Thursday, December 27, 2012
When Going Faster Becomes a Passion: Competition Upgrades for Your MINI (Part III)
Competition Upgrades for Your MINI
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.
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 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