Thursday, December 27, 2012

When Going Faster Becomes a Passion: Competition Upgrades for Your MINI (Part III)

Competition Upgrades for Your MINI

Snappier Response

  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.

Tilton High-performance Flywheel and Clutch

  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 Tilton 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, throwout bearing adapter and installation hardware costs about $1500.

Mini Cooper Forum

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.

                      Standard         Close-Ratio
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 gear shaft  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 gear shaft  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.

Next Installment: More Stopping Power is brought to you by Mini Mania Inc., the largest online retailer of Mini accessories in the US. Check out our selection of  Mini Cooper wheels online today!

Monday, December 3, 2012

When Going Faster becomes a Passion: Competition Upgrades for Your MINI (Part II)

Competition Upgrades for Your MINI

High Performance Cylinder Heads

  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 headbolts, which will add about $200 to the costs. Professional installation is required so the total cost will be around $2500.

Exhaust Header

  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.

Next Installment: Competition Upgrades for Your MINI (Part III) - Snappier Response

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