Making the MINI Cooper Supercharger More Super

As we discussed, the MINI Cooper supercharger works by forcing more air into the engine. It seems logical to assume, then, that the more air you can get it to push, the more horsepower the engine will produce. In fact, this assumption is true.

The supercharger blower is driven by a shaft connected to a pulley which in turn is rotated by a belt that is driven off the engine drive shaft. (This is the same belt that drives the alternator, and the water pump.) Every time the drive shaft makes a complete rotation, the belt around the supercharger pulley is moved a certain distance.

If you remember your basic geometry from high school, the distance around the edge of a circular object like a pulley—the circumference—is determined by the diameter of the pulley. The smaller the diameter of the pulley, then the smaller the circumference of the pulley. With a smaller circumference, less movement of the belt is required to cause the supercharger shaft of the pulley to make a complete rotation.

Or you can think about it another way. If we put a smaller pulley on the supercharger, then the supercharger will spin more times during the same number of revolutions of the engine. And the faster the supercharger spins, the more air is pushed into the engine.

That’s the basis for our next horsepower improvement. By installing a smaller pulley (and the shorter belt that will be required to go with it) we can increase the speed of the supercharger and the amount of air being pushed in. Tuners call this “increasing the boost.” Not surprisingly, since the principles are simple, aftermarket suppliers have developed smaller pulleys that you can substitute.

Of course, there are some limits to how much boost an engine can absorb without blowing itself to pieces, so there are limits to how small a pulley can be used effectively. For this reason, BMW may be reluctant to honor its warranty if you replace its very conservatively designed pulley with one that produces more boost.

Most reputable suppliers supply pulleys that are small enough to make a difference in horsepower, but aren’t so small that they could blow the engine. As long as the pulley diameter isn’t reduced by more than 15 percent, there should be no problems, If the pulley is replaced by one that has a radius of less than 85 percent of the original, it will spin the supercharger in excess of its maxium rated specification, putting the engine itself at risk. Even if the engine isn’t pushed hard, if the pulley is too small, the belt angle will be so acute that the belt life will be significantly shortened.

Replacing the original pulley on the engine is not a simple job, since several other components have to be removed to get access to the pulley, and a special tool is needed to remove the pulley. Even with the special tool, an experienced mechanic may take several hours to do the job the first time. With a little practice, the job still takes about an hour.

So if you decide to replace your pulley with the smaller one, you should probably find a shop that has experience in replacing MINI pulleys. The replacement pulley and belt will cost about $200 and the installation about two to three hours of shop time. In terms of horsepower improvement per dollar, this is probably the most cost-efficient change you can make to the engine.

A Shopping List of Basic Improvements for the MINI Cooper

One of the great things about the MINI Cooper is the robust nature of the basic engine. It is capable of producing much more power than it does right out of the showroom, even with the supercharger. There are some good reasons why it doesn’t.

For one thing, an engine that produces better performance costs more money. Materials used in a high-performance engine are better and the engine is assembled with greater care, both of which add costs.

However, most people really don’t care enough about performance to want to pay the extra price. Also, with higher horsepower, the engine doesn’t produce as many miles per gallon, and is more difficult to tune to meet emission limitations, so the product designers, even in the MINI, simply had to make some compromises.

But you don’t have to compromise. Aftermarket suppliers (the companies that make products that are bought by owners after the car is bought from the dealer) have developed a number of products that will allow you to improve the engine performance in your MINI to get better pick-up, higher speed, and simply more driving satisfaction under all speeds and situations.

If we think about that basic air/fuel/spark/exhaust equation, we want to do four things. We want to increase the amount of air entering the supercharger and we want to increase the compression capability of the supercharger, both to get more air into the combustion chambers. Then, we want to have the ECU take advantage of that added air flow to by altering the fuel mixture and timing for performance efficiency, and we want to make sure that all the smoke from the combustion can get out of the combustion chambers quickly.

We can make those improvements by upgrading the throttle intake and supercharger pulley, reprogramming the ECU, and upgrading the exhaust system. Since each of these components operates as part of the overall internal combustion cycle, you’ll get the best performance from each if you upgrade them all at once.

However, if you can’t do that, you can upgrade these components in any order that appeals to you and you’ll still notice the improvement that each component contributes to overall engine performance. And when you’re done, you’ll really have an engine you can brag about. We’ll discuss the upgrades in the order that they occur in the operation of the engine and describe the upgrades you should consider.

Mini Cooper Cold Air Intake System

Before we can have combustion, we have to have air. If we can increase the amount of air flowing into the supercharger intake, and keep it as cool as possible, then we’ll be helping the supercharger do its job by giving it more air to breath.

We can do this by replacing the standard air intake system with an upgraded “Mini Cooper Cold air intake system.” The standard cold air intake system in the MINI is really not all that complicated. Air flowing into the engine compartment through the grille is channeled into an air intake box on the top of the engine. In the box an air filter removes dust and dirt that would create undesirable wear in the engine. From the air intake box, a duct directs the filtered air into the supercharger. In addition to capturing and filtering the air, the ducts on the standard air intake system have been tuned, like you might tune an organ pipe, so that the air flowing through it produces as little noise as possible.

The standard Mini Cooper cold air intake system easily can be swapped for an upgraded one that has been designed with performance in mind. Several aftermarket equipment manufacturers make replacement cold air intake systems for the MINI, but we’ll describe two typical designs.

The simplest way to improve the system is to replace the stock air filter and air intake box with a higher-quality filter that has been designed specifically to increase air flow while still providing the same filtering functions. K&N makes a high quality filter that is used in combination with high-velocity ducts in several of these kits to replace the basic system. Since it can be cleaned and re-used, we don’t have to buy a new filter every time the old one gets dirty.

An alternative approach that not only improves filter performance, but also provides additional air flow into the intake is now being manufactured by several companies. In this system, the entire air intake box and filter is removed and replaced by a conical filter surrounded by an L-shaped divider. This system helps increase power not only by improving filter efficiency, but also by increasing the flow of air into the system.

This system is designed to do a more efficient job of directing the air from the front Mini Cooper grille into the engine’s air intake. In addition, by being open at the top and back, it captures air from the grilles below the windshield, which are in an area of the body where air flow creates high pressure. By ducting some of that air into the supercharger in addition to the air coming in through the grille, the system naturally allows more air to enter the supercharger intake.

To make sure there is as little as possible to obstruct the air flow once it gets into the air box, the cold air intake system incorporates a reusable high-flow cone-shaped air filter. K&N’s popular re-usable high-performance filters are frequently used in this application. The design of the upgraded cold air intake system offers one other advantage. While providing a direct path for air from outside the car to flow into the supercharger, it blocks off the hot air swirling around the engine. As we learned in science class, hot air is thinner than cold air, so the cooler the air going into the supercharger, the more efficiently the supercharger can do its job.

Though generally similar in design, these systems do vary somewhat from supplier to supplier. Two features should be considered when deciding which one to buy. First, the best dividers are made of shiny stainless steel. As a result they will reflect engine heat back to the outside of the box, so the cool air coming in from outside the car doesn’t get heated up before being pulled into the supercharger. Second, the divider should have good space all the way around, so that air can flow into the entire surface of the filter without any restrictions.

A typical cost for the parts for an upgraded Mini Cooper cold air intake system is about $200. The design is simple, and can be installed by anyone with the instruction sheet and the proper wrenches in a few hours or less. If it is done in a MINI service shop, it shouldn’t take more than half an hour of shop time.

If you own a Cooper model, you can still make improvements in air flow into the engine. A less-restrictive reusable flat filter is available for about $50 to replace the stock filter. Aftermarket developers have also re-engineered the ram air intake duct and air box cover to improve air flow. Installing this improved ducting system in conjunction with a reusable high-performance flat filter will cost about $200 and make a measurable improvement in your Cooper’s horsepower.