MINI Cooper Competition Upgrades
* Listed pricing in the following article is as of 11/12/2013 and are subject to change.
When you get to the point that you want to take your MINI driving to the next level and get seriously involved in auto-crossing and track events, you’ll need to add some safety gear to the car to meet most organizations’ requirements. Also, as you reach a point in your driving ability where you’re starting to reach your MINI’s limits of acceleration, handling, or braking, you may also want to make some mechanical upgrades to improve the car’s performance
Either way, aftermarket suppliers have just what you need. In this article, we’ll discuss means of further increasing your horsepower and response, gaining stopping power and improving your MINI’s handling. We will also discuss how to meet the safety requirements of your racing organization if you’ve decided you want to take your MINI out onto the track for some serious competitive motoring
Finding Even More Horsepower
By now, in your desire for more power, you’ve hopefully changed out the air intake, remapped the ECU, replaced the Cat-Back exhaust system and replaced the supercharger pulley if those changes are permitted by the group that organizes the events in which you participate. In addition, you may have replaced the spark plug wires, added a spark booster, and installed a water-to-air intercooler to increase your net horsepower for enthusiastic driving.
Be assured, there are still more parts on the shelf that will help you get the maximum available horsepower out of that sturdy four-cylinder engine under your hood. You can substitute a high-performance camshaft and if you’re really serious, replace the cylinder head to improve the engine’s ability to breathe. If your car is going to be used just for track or autocross competition or you’re willing to do a little work before big weekends, a high-flow exhaust header and catalyst can give you a solid boost in performance.
High-Performance Camshaft (view all)
The MINI engine has what is called a “single overhead camshaft” (abbreviated SOHC). The camshaft is the component on the engine that is connected to the driveshaft by a roller timing chain (it looks like a bicycle chain) and rotates with the crankshaft to cause each of the intake and exhaust valves to open and close at the right times. The MINI uses a single camshaft to control both intake and exhaust valves on all the cylinders, and it is mounted in the top of the engine, so it is called a single overhead camshaft.
The actual camshaft consists of a shaft that runs from the front to the rear of the engine. On this shaft are mounted twelve cams, three for each cylinder. As these eccentrically shaped cams rotate on the shaft, they push on two rocker arms for the two intake valves and a dual rocker arm for the two exhaust valves on each cylinder, causing the intake and exhaust valves to open and close. A high spot on the cam pushes the valve open, and then as the cam continues to rotate, the lower spot allows the spring on that valve to push the valve closed
As the intake valves open, they allow the air/fuel mixture to enter that cylinder. As the intake valves close, the exhaust valves open, allowing the combustion gases left after the explosion to escape from the cylinder.
By changing the size and shape of the cams, it is possible to change the points at which the valves open and close, and the time during which they are open. Consequently, changing the camshaft is a standard way of upgrading the performance of an engine. A camshaft that causes the valves to stay open longer is said to be “hotter”—producing more power—than one that doesn’t allow as much time for the fuel/air mixture to enter the cylinder, or the exhaust gases to escape.
The trade-off here is that additional horsepower can only be gained by negatively affecting the smoothness of the engine operation, especially when idling, so a hot cam is often said to be “lumpy.” To some extent, the design of the camshaft also has a negative effect on gas mileage, since more fuel is allowed to enter with each rotation of the engine.
As one might expect, there are several different variations of cams available, with the hottest being very good on the race track, but totally unsuitable for street use. However, there is a middle ground with several aftermarket camshafts that have been engineered to provide better engine performance without noticeable negative effects on the smoothness of the engine. These camshafts are ideal for the MINI that is used primarily for street use by an owner who would also like improved performance for occasional track days and autocross events
To select the one that’s right for you, rely on the advice of an established supplier and tell them what your objectives are in replacing the camshaft, as well as what else you’ve already done to the car. It’s possible they may be able to recommend other changes that you can make to increase performance and horsepower before going to the expense of buying and installing a new camshaft. If they think a higher-performance camshaft is the right answer, they should be able to recommend one that will meet your goals
Replacing the stock camshaft with a high-performance camshaft is an effective way of improving engine performance while still using stock valves and valve springs. This way, engine performance can be improved without the expense of disassembling the motor and changing the cylinder head
A high-performance camshaft typically is available for the Cooper S for about $500 and for the Cooper for less than $600. In both cases, there is generally a core charge of about $100 which is refunded when the original camshaft is returned to the supplier. Installation should only be undertaken by an experienced professional shop.
High Performance Cylinder Head
The cylinder head is the portion of the engine that channels the fuel-air mixture into the cylinders and the exhaust gases out of the cylinders, and controls the flow of fuel and exhaust with the valves and valve springs that are installed in the cylinder head. The cylinder head also includes the upper portion of the cylinders where the fuel/air mixture is compressed and the explosion takes place that pushes the cylinder downward.
The design of the cylinder head, and the quality of its manufacturing determine how easily air can enter the engine and how efficiently exhaust gases can be removed from the engine. The design also determines the pattern of the compression and explosion in the cylinders.
For the owners who want maximum performance from their MINIs while still using them primarily as daily drivers, additional increases in horsepower and torque can be achieved through substituting a high-performance cylinder head.
In high-performance cylinder heads, the passageways through which the air and gases flow are engineered so there is a minimum of obstruction. To further improve flow, the manufacturing process is as precise as possible, and then is finished off by hand-polishing, so that the surfaces of the air and exhaust passages are very smooth to reduce turbulence in the flow. Finally, the shape of the upper portion of the cylinders is altered to increase the explosive power of the fuel-air mixture
One product that has proven its benefits is the Stage 1 Performance Cylinder Head, available through some aftermarket MINI suppliers. Because of the complexity of its design and intricacies of its manufacturing, this cylinder head isn’t cheap, but it has been shown to add 20 to 30 horsepower to the engine’s performance
The Stage 1 head sells for about $1900, and there is a $1000 core charge, which is refunded if the original head is returned to the supplier within 30 days. Installation also requires a head gasket kit and new head-bolts, which will add about $200 to the costs. Professional installation is required so the total cost will be around $2500.
Exhaust Header (view all)
Breathing is still the means to the biggest improvements in horsepower and torque. Air has to get into the engine, and the exhaust has to get out. Because of environmental protection requirements in many states, we’ve left until last one of the critical links in that path: the exhaust header.
The exhaust header is that set of curly pipes that connects the exhaust side of the cylinder head to the exhaust pipes and muffler. Four pipes coming off the exhaust side of the head, one for each cylinder, channel the exhaust gases from the cylinders into common pipes that flow into a single pipe and into the exhaust system
On the MINI, the engineers have attached the catalytic converter directly to the bottom of the exhaust header. The cat then attaches to the “cat-back” portion of the system. (In many other cars, the cat may be further back in the system)
The design of the exhaust header can make a big difference in how smoothly exhaust gases flow out of the cylinders. Any constraints on this flow and the pistons have to do more work as they push upward in the exhaust cycle, which means a drag on horsepower and torque.
As with many other parts of the engine, manufacturing costs and engineering constraints have prevented MINI engineers from designing the high-quality system that would fully optimize exhaust flow. Fortunately, aftermarket manufacturers have jumped into that gap, making a stainless steel exhaust header that includes a catalytic converter which is a bolt-in replacement for the factory part.
Tests on one of the better versions of this performance component show that replacing the exhaust header can produce a significant increase in torque, especially in the mid-RPM range. In practical terms, that can mean better acceleration out of corners and higher speeds on entrance into the straights, which means lower lap times.
Because the catalytic converter is an integral part of the emissions control system on modern cars, in most states it is illegal to remove that portion of the system or replace it with a non-factory substitute. As a result, it isn’t legal to replace the exhaust header on MINIs that are going to be used on the street.
However, if you’re building a car that is designed primarily for competitive events and isn’t used on the street, putting on a high-flow exhaust header is a sensible and relatively inexpensive performance enhancer. If you only use the car occasionally for competition and don’t mind doing a little wrench work before the big weekend, the bolt-in design of this system means that it can easily be swapped in for competition and then removed for street use. Since many of the MINI aftermarket headers include a catalyst, you don’t even have to fret about increased emissions. The aftermarket header won’t produce any greater emissions out the tailpipe than the stock header.
With a catalytic converter attached, a high-performance exhaust header is available for about $750, and you can install it in your garage with standard tools. A new gasket, costing less than $10, will be needed when you make the exchange.
If you’re really getting into doing track days or autocrossing, and haven’t considered replacing the clutch and flywheel with a quick-reaction clutch and lightened flywheel as we discussed in the preceding section of the book, now is the time to consider doing that upgrade. If you can make your downshifts into corners quickly, losing as little time as possible coasting between gear changes, your lap times will improve.
A performance clutch and lightened flywheel is key to this aspect of your driving. For the same reason—improving your performance through the gears—a close-ratio gear kit can help reduce your lap times.
High-Performance Flywheel and Clutch (view all)
If you haven’t yet made the change, and are serious about finding every tenth of second that you can, you may wish to consider doing what the pros do and replacing your MINI clutch and flywheel with a high-performance clutch and flywheel.
This substitution offers several advantages over stock components and less-expensive upgrades. Going from front to back, the system starts with a light, balanced flywheel weighing only 11 pounds, which means the minimum amount of inertia and quicker response. Behind the flywheel is a smaller-diameter lightweight clutch, which means less weight in the car and a reduction in rotational inertia as with the flywheel.
The clutch disc itself has a cera-metallic surface, which means quick pick-up with no clutch fade over a long race. Finally, manufactured to racing standards by a well-respected racing supplier, this whole system is guaranteed to stand up to racing demand and provide long-lasting performance.
The only drawback to the competition clutch is that it does away with the vibration-damping springs with which the stock flywheel is equipped. As a result, your Tilton-equipped MINI will be louder at idle than a stock MINI. But then quiet idling isn’t the point, is it.
As you might expect, the Tilton materials and manufacturing quality are going to cost a little more than typical clutch and flywheel upgrades, but the performance and especially the durability make it the best choice when you’re seeking to be the very best. The full kit, including flywheel, clutch disc, pressure plate, throw-out bearing adapter and installation hardware costs about $1500.
Straight-Cut Close-Ratio Gear Kit
When the engineers are selecting gear ratios for a manual gear box, they’re generally going for the best gas mileage possible. That means that gears are selected to produce reasonable torque at the lowest possible engine speed, which may not be what you’re looking for in a track car. They’re also assuming that the average driver may not be all that precise in shifting, so they’ll use bevel-cut gears to reduce the possibility of grinding gears when shifting
In racing, you’ll sometimes hear drivers bragging about their “close-ratio” gearbox. They’ll also talk about using “straight-cut” gears in the gear box.
The first term—close-ratio gears—describes a gear set that has different ratios than the standard gears that are installed in stock MINIs. The purpose of installing a close-ratio gear set is to keep the engine well up in the power band (which as you may remember really starts about 3500 RPM) at the range of speeds common on a road track. It is especially important to making sure that you have all the torque possible for those all-important part of the turn where you are accelerating, because the car that accelerates faster out of the corner will be the one that’s ahead at the end of the next straightaway.
For comparison, the following table shows the ratios of the standard five-speed gear box and those in an aftermarket gear kit available in the aftermarket. To understand this table, remember that the lower the ratio, the slower the engine is turning relative to the driveshaft at the differential. So, for example, in first gear, the engine is turning 3.42 times as fast as the shaft on the end of the transmission. You’ll note there’s little difference between the gearing on the standard gearset and the close-ratio set, since first gear is only used to get out of the pits
However, the differences become more obvious as you move up through the gears. There is a significant gap between first and second gears on the stock gear box, going from 3.42:1 to 1.95:1. However, on the close-ratio set, the ratio only changes to 2.333:1. From this, you can tell that the engine speed will be much closer between first and second gear on the competition box than on the stock box, which of course is why this is referred to as a close-ratio box.
You can also see that the engine is going to be turning over about 15 percent faster in second gear with the competition box than with the standard box, so when the stock engine is turning at about 3000 RPM in second gear, the engine in the close-ratio equipped MINI will be turning at about 3450 RPM at the same speed, or right at the point where torque really starts to increase.
The difference is very marked in fifth gear. On the stock box, the engine is actually turning slower than the final drive, the ratio that is called “overdrive.” This overdrive ratio is excellent for highway driving where you don’t need any real torque because you’re cruising at a constant speed, but you want the best gas mileage. However, it would be totally unsuitable for the track. In contrast, the close-ratio box has a gearing that about 40 percent higher. So when the stock engine is loafing at 2500 RPM in fifth—perfect for highway driving—the competition engine will be running at 3550 RPM to produce the same speed, right at the beginning of the power band.
With this gear box, the engine easily can be kept in the power band at normal racing speeds in all types of corners. To stay equal in acceleration out of corners, the stock MINI would have to be able to put another 100 horsepower to the wheels.
First 3.42:1 3.417:1
Second 1.95:1 2.333:1
Third 1.33:1 1.788:1
Fourth 1.05:1 1.429:1
Fifth 0.85:1 1.208:1
The second feature of this competition gear kit is the design of the gears. In a standard box, the gear teeth are cut at an angle to the gearshaft, so that two gear teeth are engaged at any one time. These are called “helical gears.” By contrast, “straight-cut” gears have teeth edges that are parallel to the gearshaft. The helical gears significantly reduce the chance of the driver grinding gears when shifting and they are also noticeably quieter in operation than if the gear teeth were parallel to the shaft
However, in racing the helical gear is slower to engage than the straight-cut gear. Also, it doesn’t transmit power as effectively as a straight-cut gear. Since a race driver should be able to shift more precisely, and gear noise is not an issue, for competition the straight-cut design is preferred.
Most MINI owners who use their cars on the street as well as for competition probably wouldn’t enjoy the straight-cut gears in daily use. They also require higher rev levels for given speeds because of their ratios, so the competition box would deliver much lower mileage and greater engine wear. However, for MINIs that are being built up primarily for track use, they are worth serious consideration because of the competitive edge they provide.
The gear kit with the ratios described above is available for about $3500. Since it requires removal of the gear box and replacement of the gears, installation is best left to a service shop that is experienced in MINI work.
More Stopping Power
“The longer you can go fast, the faster you’ll go.” If some grizzled old racing or autocross veteran didn’t say that, they should have. One of the basics of racing, whether on a road course or autocross course, is that brakes matter because they allow you to maintain higher speeds longer on the straights before you have to brake for the corner.
On a road course, one of the best ways of getting ahead of your competitor is by out-braking them on the corner. You stay door-handle to door-handle with them down the straight, then wait to hit your brakes until after they do and the corner is yours. On the autocross course, the longer you can stay at top speed before braking for the corner, the faster your time will be around the course.
You can have all the power you want, but if you can’t make your car stop more quickly than your competitor, you’ll have trouble gaining that competitive edge. It all comes down to having brakes that better than your competitor.
In the previous sections we recommended fitting your car with 17-inch wheels when you replace the stock wheels, so that you could install a set of competition-grade brakes when you were ready. We also recommended that you upgrade your brake pads for both street and track use, so you’ve already gotten a taste of the difference made by having good, sustained stopping power.
As you get more advanced, and start probing the limits of your MINI’s stopping ability, it’s time to consider upgrading the brakes. Rest assured that there are a variety of options in the market for you to consider.
The easiest and least expensive upgrade is to install high-performance or racing brake pads. Another excellent upgrade is to replace your brake lines for improved pressure. For better performance and quick recovery, you can replace your brake rotors with drilled and slotted brakes. And for the serious performance driver, there are several complete brake systems that incorporate upgraded versions of all these components.
High-Performance Brake Pads (view all
To gain that competitive edge in braking, the easiest conversion is to a set of high-performance brake pads. For example, Ferodo makes a set of high-performance brake pads that offer a higher co-efficient of friction than typical street pads do, which means a more effective bite and quicker brake action when you press the brake pedal. Made of a denser compound, they also have less compressability, which improves pedal feel and reduces pedal travel.
Obviously, with these attributes, you’ve got to be a little more sensitive to your braking in normal street use, but they certainly can be used on a full-time basis if you want the assurance that excellent braking capability will give you.
On the track or autocross course you need to be able to use your brakes hard ten or fifteen times every lap. For these applications, it is good to know that the material in these brakes maintains its high friction level even as the brakes really begin to heat up.
Upgrading your brake pads is one of the least expensive means of improving braking performance. Improved high performance pads are available for about $150 a pair, or about $300 for both front and rear. It’s an easy replacement to make, as well. Just remove the wheel, release the retainers, slide out the old pads and slide in the new ones. Then replace the retainers and wheel and you’re ready to go.
If you want to take the next step up, Ferodo also offers racing-grade brake pads for the MINI’s front wheels, where more of the braking action takes place. These pads excel on all the criteria of good brakes: high friction levels, higher initial bite, better performance at high temperatures, and quicker pedal action.
The only drawback is that they don’t produce much friction until they are heated up, and they are quicker to bite, so we don’t recommend them for street use. However, all you need to do is upgrade the front and rear pads to a good dual-use level, then swap in these pads before you’re ready to go out on the course. Fifteen or twenty minutes at the end of the event to swap them out, and you’ll be ready to hit the road again. A set of excellent racing-quality pads for the front wheels is available for about $200.
Stainless Steel Brake Lines (view all)
When we first described how modern hydraulic brakes work, we noted that there is a master cylinder connected to the brake pedal, which pushes fluid down through the brake lines to the slave cylinders at each wheel, which in turn push the pads against the rotors. At the very last point before the fluid gets to the slave cylinders, it passes through four flexible lines that link the slave cylinders to the solid lines on the chassis. These lines have to be flexible so that the front wheels can turn, and all wheels can go up and down over bumps and in corners.
These flexible brake lines, or brake hoses, each about 12 inches long, are one weak link in the whole braking system. Stock hoses are made of rubber. As the fluid is pressed into these hoses, under hard use the rubber will expand before the pressure of the fluid builds up in the slave cylinder and pushes the brake pad in.
The MINI engineers incorporated rubber hoses, despite these limitations, for the simple reason that most drivers never use their brakes very hard, and the rubber hoses are less expensive. However, any serious race car builder knows the opposite: the racing driver will use the brakes hard, over and over again, and good stopping power is never too costly.
The alternative for competition and active street use is a flexible brake line made of a high-performance polymer called PTFE in place of the rubber, which is in turn surrounded by braided stainless steel strands to prevent expansion. With these lines, all of the pressure from the master cylinder is transferred to the slave cylinder to apply the brakes.
Braided steel brake lines have become the standard in all racing applications. However, because of their durability and performance they should also be considered for street use by the enthusiastic driver; other than the purchase price and installation time there is no downside to making this substitution.
A set of braided stainless brake lines for all four wheels that fits the standard MINI brake calipers is available for less than $175. Installation is also straightforward and can be done by the home mechanic. However, since brakes are a critical item, and installation does require bleeding the brakes, the installation might be left to a professional brake shop.
Brake Conversion Kits
To make a car go, they say, there’s no substitute for horsepower. To coin another phrase, to make a car stop, there’s no substitute for surface area. The bigger the brake rotors and pads, the more effective the brakes are going to be at reining in those horses you’ve been stuffing under the bonnet.
If you want to maximize your stopping power, you can opt for a complete conversion kit for the front disc brakes—the ones that most of the stopping—that will meet your needs for high-performance touring and autocross use, for frequent track days, and full-on racing. Of course, the look of custom aftermarket brake calipers and discs peeking through the wheel spokes give your MINI that serious competitive look.
Conversion kits from four different manufacturers—Wilwood,Stoptech, AP Racing, Brembo—illustrate the range of performance and price that are available in the MINI aftermarket. All four of these kits incorporate larger-than-stock rotors for increased surface area and better stopping power.
One note that we should make upfront: Because of their larger diameters, these kits can’t be used on 15- or 16-inch wheels and won’t fit all types of 17-inch wheels. Check with your supplier to make sure the kit you buy will fit the wheels you have on your MINI.
The Wilwood Big Brake kit is probably the most versatile of these kits as well as being quite inexpensive. The kit will fit the stock 17-inch MINI wheels, as well as most other 17-inch aftermarket wheels. It includes stress-flow forged brake calipers, The calipers on this upgraded system incorporate four separate pistons to provide more aggressive and consistent stopping power than the standard two-piston calipers on the stock MINI. The brake pads in this kit are “Q” compound polymatrix brake pads for reduced noise and dust levels while providing high resistance to fade, long wear, and low rotor abrasion.
Vented iron rotors of 12.19 inch diameter add stopping power because they have a larger surface area than the stock rotors. Drilled and slotted, they also dissipate heat more readily than stock rotors, as well as dispersing brake dust to avoid irregular build-up of dust on the rotor faces. The kit includes high-quality mounting brackets and fastening hardware and is available for about $900 on some websites.
Stoptech’s Big Brake conversion kit is a very popular kit that is a good upgrade for enthusiastic road use, and is frequently seen at MINI track days. The 12.9 inch (328mm) diameter rotors in the Stoptech kit also provide more stopping surface than the stock MINI rotors. In addition, they are vented for cooling and slotted to facilitate removal of brake dust. High-performance brake pads, stainless steel brake lines and all mounting hardware are included with the kit.
The Stoptech Big Brake conversion kit is available with anodized calipers for about $1700 and with vivid red calipers for about $1900.
If you’re going to be doing serious track-day driving or racing in a class that permits brake conversions, the AP Racing “Formula” Big Brake conversion may be a better choice for slightly more money.
The rotors on this kit measure 304mm in diameter and 24mm in thickness and are both vented and cross-drilled for efficient cooling under heavy use. Calipers incorporate four pistons for better stopping performance, dirt seals, and anti-rattle clips and are coated with a bright red anti-corrosion finish. The kit also includes stainless-steel brake lines, DOT 5.1 racing spec brake fluid, and all mounting hardware.
The AP Racing kit is available for just under $2000 from most suppliers, and is easily installed by a professional shop or experienced amateur mechanic.
At the top of the pecking order is the Gran Turismo Brake system made for MINIs by the Brembo company. Brembo is the brand most often seen on the most expensive performance cars on the road, including Aston Martin, Jaguar, and Ferrari, and this quality is obvious in the MINI kit
The brake rotors on this kit are a two-piece design that employs an aluminum hat to create a “floating disc” to reduce heat transmission and improve stopping power and pedal feel. Rotors are 320x28mm and are both vented and cross-drilled for cooling. Calipers, available in black, red, or silver incorporate four pistons for efficient performance.
In spite of its larger size, the Brembo system is lighter than the stock factory system. The reduction in unsprung weight is important, since it means that the brakes can be upgraded without the loss in horsepower at the wheels that can result from installation of heavier upsized brake systems
The Brembo Gran Turismo front-brake system is priced at about $2600 from suppliers. Installation can be done by a good tuning shop or an experienced amateur mechanic.
If you do opt to install one of these upgraded systems on your front brakes, we would recommend that you also upgrade the pads and rotors on your rear wheels to keep front-to-rear braking in balance. To complete the look, you can buy specialized spray paint to paint your rear brake calipers to match the front calipers.
Reducing Wheel Spin in Cornering
Perhaps the most important upgrade that we haven’t yet suggested is the limited-slip differential. The main reason for not recommending the limited-slip diff until now is because it offers such a comparative advantage that it isn’t allowed in any of the autocross stock or street or road-racing spec classes in SCCA or BMWCCA competition.
On the other hand, if you satisfy your urge for spirited driving primarily at open track days or on less-traveled back roads, you may very well want to consider this MINI modification. But let’s start at the beginning with why a limited-slip diff is a good thing.
Why Do We Need a Differential, Anyway?
If you go back to basic geometry, you’ll remember that the further you are from the center of a circle, the greater the distance around the circle. If you think about your front wheels, you can see that the same principle applies. When you drive around a corner, the outer wheel is going to have to travel further than the inner wheel. That means that the outer wheel has to revolve faster than the inner wheel as you turn.
To accomplish this feat through the power wheels requires some means to put the power to one front wheel while allowing the other wheel to spin freely so that it doesn’t scuff and slow you down. This is done with a device that dates back almost as far as the first four-wheel automobile, the differential.
In the simplest terms, the differential is a set of gears that transmits the power from the drive shaft to the wheels. These gears are designed so that the internal “pinion” or “spider” gears attached to each wheel rotate together when the car is going straight and both powered wheels are spinning at the same rate. However, when the car goes around a corner, the spider gears can spin at different rates to allow one of the wheels to spin at a different speed than the other. (For more information on how a differential actually does this, www.howstuffworks.com/differential.htm provides an excellent animated explanation of the inner workings of the differential)
The problem with this set-up is that the power is always divided equally between the wheels. However, since the gearing of the differential allows one wheel to rotate at a different speed than the other, the actual power applied to the pavement at each wheel depends on how much grip that wheel can get. If, for any reason, one of the wheels has less grip than the other, the torque may be too much for that wheel and it can break loose and begin to spin.
This can happen in regular driving conditions if one wheel hits an icy or slippery patch, especially when starting up, when a significant amount of torque is required to get the car underway. Under these circumstances, the wheel with the least grip starts to spin and we get nowhere. At that point, we’d be happier if the wheels were locked together, since some power would still get to the wheel with more grip and we’d be able to get out of the slippery spot.
In racing, a similar thing can happen on hard cornering. As the car leans away from the inside of the corner and the weight transfers to the outside wheel, the inside wheel doesn’t press as hard on the pavement. This will have the same effect as if the pavement under that inside wheel was more slippery than the pavement under the outside wheel. Take enough weight off a powered wheel, and the wheel will begin to spin.
In racing, what this means is that with a regular open differential, the amount of torque that can be applied to the wheels will be limited by the cornering speed. Exceed the cornering limits and the inside wheel will start to spin.
Once the inside wheel starts to spin, all you can do is back out of the throttle until the car gets more weight on the wheel and the torque being applied to the wheel decreases. Then the spinning tire will regain its grip and get you around the corner, but much slower than you might have wanted.
Limited Slip Differentials
To overcome this limitation, racers first developed differentials with internal (spider) gears that could only spin at different rates for a short time before an internal mechanism caused them to lock together, putting power back to both wheels. These “locking differentials” or “lockers” were quite effective, but jarring in their operation and not particularly useful for anything except racing.
More recently, racing engineers have developed “limited-slip” differentials (LSD). The most common form of LSD incorporates a clutch and spring mechanism operating on the spider gears.
In the clutch-type LSD, when the car is going straight and the wheels are rotating at the same speed, the spider gears spin together with the wheels. However, when one wheel tries to spin faster than the other, the springs press the clutches against the spider gears, restraining the tendency of the wheels to rotate at different speeds and thus assuring that some power is still transmitted to the wheel with grip.
There are also limited-slip devices that can be installed in an existing open differential to accomplish the same objectives. These devices are simpler in design than the clutch-type LSDs,and hence less expensive, though they aren’t quite as smooth in their operation.
The Phantom Grip is an example of the simple device that can be installed in the existing differential. This device consists of two disc plates that with springs between them, so that the faces of the plates press against the spider gears to constrain slippage under moderate acceleration. In racing and autocrossing, this action is particularly good to increase possible exit speeds out of corners. Under hard acceleration, such as getting away from the starting line as quickly as possible, the disc plates pivot to lock the spiders completely, causing both wheels to exert equal power to the road.
The Phantom Grip is available with standard springs that work effectively for occasional track or autocross events while still providing smooth operation for day-to-day driving. Race-quality springs can be used to upgrade the operation, though at the expense of smoothness in daily driving, for the owner who primarily races his or her MINI.
The Phantom Grip sells for about $400. The race spring upgrade tuning kit sells for about $40. Since the differential must be removed and disassembled to install the device, the installation is best done by an experienced tuning shop.
The more common, though more expensive, approach is to replace the existing differential completely, with a clutch-type limited-slip Several versions of the clutch-type limited-slip diff are available.
The Quaife company, one of the most well-known differential manufacturers, makes a clutch-type limited-slip differential specifically for the MINI Cooper S. Smooth in operation, it is very effective for the MINI that is used as a daily driver, but still raced or autocrossed seriously. The Quaife LSD sells for about $1200.
For the serious racer who wants more responsiveness, and a lighter-weight differential, a slightly more expensive Salisbury-clutch LSD is available. This style can easily handle up to 300 horsepower, enough to satisfy the most serious MINI racer. This high-performance racing LSD sells for about $1400.
For the racer or autocrosser who drives a MINI Cooper in a non-supercharged competition class, a limited-slip differential is manufactured for the five-speed transmission. This replacement is manufactured to racing-quality specifications, but offers smooth operation that makes it ideal for the dual-use driver.
An added benefit of this differential for the Cooper is that it can be supplied with a final drive of your choice, providing different choices for short- or long-track use. This LSD is supplied with a 3.71:1 final drive as stock, and is available in 4.29 and 4.54 ratios as well, though these changes require adding a spider (pinion) gear. This high-quality LSD for the MINI Cooper sells for around $2000.
Better Cornering Performance
If we haven’t been clear enough up to this point, we’ll emphasize the point again. Fast times on the track and autocross course, and satisfying jaunts down the backroads are all about cornering. The faster you can get the car around the corners without losing control, the better your times will be and your everyday driving will also be more safer and more fun.
We’ve already discussed basic and advanced improvements to MINI handling to gain an advantage on the corners. These improvements have included better tires, lighter wheels, shorter and heavier springs, more responsive shocks, stiffer sway bars and adjustable rear control arms, and now limited slip differentials. Nevertheless, there are still a few other modifications that will provide small but significant improvements in your MINI’s handling. These components come under two categories: camber/caster kits and bushing kits.
Front Camber/Caster Plates
The MINI’s ability to put power to the pavement through turns and its ability to steer and turn into corners responsively is determined by the “camber” and “caster” of the front wheels.
Camber is the angle between the side of the wheel and a vertical line at right angles to the pavement. This angle determines how much of the tread will be in contact with the pavement, especially under cornering conditions when the weight of the car is pressing down on the wheel, or lifting up on it.
Typically in racing, you will want to set the camber so that the tire leans in just a little bit. This is said to be negative camber, though the angle may be only a degree or less from vertical. With negative camber, the full width of the tread will still be in contact with the pavement when the wheel is unweighted.
Caster is the angle of the line around which the wheel turns, relative to a vertical line. Think of the fork on your old bicycle. If you remember, it slanted backwards from the bottom to the top. Since it was so effective in keeping your bike going in a straight line, you could even ride “no-hands” and still keep going straight.
The principle is the same on your MINI. A slight caster angle is desirable for normal driving, because it causes the wheel to come back to straight after a turn while the steering wheel slips through your fingers. This also makes long-distance driving easier, since you don’t have to be constantly adjusting the steering to keep the car going in a straight line.
However, in racing it will be easier to steer your MINI into the turns if it has a smaller caster angle than would be desirable for road driving, though you wouldn’t want to have that angle all the time.
Serious racers want to have the ability to adjust camber and caster in order to fine-tune suspension. For this purpose, aftermarket suppliers have introduced camber/caster plates that replace the factory strut tops and enable easy adjustment of both camber and caster.
The aftermarket camber/caster plates have two purposes: First, they allow you to fine-tune your suspension adjustment so the front wheels have exactly the right camber and caster for good cornering on the track. Second, they allow you to change back and forth between competitive caster/camber angles for racing and more comfortable caster/camber angles for street use.
These plates are installed in place of the tops of the strut towers, and hold the upper mounts of the kingpins to which the wheel is fastened and around which the wheel turns. All you need to do to adjust the camber and caster is place the wheels on alignment turntables so they can turn easily, loosen and readjust the camber and caster angles, then tighten up the plates. You don’t even have to jack up the vehicle to make the adjustment.
Two versions of camber/caster plates are available, one version for use with coil-over shocks, and the other for use with the separate shock and spring set-up. Either way, the cost of the plates is about $500. Installation is pretty straightforward, but you will probably want the assistance of a good race tuning shop to help you get the correct camber and caster angles dialed in and marked for racing and street use.
High-Performance Suspension Bushings
There are a number of points at which the chassis of the car is connected to the wheels. These include the front wishbones, steering rack, trailing arms, control arms, and anti-roll bars. The problem is that at each of these points one part must move relative to another part as the car goes over bumps, as weight shifts in turns, and as the wheels turn relative to the body.
If the metal parts were simply connected to each other at these movement points, they would soon wear out and break. Even worse, every time they moved, the pieces would rub across one another, creating noisy rattles and squeaks, and transmitting road noise directly into the car.
To solve this problem, auto engineers put “bushings” in between the moving parts, so the metal pieces don’t come directly in contact with one another. In the MINI that is designed for typical street and road use, these bushings are made of a fairly soft rubber-like material. This is a good thing, since the bushings soften the ride while effectively reducing sound transmission and shake. (Auto engineers collectively refer to these bad things as “noise, vibration, and harshness” or “NVH.”)
However, as we’ve noticed, a soft, quiet ride is often the antithesis of good handling and speed on the track, and such is the case with the bushings. The soft bushings are very flexible, so the car’s handling and turning aren’t as precise as the competitive driver will want.
To reduce flex without compromising noise suppression, serious competitors usually replace the soft bushings with polyurethane bushings. The material in these bushings is significantly less flexible than the rubber that is normally used, providing a much greater feeling of control in steering and handling.
Powerflex polyurethane bushings, one excellent brand, are available to replace each of the rubber bushings in the suspension. Prices vary from around $30 for the anti-roll bar and steering rack mounts, to $40 for the rear trailing arms, $70 for the front wishbone, and $160 for the eight pieces required for the rear control arms.
If you want to completely control the flex in the rear suspension for racing and serious track activities, you may wish to go one step further and replace the rear trailing arm bushings that control rear suspension movement with high-performance solid rear trailing arm bushings. Solid bushings precision-machined to exacting tolerances aren’t cheap, at $495 a pair, but they are the final step in upgrading the MINI suspension to racing quality.
Safety Items for the MINI
If you’re planning to race your MINI in one of the growing number of organizations that sanction specific classes for MINIs, then you’ll be required to install a proper race roll bar to protect you, and your passenger, in the event of a roll-over. For for complete protection, especially in a MINI that is being built specifically for racing, you may want to go to a full roll cage, that provides greater roll-over protection and bars across the doors that protect against impact from the side.
Even if you’re going to be using your car primarily for street use, but expect to put in several enthusiastic days a year at club track days, the added roll-over and side intrusion protection may be desirable.
Several alternatives are available. At a minimum for racing in most organizations, you’ll want a basic bolt-in roll bar that can be installed using supplied back-up plates and grade-five mounting hardware. This set-up provides good protection in the event of a roll-over and can be removed when you’re not actively campaigning the car. Even with this style installed, you still have access to the back seats for street use. Two styles are available, one for cars with and one for cars without a sun-roof. The complete set-up is only about $350, which is a reasonable price for added roll-over protection, and wouldn’t be out of place in a MINI that is used primarily as a street car.
An upgraded version of the roll-bar style protection includes a diagonal brace for better protection and a harness bar at should height to which the shoulder harnesses are fastened. This version can be used only on cars without sun-roofs, but is available in a design that allows you to leave the roll bar installed but unbolt the diagonal and harness braces when you want to use the back seat. This is a desirable design for about $490.
A third style of roll bar has the rear diagonal brace and harness bar welded in for better structural rigidity. The drawback of course is that the rear seat is not accessible when the roll bar is installed. Available for MINIs without sunroofs, this alternative costs about $500.
For complete protection, especially in serious racing competition, a full roll cage, with supports at the front and sides of the cabin and with bars that go across the doors for side-intrusion protection, is the best option. A well-designed roll cage providing full protection for the driver and co-driver, is available that can easily be bolted in without any welding required.
In this design, the diagonal cross brace and harness bar behind the front seats are welded in, so the back seat is pretty much unusable However, the door bars do unbolt to provide easier entry when the car isn’t being raced. The full roll cage is available for both sunroof and non-sunroof cars for about $850.
Pretty much every racing organization will require five-point seat harnesses for the driver. Not only do these harnesses provide protection in an accident, keeping the driver (and passenger if also belted in) from being thrown about the interior, or worse, being partially ejected from the car, but they also help stabilize you when you’re driving enthusiastically.
The “five-point” term refers to the fact that these harnesses have a belt over each shoulder, generally fastened to the roll bar or cage, a seat belt fastened to the floor on both sides of the seat, and an anti-submarine belt that fastens to the buckle and in the floor in the center front of the seat. This fifth belt prevent the lap belts from rising under heavy braking or collisions and prevents the wearer from sliding forward under the seat belts.
A full five-point seat harness from a manufacturer like Sparco, one of the best-known harness suppliers, sells for about $200, plus another $10 to $15 for installation hardware. It is worth emphasizing that seat belts should be installed by a specialist with racing experience, since a badly-installed harness system can be worse in some types of accidents than if the driver weren’t belted in at all.
Finally, most race sanctioning organizations require that the car have a window safety net. When you’re racing, you’ll be required to have your windows down, so that in the event of accident you can get out quickly, or be removed from the car, even if the door is jammed shut. To keep any important body parts, like an arm, or your head, from going out the window in a serious accident, you’ll need to have an easily-removable safety net covering the window. A good safety net for your MINI is available for about $60.
In addition, you’ll need clear protective headlight covers to keep glass from getting on the track in the event of a front-end collision. You can use racers’ tape if you wish, but clear plastic covers are preferable and can be left on to protect against gravel and stone impact on the highway. These are available for your MINI for about $50.
Driver Safety Equipment
Autocross and track day events require only that the driver wear an automobile safety helmet with a current Snell approval rating. If you move up to wheel-to-wheel club racing, however, you’ll need full safety gear. In addition to your “brain-bucket” you’ll need a fireproof Nomex racing suit (consisting of either a three-layer suit or two-layer suit and Nomex underwear), fireproof driving gloves, and leather or Nomex-lined driving shoes.
Many of the racing equipment catalogs offer a packaged-deal for novice club racing including suit, gloves, and shoes and helmet for under $500. However, if you want a good-looking outfit so you can really look the part as envious friends watch you slide into your MINI in the paddock, you’ll probably be looking at $750 to $1000 for your gear, in addition to your helmet.