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Making Your MINI More Responsive
Making Your MINI More Responsive
In an earlier section of this book, we suggested that you could enhance the performance of your MINI by making a few changes to the engine, upgrading the air intake, supercharger, ECU, and exhaust. We think that any MINI on the street can benefit from the added horsepower and more satisfying engine response that these upgrades will supply.
In this chapter, we’re assuming that as you try out the various driving activities available to MINI enthusiasts—touring and rallying, track days, or autocross—that you will start to think about making some additional improvements to your MINI. As your driving skills increase, you’ll discover some limits to the street-stock car that can easily be addressed with readily available substitute parts and accessories.
Let’s consider them in the order that we think makes sense. As you take a more active interest in your driving, and become more conscious of the car’s behavior, you’ll be ready to appreciate more responsiveness. Responsiveness is found in the feeling when you step on the brakes, shift gears, and accelerate. There are some really great ways you can improve that behavior.
Getting More Bite from Your Brakes
If you’ve had the chance to spend a day out on the track, we suspect that towards the end of the day you were starting to smell a fragrance that was new to you: the odor of hot brakes. You may also have noticed that you were having to push harder on the brake pedal and your car was taking longer to slow down. And then there was the fine dirty dust that was coating those slick new wheels.
What you were learning was that brake pads intended for street use just don’t cut it for a hard day of track use. So if you’re thinking seriously about going back out on the track, and are already thinking about how you could go faster on some of the corners, it’s time to switch to performance brake pads and to consider upgrading your brake rotors.
Upgraded Brake Pads
Switching brake pads is not an expensive or technically difficult chore. If you’re thinking about going faster, then you’ll want to be able to stop more quickly, and that means new brake pads.
Disc brakes seem like simple devices. When the brake pedal is pressed, a “master cylinder” behind the pedal forces brake fluid through the brake lines to the “slave cylinders” on each wheel. The slave cylinders in turn push against the backs of brake pads in the brake calipers on each wheel and these pads push against both sides of the brake discs (discs also are often referred to as “rotors”).
Since the brake rotors are connected directly to the wheels, the friction created by the pressure of the pads against the rotors slows down the wheel, causing the car to slow down. The rubbing of the brake pads against the rotors generates heat as a byproduct of the process.
How well the brakes work, especially over a long period when they are being used frequently—such as descending a long hill on curving roads or when speed changes from very fast to very slow take place frequently, such as in racing—depends on two factors. The first is the co-efficient of friction between the material in the disc and the material in the pad. The second is the ability of the pad to maintain that friction as the heat generated by the friction causes the rotor to heat up.
Both of these factors are determined by the material with which the brake pad is made. Brake pads are made of a variety of different materials, including organic, metallic, and ceramic materials, each with its own co-efficient of friction and ability to function effectively at varying temperatures.
On the street, you don’t use your brakes very hard or very frequently. As a result, brake pads designed for street use are typically made of materials that are softer and have lower co-efficients of friction so that they will slow the car down gradually and progressively, rather than an abrupt or grabby manner.
However, when you’re driving in a spirited manner at a track day or on the autocross course, you don’t want to slow down gradually. If you’re going to get good lap times on the autocross course, or be able to really drive at higher speeds on the track, you want the car to slow down in as short a distance as possible, something that the standard street pads aren’t really designed to do.
Also, if you put to the brakes to hard, continuous use—on a long section of curving roads, on an autocross course, or most definitely at a track day—the softer material on the street pads will become less effective in slowing the car down. As heat builds up in the disc from continuous use, the co-efficient of friction between the pad and the rotor decreases. In practical terms, that means you have to exert more and more pressure on the pedal to get the car to slow down, and stopping distances become longer and longer.
The easiest way to correct this is to substitute brake pads that have a higher co-efficient of friction and can stand up to heat for longer period of time. To meet these objectives, there are a wide range of alternative pads that you might buy. You can buy pads that are designed specifically for high-performance racing, and several gradations below that level.
Here, you don’t want to go overboard. A set of pads that would be perfect for a driver at the national championships is not what you want to put on your car for the occasional track day. That additional bite that will slow a race car down quickly from high speeds would throw your maiden aunt into her seat belts, something she probably wouldn’t appreciate.
In addition, racing brake pads that function very effectively at high temperatures don’t work well when they are cold. These types of pads are excellent for racing conditions where they can be heated up before being used hard, but are totally unsafe for daily driving where brakes are used more infrequently and are almost never heated to any degree.
The good thing is that there pads available for all levels of use. if you do anticipate the occasional high-spirited outing, but normally use the car for more typical purposes, you can easily find an all-around brake pad that will perform better on the track or autocross course, give you better responsiveness on the road, but will still be safe and comfortable for daily use.
You want one that will give a better “bite” than stock street pads and handle higher temperatures, but not be a full-on race pad. When selecting the pad, look at three factors: co-efficient of friction or “bite,” ability to function at high temperatures, and price.
One example of a good compromise is the “Greenstuff” brake pad set made by EBC, which produces adequate stopping power at lower temperatures than stock pads, thus reducing heat build-up, and will function effectively up to 800 degrees F. A side-benefit of these slightly harder pads is that they’ll generate less brake dust under normal or extended usage, so that your wheels don’t have to be cleaned as often. A set of these for the MINI will cost around $90 for the front pair and $80 for the rear pair.
Drilled and Slotted Rotors
Heat that causes deterioration of the brakes is not just a factor of the brake pads; it is also generated by the brake rotors, or brake discs, against which the brake pads rub. Build-up of brake pad residue on the rotors is a second source of deterioration of brake performance during a series of sessions during a track day.
Excellent replacement brake rotors are available that do a much better job of solving all of these problems than the original brake rotors with which the car comes equipped. Two different styles are good examples of the alternatives available to you, at two different price levels.
At the basic cost level, you can improve your brakes by substituting drilled and slotted single-disc rotors for the stock rotors on your MINI. Drilled brake rotors are permitted in BMWCCA spec class racing. These rotors are drilled through their surface at a number of points across the area where the pads rub against the rotor, and also have slots machined into them that extend diagonally across the disc from the inside to the outside.
Drilling the rotors has a major advantage in giving heat a place to dissipate, thus reducing the amount of heat build-up in spirited driving. Drilling as the secondary advantage of reducing the weight of the rotors, thus reducing the unsprung weight that has to be spun up and moved around when the car is in motion.
The grooves also help dissipate the heat from the rotors by creating a draft effect across the surface of the disc. In addition, they provide a channel through which the dust generated as the brake pads rub against the rotors can be removed from underneath the pads. By keeping the pads clean, the grooves increase the frictional efficiency of the brakes.
The increased efficiency and reduction in heat build-up can make a big difference in maintaining your brake performance throughout an entire track day, and they help make sure that the brakes are as good on your last run of the day as on the first. In addition, they are inexpensive insurance to keep your brakes operating efficiently on the road over a long day of enthusiastic back-roads touring as well as on steep hill descents.
A good quality pair of drilled and slotted front rotors is available from catalog suppliers for about $180, and the slightly smaller rotors that fit the rear brakes is available for about $150. Installation of these rotors is a bolt-on, bolt-off affair.
If you’re a little more serious about improving your brake performance, but don’t want to replace your entire brake system to get more brake surface area, there is a slightly pricier option. You might wish instead to consider a high-quality pair of double-surface cross-drilled brake rotors, such as those manufactured by Brembo Brakes.
In addition to having the advantages of weight reduction, heat reduction, and pad cleaning offered by other cross-drilled brakes, the Brembo rotors are constructed in a different way than standard single-plate rotors.
Like the brake rotors on all professional race cars in NASCAR, ALMS, and Formula 1, these rotors are constructed of two layers of metal. The two metal plates of the rotors are joined by braces in between the surfaces that create openings along the edge of the rotor.
The openings created by the cross-drilling, and the openings on the edges of the rotor work together to help ventilate and cool the brake rotors. In addition, this construction strengthens the rigidity of the rotor, reducing the chance that the rotor will become distorted under heavy use.
These Brembo rotors have an attractive gold anti-corrosion finish that helps eliminate surface rust that can interfere with brake performance. The gold rotors also enhance the look of any road wheels, and give the MINI a more aggressive look. They are available for about $320 for the front brakes and about 4220 for the rear brakes. They are quite simple to install in a bolt-off, bolt-on operation and fit all standard MINI wheels.
Short Shifter for Shifting Satisfaction
One of the most satisfying aspects of performance driving is to run the car up through the gears with a series of crisp flicks of the shift lever. In the stock MINI, the transmission and gear shift linkage are designed to make this process fairly seamless, but the “throws” — the distance the shift lever has to be moved between gears — are fairly long.
The problem here is that while you are moving the shift lever from one gear position to the next, you have to have the clutch pedal in and the clutch disengaged. That means that the car is simply coasting. And coasting means you’re wasting time.
If it was possible to shorten the distance that your hand has to move to shift gears, then the time lost coasting between gears would be reduced. And that means you can get back on the throttle sooner during each shift of gears. It also means that your “heel and toe” will work more effectively on downshifts, because there will be a shorter period of time between blipping the throttle and actually changing down into the next lower gear.
Reducing the shifter distance is a straightforward improvement, taking advantage of the principles of leverage. All that is required is installation of an extension on the lower end of the shift lever. This changes the leverage between the shift lever and the rod that connects it to the transmission, so that you don’t have to move your hand as far to make the gear change. This modified gear lever is often called a “short shifter.”
A “short shift kit” to make the change is available from aftermarket suppliers for approximately $90. This kit consists of the extension to the shift lever and a modified dust cover plate for the box that encloses the shift lever mechanism under the car.
Working underneath the car, the mechanic removes the original dust cover plate, disconnects the shift rod, adds the extension, makes some alterations to the shift lever case, and then installs the new dust cover plate. The mechanical work is not complicated, but it does involve raising the car, removing the exhaust system and modifying the the case. Most owner will prefer to leave the work to an experienced modification shop.
Performance Clutch Kit
The clutch is that essential little piece of friction material that connects all that power being generated by the engine to the transmission and driveshaft that make the wheels go round. It’s surprising to think that all that power is transmitted through two discs pressed up against one another: no chains, no gears, just pressure and friction.
On the other hand, if the car didn’t have a clutch, there would be no way of disconnecting the running engine from the transmission so that you could change gears or, for that matter, stop the car while keeping the engine going. For a quick primer in how a clutch operates, check out www.howstuffworks.com/clutch.htm.
Yet, it’s the contact between the two plates, the springs that hold the plates together, the friction material on the clutch plate, and the release mechanism that pulls the plates apart that can make all the difference. These parts determine how quickly and smoothly the plates separate to allow a quick gear change, and how quickly and tightly the plates go back together to put the power back to the wheels. And that speed and efficiency makes a big difference in performance and driving sensations when the car is pushed close to its limits on the track, in and out of the cones on an autocross course or on the curves of a scenic backroad.
Like many other performance parts, clutches represent trade-offs between price and performance, and between speed and comfort. The clutch with which the car is equipped from the factory is a good component, but is built to a budget. More important, it is built with the average (or below-average) driver in mind, so its design and choice of materials err in favor of comfort and longevity, rather than performance and speed.
If you’re thinking about using your MINI Cooper or Cooper S in a more enthusiastic way than that average driver, you may be willing to pay spend some money to improve the performance of your clutch. Of course, you should also be willing to accept the need to be more quick and precise with your gear changes than that average driver so that you don’t start off, or go through gear changes, in neck-snapping fits and starts.
If you are, a performance clutch kit may be your ticket. But it isn’t a simple, “either-or” choice, since there are several levels of upgrade available. One typical catalog, for example, offers a “high-performance street kit,” a “casual autocross kit,” a “casual drag race kit,” a “race kit” and a top-of-the-line “high performance flywheel/clutch system.”
Choosing the one that’s right for you is largely determined by what you want to do with your car. Aside from the top-of-the-line system, the prices aren’t significantly different for various applications. These kits sell in the range of $400 to $600, not including installation.
All of the kits consist of the clutch disc that is pressed against the flywheel when the clutch is engaged, the pressure plate that pushes the clutch against the flywheel, the throwout bearing which pushes the clutch disc away from the flywheel when the clutch pedal is depressed, and the alignment tool needed to install the parts.
It is the type of friction material that makes the difference among the clutches designed for different applications. In the “street kit” level, a steel-backed woven organic material is used, that allows a small amount of slippage before hooking up. This slippage, though less than that of the stock clutch disc, will smooth out the clutch engagement when starting off from a stop. Allowing the clutch to slip slightly can be important in situations such as starting from a stop on a hill.
The higher performance clutches will engage more positively, since the intention is to get off from a stop as quickly as possible, and spend as little time as possible with the flywheel spinning but not connected to the transmission while shifting gears. On these clutches, kevlar, carbon or ceramic materials are used on the clutch disc, which allow for less slippage than stock disc materials as the clutch is engaged.
At the level of performance and activities being discussed in this chapter, where you’re using the car as a daily driver, but taking it out occasionally for a track day, autocross, or long-distance tour, you will probably be quite happy with a high-performance street kit. The organic disc material will provide more grip and quicker engagement and disengagement than the stock clutch, but still allows a little slippage.
If you are considering changing the clutch and your budget will tolerate the added cost, you may also wish to change the flywheel at the same time.
The flywheel on an automobile engine is a large disc at the rear of the engine that has a great deal to do with how smooth the engine runs. It is fastened to the end of the crankshaft, the shaft that is rotated as the pistons go up and down inside the cylinders. The momentum of the flywheel is used in order to smooth out the operation of the engine as its speed changes.
As the engine builds up speed, part of its energy is used to spin the flywheel, which being large and heavy, requires some time to build up momentum. Once the flywheel is spinning, that momentum keeps it spinning for a short time, even after you let off on the gas and the engine is no longer producing as much power. The flywheel keeps the engine from speeding up or slowing down too abruptly, which in turn smooths out the car’s changes in speed, keeping your passengers more comfortable.
On most brands of automobile, the flywheel consists of a single metal disc with a center hub that clamps to the crankshaft. It also has a toothed gear around the outside edge against which the starter gears engage when you press the starter button.
For smoother performance in engine and clutch operation on the MINI Cooper S, BMW engineers have taken the basic flywheel design one step further. The MCS flywheel is what is known as a “dual-mass” flywheel. Instead of using just one disc, this flywheel consists of two discs with a spring mechanism in between. The spring mechanism allows the disc fastened to the crankshaft to rotate as much as 70 degrees, almost a quarter of a turn, before the disc that engages the clutch is put into motion.
This design permits smoother clutch engagement, and also damps out much of the noise and vibration that is generated between the clutch and flywheel. Here again, the designers have balanced comfort against performance, giving up a little performance to get a little comfort, which the average driver and passenger wants.
However, if you want to push the balance a little towards performance, and are willing to live with a bit more noise, especially when the car is not in gear, you can replace the stock flywheel with a lightweight single-disc flywheel. An aftermarket aluminum flywheel can weigh less than half the weight of the stock flywheel without risking engine longevity.
With a lighter flywheel, you’ll get quicker acceleration and deceleration. These are two positive benefits if you’re trying to get around an autocross course or track as quickly as possible, since best time of day will go the the car that can speed up on the straightaways faster, and slow down into the turns more quickly.
Since it seems as if every drivetrain modification in the typical tuner catalog promises increased horsepower, we should probably make sure that you understand that lightening the flywheel does not increase horsepower, since it doesn’t alter the engine operation. It simply increases the responsiveness of the car when you get on and off the throttle. Many drivers will mistake this responsiveness for added horsepower. The lightened flywheel will help reduce your lap times, and make driving more fun, but it won’t increase horsepower, grow hair on that bald spot, or perform any similar miracles.
A lightweight aluminum flywheel will cost approximately $500. Since it is necessary to remove the clutch to replace the flywheel, if you’re already substituting a high-performance clutch, it is considerably less expensive in the long run to change the flywheel at the same time.
Improvements in Responsiveness