About Turbochargers
Many people incorrectly think a larger turbocharger alone will generate more boost and horsepower. This is not always true. A larger forced-induction unit must accompany many other changes in the engine. Maximum boost is limited by a pressure relief valve called a wastegate. The wastegate acts to release exhaust gas pressure, slowing the turbine so the engine doesn’t suffer from too much boost. Installing a larger turbocharger without making adjustments to the wastegate will result in no increase in maximum boost levels.
Another consideration is power lag. A larger turbocharger requires more time to reach boost. Small turbos, like those found in Ford’s Ecoboost lineup, spool up quickly and reach boost at lower rpm. The numeric digits used to describe turbos (K24, K26, K27, etc.) usually correspond to the actual size of the exhaust fan wheel inside the turbocharger — the hot side. There is also a wheel on the intake — the cold side — that compresses the air to create the actual boost. Changing the sizes of the two wheels alters the overall personality of the turbocharger and can be used to tailor the turbo response.
A small turbine wheel in the exhaust, combined with a small impeller wheel on the compressor side, will allow the turbo to spin up quickly and generate a quick throttle response, but will yield less power on the top end. A small turbine in the exhaust with a large blower will generate a good compromise between throttle response and top-end power. For the best top-end performance, a large turbo wheel can be combined with a large blower wheel, but low-end throttle response will be low.
How much boost?
There’s no such thing as a free lunch.
Turbo systems come to full boost and then bleed off excess with a wastegate. This does create great power in the upper rpm range, but it also means you’re running at highly boosted levels for extended periods of time. A centrifugal supercharger only reaches max boost at the highest rpm. Thusly, you can run much higher peak boost levels on a centrifugal supercharger than you can with a turbo or screw-type supercharger system.
Running a boosted engine puts a lot of stress on the internals of the engine, as you are pushing more power through the drivetrain than it was likely designed for. The real killer for engines, though, is detonation. If your fuel octane is too low and the compression of the engine too high, the fuel will explode prematurely, the result is engine knocking otherwise known as or detonation. Detonation occurs if the the piston is still rising and compressing the mixture and ignition occurs, so the pressure builds and has no release. The pressure pushes down on the piston as it’s rising. Detonation will destroy pistons and blow out head gaskets.
To prevent destruction, you must reduce your boost levels so that the engine no longer detonates. The engine management system normally adjusts timing and ignition to reduce detonation in the cylinders. However, running really high amounts of boost with lower octane fuel can overwhelm the stock system. The higher the boost you wish to run, the higher the octane of fuel you will have to buy.
When you install a forced-induction system onto an engine, you increase the amount of air injected into the combustion chamber. This will likely cause the air/fuel mixture to become lean. To compensate you must increase the amount of fuel that is combined with the air. Combustion achieves its maximum efficiency at an air/fuel ratio of 14.67:1. This ratio is optimal for good fuel economy. For maximizing power an air/fuel ratio set from about 14.2:1 to 14.3:1 is optimal for a normally aspirated engine. On boosted engines, the maximum power ratio is closer to the range of 12.2:1 to 12.4:1. An engine that runs too lean, will increase the likelihood of detonation and also will increase the operating temperature of the cylinder head.
Engines that start with a high compression ratio — E36 M3s for example — cannot be boosted as much as engines with lower ratios. You can generate more horsepower by maximizing boost from the turbo than you can with higher compression.
Bearings and clearances wear out over years of use. As such turbochargers and superchargers don’t usually pair well with a tired engine. The chances you will blow out your head gasket increase as the engine gets older.
Which forced-induction unit you install depends on your goals. Ease of installation and budget limitations aside, you don’t have an easy task ahead of you. Ask 10 car enthusiasts what their preferences are, and you’re bound to get 10 completely different answers. Turbo fans and supercharger fans agree on little, other than speed.
Some basic truths. If you want a drag car with lots of power off of the line, a supercharger system is in your future. If you’re looking for top speed a turbocharger system will be hard to beat. If you like to feel the rush of power as you hit boost and go with a turbocharger. If you want a somewhat stock feel with a big push on the high-end, then go with a centrifugal supercharger. If you want even power across the entire rpm range, then install a twin-screw supercharger.
In terms of installation ease, the supercharger wins hands down . Turbocharger systems can be made to perform better and tweaked to your needs and wants, but they generally require more time, money, and installation effort.