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Monday, July 31, 2006

 

How to supercharge any car - the real story

It seems that my writings ridiculing a dubious gas mileage gadget have attracted some readers to this blog who are wondering what it really takes to supercharge any car. Well, putting in a real supercharger is not particularly easy or cheap, and some cars have features that will complicate matters. But it is possible to write a generic set of supercharger instructions, so as to give you the general idea of what's involved with installing one.

First, I should start off by explaining what a supercharger is. A supercharger is an air pump that pushes more air into the engine. Drawing more air into the engine allows you to burn more fuel, which lets you make more power from the same size engine.

The supercharger needs power, and a fair amount of it. The power required to drive a street supercharger can be as little as five horsepower or as high as sixty. The supercharger on Top Fuel dragsters require hundreds of horsepower to drive. You could drive one with a turbine in the exhaust – a device that does this is known as a turbocharger. These have some other complications, so I won't talk about them in this little article. Here, I will focus on superchargers that use a belt drive, the most common way to drive a conventional supercharger.

Here is what it takes to install a supercharger, in 5 not particularly easy and grossly oversimplified steps.

1. First, you pick a supercharger. Adapting a piece of ventilation equipment isn't going to cut it; you need one that is optimized for delivering the pressure you want at the airflow levels you need. Selecting a supercharger involves calculating how much air you want to flow into your engine and what pressure you want this air to be. Once you have these numbers, you compare your engine's needs to graphs called compressor maps that you can get from the supercharger's manufacturer. There is no one size fits all supercharger, or even one that fits a wide variety of cars from 1.5 to 5.1 liters. If anybody claims to have a single “supercharger” that will be equally effective across a size range like that, the only way it could be equally effective would be if it fails to do anything for any sized engine.

At this point, you will have to decide whether you want a centrifugal supercharger or a positive displacement one. Positive displacement superchargers put out nearly constant boost at any engine speed, making them a good choice if you want low RPM torque. Centrifugal superchargers are lighter and more efficient, but the boost builds with engine speed. So a centrifugal supercharger is not going to do much at low RPM. Their extra efficiency, however, means that theoretically you should get more performance for the same amount of boost.

2. Next, you have to find a good way to get the supercharger physically on the engine, positioned in such a way that you can set up a pulley to drive it. There are two ways to go about doing this – you can make a new intake manifold and attach the supercharger to that, or you can attach the supercharger to a bracket like it's an overgrown alternator. Usually positive displacement superchargers attach to the intake manifold and centrifugal units use brackets, but there's no absolute rule about which way to do this.

3. Then you need to rig up pulleys to turn your supercharger. You may be able to just line the supercharger's pulley up with the other pulleys and put on a longer serpentine belt. Or you may need to add a new pulley to the crank and use a large, toothed belt called a Gilmer belt to drive it. This will depend to some extent on just how much power your supercharger needs to drive it.

In theory, you could build an electric supercharger, but you'd need a very large motor driven with wiring the size of battery cables if you wanted it to make any horsepower. It would also put an enormous strain on your charging system. Remember, an effective street supercharger is going to need somewhere between five and sixty horsepower to turn it. You can't get that kind of power out of a motor the size of a D cell battery, or pack enough current to drive it through a 12 gauge wire. I've also heard of someone working on a hydraulic supercharger, but I never heard of such a thing turning up on the streets or the racetrack.

4. Next you will have to deal with plumbing. If you've mounted your supercharger on a bracket, you will need to run a tube from the discharge side of the supercharger to the throttle body. You can also put an intercooler somewhere in this length of tube to cool down your incoming air, which will be hotter after it leaves the supercharger. Cooling down the incoming air will boost efficiency and make your engine less likely to ping. Some superchargers will require external oil supply lines as well; most don't. You will also need to find some way to get filtered air to the supercharger's intake, even if you have a manifold-mounted supercharger. And if you've moved the throttle body, you may need to change the throttle and transmission kickdown linkage. Or maybe not.

5. Then you will need to tune it all. At this stage, you may need to add a larger fuel pump or larger injectors to get more fuel into the engine. If you have an injected motor, you'll benefit from tuning things with either an adjustable fuel pressure regulator or ECU mods. Cabureted motors will likely need new jetting and recurving the distributor. You may even need different pistons if you are particularly unlucky - or going for all out power.

You may need to move a few things in the engine compartment out of the way for some of the above steps. The most common one is having that pesky hood refuse to close until you cut a hole in it, but there are all kinds of other things that may need relocation assistance. A recent Grassroots Motorsports project where they tried to put a supercharger kit for a BMW Z3 roadster onto a 318 (BMW, not Mopar) required relocating the master cylinder reservoir, for example.

If you are putting in a supercharger from scratch, any one of those steps could be the subject of a whole magazine article. If you are buying a kit, the kit's manufacturers will hopefully provide detailed instructions that make these steps no harder than changing your alternator. Any real supercharger is going to be somewhat complicated and expensive to install. But as you can easily get 50% more horsepower, many would say it's worth it.

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