Four-cylinder engines are short on displacement, so to maximize their performance potential they have to be wound up and geared down. High rpm power is achieved with lots of valve opening and valve timing, but the trade-off is a loss of low-end torque and throttle response. That is one reason why six-speed manual gearboxes are common on sport compact cars.
So how do you boost the power of a small displacement four cylinder engine? You make it breathe like a much larger engine by bolting on a supercharger or turbocharger. You can also upgrade the induction system with a performance intake manifold, oversized throttle body and cold air intake system, but you won't see the gains that a supercharger or turbocharger can deliver.
The power gains that are possible with forced induction are limited only by the strength of the engine block, pistons, connecting rods and crankshaft. Turning up the boost pressure produces more and more horsepower. Most four cylinder and V6 engines can safely handle up to 8 pounds of boost with no other modifications. But when you start forcing 10 to 15 to 20 pounds of boost into an engine, it really stresses the block, head gasket, pistons, rods and crank. If you want to make serious power (say 500 hp and up), upgrading the stock connecting rods (which are often cast iron or powder metal) to some type of high strength steel i-beam or h-beam aftermarket rods will probably be necessary. The stock cast aluminum pistons should also be upgraded to forged aluminum pistons.
For really high horsepower engines, say 800 to 1000 horsepower or more, major block upgrades will be needed, such as installing steel sleeves in the cylinder bores or upgrading to a billet and reinforced aftermarket block.
A naturally aspirated engine can suck in only so much air with every intake stroke of its pistons. As rpm increases, the volume of air ingested by the engine goes up, but breathing efficiency starts to drop off due to restrictions in the intake plumbing, heads and valves. The amount of power produced by the engine ultimately depends on how much air it takes in and how efficiently it breathes.
Four-cylinder engines don't have much displacement. A couple of liters are not a lot to work with. One way to make a little engine breathe big is to give it a bigger set of lungs with a blower. Forcing air into the engine under boost pressure increases air flow and breathing efficiency.
It doesn't take much boost to make a big difference in horsepower and torque. Eight to 12 pounds of boost pressure generated by a supercharger will typically boost an engine's power output 30 to 40 percent or more! Want more power? Just turn up the boost pressure.
For example, one aftermarket supplier of Honda supercharger kits said their bolt-on system takes a stock Honda Civic Si from 160 hp up to 277 hp measured at the flywheel. That is an improvement of more than 100 horsepower using seven to eight pounds of boost and an air-to-water intercooler on an otherwise stock engine.
The main difference between superchargers and turbochargers is the way they are driven. Superchargers are belt driven and create boost (as needed) at all engine speeds. Consequently, low-end torque is greatly improved and throttle response is excellent. Turbochargers, on the other hand, are spun by hot exhaust gas. It's a free ride, so to speak, because thereis no drive belt and no parasitic load on the engine. But a turbo takes time to spool up. This creates a certain amount of throttle lag and doesn't produce much extra power until the engine passes 2,500 to 3,000 rpm. Decreasing the size of the turbocharger improves throttle response, but the trade-off is less boost and power at high rpm. That is why some V6 engines run twin-turbos instead of a single larger turbo. It gives them better throttle response as well as high-end power.
When air is compressed, it gets hot whether it is being pumped by a supercharger or turbocharger. Turbos obviously run a lot hotter because they are exhaust driven. Hot air is less dense and does not fill the engine's cylinders as efficiently as cooler air. One way to overcome this effect is to route the air through an "intercooler" or "aftercooler" after it exits the compressor. This special heat exchanger may be an air-to-air cooler mounted in front of the radiator, or a smaller and more compact air-to-water cooler attached to the cooling system. You will find both varieties used on factory turbocharged and supercharged engines.
You do not have to install an intercooler with a supercharger or turbocharger kit, but doing so allows you to run more boost pressure and make more horsepower (typically 10 to 15 percent over a non-intercooled blower or turbo). Intercooling also helps reduce the risk of engine-damaging detonation under high load, which is a major concern with forced induction systems.
Increasing boost pressure increases effective compression. This, in turn, increases the risk of detonation with low-octane gasoline. Premium fuel is recommended and, on some engines with high static compression ratios (over 10:1), it may be necessary to reduce the compression ratio to a more blower-friendly level.
The most common type of supercharger is the positive displacement "Roots" style blower. Many aftermarket supercharger kits use an Eaton blower which is identical to the Eaton blowers used on many factory supercharged cars. Inside the compact blower are two counter-rotating helical rotors with three lobes each. The rotor bearings are lubricated by a self-contained oil supply, so no external oil lines are needed.
There are also centrifugal superchargers by ProCharger and Vortech that function like a belt-driven turbocharger. Like a turbo, a centrifugal blower needs to be revved up to produce boost pressure. It also requires an external oil supply line for lubrication. A centrifugal blower is about the size of a large alternator and can be mounted in any location where there is enough space. You can use them with or without an intercooler, but an intercooler will give you more power.
The term "bolt-on" may be a bit misleading; there is more to installing a supercharger or turbocharger kit than just bolting on a blower. Increased air flow means the engine will also need more fuel and recalibrated spark timing. The stock fuel injectors on most engines do not have enough extra capacity to handle a big increase in air flow. Most aftermarket supercharger and turbocharger kits include larger replacement injectors or a parallel set of injectors to add more fuel to the engine.
Many kits also include add-on electronics to help the engine management system properly handle the increased air flow. The add-ons may include an "interface" module that goes in the wiring harness between the PCM and engine, or a whole new PCM that has been recalibrated to compensate for the added boost pressure. An interface module essentially tricks the PCM into adding fuel or changing timing advance by modifying the sensor input signals to the PCM.
Most of the supercharger and turbocharger kits that are currently available in the aftermarket are emissions legal and CARB-certified for all 50 states. This is an important consideration for any kind of street performance application. Trying to cobble together a kit of your own using parts and pieces from a variety of different suppliers can be a time-consuming, trial-and-error process - and, when you're done, there is no guarantee the engine will meet emissions or perform as well as you had hoped. That is why kits are a smart way to go. You get everything you need in one box, and all the dyno tuning and emissions testing has already been done for you.
Supercharger and turbocharger kits are not cheap. Retail prices for complete aftermarket bolt-on supercharger and turbocharger kits typically run in the $5,000 to $8,000 range. Even so, they are a bargain when you consider what it takes in time and effort to duplicate the same thing from scratch. You might be able to get the parts for less from various sources, but you still have to fabricate your own brackets and hoses, and spend hours developing the right boost pressure, fuel and ignition curves for the engine.
That might not be an issue if you are building a one-of-a-kind race motor and have deep pockets, but for most people a ready-to-install kit is more affordable and practical. Most kits also come with a warranty on the parts. One supplier we spoke with said they offer a two-year or 100,000-mile guarantee on all the parts in their kit.
Installation of a complete kit typically takes about a day to a day and a half depending on the complexity of the kit and your level of expertise. Some shops that specialize in this kind of work can complete an installation in four or five hours.
Usually, no cutting, drilling or welding is required, but there is a lot of disassembly and rerouting of hoses. Mounting brackets are provided, so all you have to do is follow the instructions and put everything together. If the kit includes a cold-air intake system you may have to cut an opening to reposition the air intake on some applications.
The only drawback with kits is limited availability. There are a variety of aftermarket supercharger and turbocharger kits available today for more popular domestic cars such as Mustang, Corvette, Camaro and Dodge, and many imports such as Acura, Audi, BMW, Honda, Mazda, Nissan, Porsche, Subaru, Toyota and VW.
One of the reasons why Hondas have been so hot in the import street performance compact car scene is because they are affordable and easy to modify. Honda owners also seem to be the most performance-minded among the non-luxury import vehicle owners.