So you just bought an import tuner magazine, and you're laughing at the Civic dynos that only show 120hp. You say because they are so old, even with modifications, these cars that should have an output of over 160hp, but actually only have 120! Well... not exactly. Dynos measure wheel HP, rather than "BHP." BHP stands for brake horse power, the power measured by an engine brake, being measured at the flywheel. It is typical for an engine to loose 15-25% of its power output at the flywheel, by the time it reaches the wheels. AWD cars have higher losses, due to the fact that they have to drive 4 wheels, and go through 3 differentials. front engine or rear engine cars have less loses since the transaxle holds everything together (transmission + diff), and there is less rotating mass to accelerate.

Other parts of the drivetrain are the clutch and differential. These parts help you get your power down to the ground, the clutch by holding tightly onto the flywheel, and the differential directs power to the two wheels. Below we list various parts of the drivetrain, and suggestions on which drivetrain parts are best for you.


Flywheel:

The flywheel attaches to the crankshaft on the engine and is usually thought of as the part in the drivetrain. Stock flywheels are heavy and usually iron. By upgrading to a lightweight flywheel you will find your engine revs faster, and your car will accelerate quicker due to the fact that it no longer has to accelerate that mass, so it can send the torque straight to the wheels. However, if your flywheel is too light your car will be very touchy, and much harder to drive (wants to stall, doesn't idle as smoothly etc). Most people that are not used to your car will stall it when they attempt to start from a stop - esspecially when matched with a harsh engaging clutch. Most sports flywheels from Nismo, TRD, JUN, HKS etc. have a good mix between power and driveability for the performance street/track car.


Clutch:

The clutch is what engages and dis-engages the drivetrain from the engine, by acting as a brade pad, pressing on the flywheel and pressure plate. When your engine produces a lot of power, the stock clutch can no longer a) clamp the disk firmly enough to keep it from slipping, and b) grip the flywheel/pressure plate well enough to keep the clutch from slipping from the torque. You know your clutch is no longer gripping when it smells like eggs, and if you find your engine revs up while you are driving in gear under high-load. If you let it get really bad the clutch won't even have enough grip to move the car from a stop. Different types of aftermarket clutches use different techniques to harness the power of your engine. Different material compounds on the clutch, such as metal or carbon, change its gripping characteristics. Different compounds will also change how the clutch engages. Usually a balance between clutch holding power, and driveability will be found. New clutch technologies (Such as ATS multi compound) are being developed to allow for an almost stock engagement feel, with a much higher holding power. When choosing a clutch, there is no need to go overboard. Find a clutch just over your TQ output goal, and use that, instead of buying a Tiilton Tripple Plate racing clutch for example, which will never reach its potential on a street car, and will be so light it is entirely un-driveable. Not to mention requiring rebuilds every 50 hours or so.

Transmission:

The transmission is what makes the engine efficent. Since gasoline engines are only efficent in very narrow RPM bands, they need to be able to vary the RPM to speed ratio. This is done using the transmission. Ever wonder why you can spin your tires so much in 1st gear but 3rd they wont spin? Sure you are going faster, but so what? You should still be able to spin them at high RPM... RIGHT? Wrong. The transmission is a torque multiplier, as the gear ratio changes the TQ output at the wheels also changes. This is why you cannot do a burnout in 6th gear, even if you are standing still. Stock transmissions can be weak, and are designed for low RPM on the highway, and very short first gear, so the car get get to a smooth rolling start with ease. Racing transmissions use long first gear, and short high gears so that all the gears are very close in ratios to each other, so the engine is always in its peak power band, although highway RPMs would be high and there is a heavy burdon on the clutch when starting from a stop. But who cares about that stuff in a race car?


Driveshaft:

The Driveshaft exists only on AWD, and FR cars, not MR or FF cars which use transaxles. By using lightweight driveshafts such as aluminum or carbon fiber (CF stock on a 350z!) you can eliminate rotating mass, similar to a lightweight flywheel. Aftermarket driveshafts are also stronger, stock driveshafts can actually explode when used with extremely high torque engines, and high grip tires. If the driveshaft is your weakest link in the system, it will be the first to give out - however usually the axles break or the tires spin well before a driveshaft would shatter or explode. Most cars do not require a driveshaft upgrade unless serious modification has been done. Axles are however more common for failing, and are basically little driveshafts to go from the differential to the wheels. You'll find these a common upgrade for just about any car that makes big power.


Differential:

The Differential transfers the power towards the wheels. On an FR car, the driveshaft runs to the back of the car, but the power must run sideways to the wheels. The differential takes the input from the driveshaft, and spins the axles forwards (think chain and axle on a go-kart). There are three main types of differentials, open, limited-slip, and locked. Most cars have an open differential, which will send power to the wheel with least resistance, so if you take a hard left hand corner, on exit acceleration the inside tire will start spinning, since the outside tire is loaded but the inside tire has much less weight on it. Drifting with an open differential is also very hard because it is not possible to drive the car out of the corner with power. A limited slip differential drives similarly to an open differential, except when one wheel starts to spin too much faster than the other (since the outside will always be faster than the inside), the system locks up, sending power equally to both wheels, either though the use of a viscus fluid, clutch, or torsen type. Many limited slip differentials take a more proactive action towards prevening inside wheel spin, and add "lock" based on acceleration and deccleration. Some stock cars come with an LSD, usually a viscus or torsen type. Clutch type has a harsher activation feeling, but is best for drifting and road racing. A locked differential would only serve usefull to cheap people, or people that drive mostly in dirt or loose surfaces. By welding the spider gears of your differnetial, both tires will spin at the exact same speed all the time. In a straight line this would be fine, but when you turn, your inside tire's turning radius is less than the outside tire, which means the outside tire must be spinning faster to travel the same distance as the inside tire. With a locked differential, the outside tire would overpower the inside, and the inside tire would start to hop since the tire is actually spinning faster than it is rolling. The hop happens because the tire winds up, and then hops over itself to release the twist that was created by the grip in the tire. It's quite unplesant and very hard on the drivetrain! When selecting an LSD, keep in mind what you want to do, if you just drive on the street, a stock LSD should suit your needs perfectly. Even for road racing, a stock LSD may be OK. Once you start geting into drifting you will surely want to upgrade to a clutch-pack LSD.