Xcceleration

Drivetrain 101

How Much Power Can Stock Subaru Drivetrain Take?

Most people who begin to modify their WRX/STI begin with bolt-on modifications, and safe, Staged tunes. And everyone wonders at some point - just how much more power can stock Subaru drivetrain take while maintaining stock longevity and reliability?

Generally, whenever you modify your car and increase the power level, you are increasing the stress on car’s drivetrain components, which were designed from the factory to reliably work under a certain amount of stress/power. So technically, with increase in power comes risk of failure.

The good news is that Subarus are very well and strong built cars. In fact, stock WRX/STI drivetrain (engine, transmission, axles, differentials, etc) is exceptionally strong, stronger than most performance cars on the market today.

WRX and newer, 08-up STIs do have a ‘weaker’ point - transmission. Stock WRX 5-speed transmissions are known to be fairly weak when it comes to abusive launches and hard shifting, suffering from stripped gears. Newer STI transmissions have problems from the factory it seems, suffering from ‘popping out of gear’ and other issues.

04-07 STIs seem to have the stronger transmissions, with ‘07 transmission being the preferred one due to longer gears.

So, what are the relatively safe power levels for the stock drivetrain components? Based on our experience, the list below represents the relatively safe power levels for WRX and STI stock drivetrain components, assuming a safe tune. Keep in mind that modifying your car to these power levels greatly increases the risk of component failure due to increased stress. It doesn’t mean that the components will fail, just that there is an increased risk, especially if you abuse the car:

WRX Engine: ~ 400 chp
STI Engine: ~ 450 chp
WRX/STI clutch: ~ 400 chp / 350 ft.lb torque - clutch will last, but not long at these power levels
WRX transmission: ~ 350 chp / 350 ft.lb torque
STI transmission: ~ 600 chp / 600 ft.lb torque
WRX axles, differentials: ~ 400 chp / 400 ft.lb torque
STI axles, differentials: ~ 450 chp / 450 ft.lb torque

As you can see, stock Subaru drivetrain components have their limits for the amount of power the car can handle from the factory. What we are saying is depending on your power level and driving, you’re going to need to upgrade components.

To sum it all up:

Subaru WRX cars are weaker than STI cars, which are well built to a point;
WRX / STI can fairly reliably take some more power than factory levels, but should consider upgrades appropriate to their power, before drivetrain failures begin to happen;
Your car must be tuned safe in order to have any degree of reliability and longevity.

If you abuse your car, it will break, without sufficient upgrades!

Call us to discuss your upgrade options, before it breaks.

Drive Shaft Problems and their Symptoms

Vibration
A damaged drive shaft can cause the vehicle to vibrate. The entire vehicle or parts of it--especially the floorboards--may shake and tremble, and the vibration will often intensify and worsen at increased speed. Severe vibrations can also indicate a bad tire; however, vibrations from tire problems usually happen during acceleration, whereas drive shaft problems cause vibration when the vehicle is moving or stationary.

Sounds
A vehicle may emit noises if the drive shaft is bad. You may hear a low squeaking sound that intensifies with increases in speed. The sound may completely disappear when traveling at higher speeds and reappear as the vehicle decelerates. This is often the result of a severely damaged U-joint, a part connected to the drive shaft.

Turning Issues
A drive shaft problem can prevent your vehicle's wheels form turning properly. You may notice that the wheels hesitate when you turn a corner. You may feel resistance from the tires when making sharp turns or U-turns. You might also experience parking problems while trying to maneuver the wheels to turn into tight spaces.

Universal Joint Movement
A U-joint, or universal joint, that moves too much or does not rotate at all can indicate a drive shaft problem. To inspect the U-joint, set the parking brake and shift the vehicle into neutral. Move the yokes of the U-joint back and forth to check for flexibility. If there is excess movement in any direction, the U-joint is unstable and could negatively affect the drive shaft. Also, visible rust on the bearing cap seals surrounding the U-joint can indicate a drive shaft problem.

FAQ

How much driveline angle is right for my application?

That's a loaded question. The best answer is....the least amount of driveline or u-joint angle is the best amount of angle. Try to achieve the least amount of u-joint angle but don't make it less than 1 degree. A little known fact about u-joints is that they require about 1 degree of operating angle to get the needle bearings rotating. If they do not rotate they will fail. Too much angle will also cause them to fail. The type of rear suspension also plays a big part in setting the angles as well as the engine/transmission angle. Hard acceleration as in the case of a drag race car requires a different setting than a street driven car. Traction bars, ladder bars, 4 links, independent rears all have special needs and requirements.

Is driveshaft balance really necessary?

YES! Driveshaft balance is necessary for smooth driveline operation and to eliminate the destructive effects that an unbalanced or poorly balanced driveshaft will have on your transmission and rear end. All Driveshafts are High Speed High RPM Balanced to meet or exceed the top rpm requirements of your engine and transmission, especially in the case of an overdrive transmission where the driveshaft rpm is greater than the engine rpm in high gear. This is very important and should not be overlooked when purchasing a new driveshaft. True high speed balance means balancing at a High RPM.

DRAG RACING DRIVESHAFTS AND STREET SHAFTS

With today’s higher HP/Torque motors, higher RPM drive trains and sophisticated suspensions you need to know what you’re doing to make a shaft today. We have driveshafts manufactured in standard carbon steel, Chromoly,Carbon fiber or 6061-T6 aluminum. All of our carbon steel and Chromoly shafts are welded to demanding specs using unique slow feed lathe mounted welders using a special powdered core wire that has a higher tensile strength than conventional welding. Every shaft is balanced on state of the art computerized two-plane balancers and weights are attached with special rivet type mounts. All this is great. But before any shaft is made we carefully make sure the shaft is designed to spec before having it manufactured, we could just make a simple flyer to ask what’s needed and we do have that for lower powered cars. We feel at a higher level of competition cars we need to talk to you and give you a personal assessment of the car to assure its proper part.

We will need to know the following:

HP/Torque of the motor
Transmission
Rear ratio
Rear tire size and height
Top MPH the car will see
Weight of the car with driver

With this info we will be able to check for overall strength, critical speed and durability needed. All shafts are hand made and carefully packed for shipping. If you’re looking for a racing shaft give is a call and let us make you a custom shaft for your application.

To check for critical speed, calculate this equation:

Top speed x 336 (a constant) x rear ratio (like 4.10), then divide it by the tire height (28” tall tire).

Here is an example:

Top speed is 160mph x 336 = 53760 x rear ratio of 4.10 = 220416, then divide this number by the tire height, 28”. This car would have top RPM of 7872. Now the shaft being designed will need to be able to so this RPM.

STREET ROD DRIVESHAFTS

We have shafts manufactured from simple carbon steel to Chromoly and finish up the line up with 6061-T6 aluminum. These shafts can be made in any style or arrangement. Where we differ from most companies is that we have an understanding of the entire car.

DRIVESHAFT VIBRATION: 101 (not for CV axles)

Most people have trouble determining if the vibration in their vehicle is coming from the DRIVESHAFT or not. There are typically two types of vibration most vehicles would have. A fast cycle vibration or a slow cycle vibration, to help understand this lets say we put a bucket of water on the passenger’s floor. If the vibration in the vehicle produces small ripples on the top of the water this would be considered a fast cycle vibration. This type of vibration is usually a drive train vibration, things like the driveshaft, motor or torque converter. If the vibration puts waves on the top of the water or splashes, this type of vibration is a slow cycle vibration and usually is an axle or tire vibration. People have trouble determining where the vibration in their vehicle is coming from. Try the test below if your not sure.

JACK TEST

A simple way to do this is out the vehicle in question up on jack stands (make sure the vehicle is completely secure), run the vehicle up to the speed you have the vibration. Make sure you use the brake to stop the drive train before you put the car in park if it’s an automatic. If the vibration is a fast cycle vibration you may want to have the DRIVESHAFT checked for balance. This may make no sense to you but you may try indexing the shaft 180 degrees (just pull the shaft off the rear yoke and put it on the opposite way). What this does is change the resonant frequency property of the driveline and in many cases it takes the vibration away. If you have a slow cycle vibration take the tires off the car (make sure you put lugs back on the axle to keep the brake in line) and run the vehicle again. If the vibration is gone you now have to find out if it’s the rim or the tire and good tire shop can help you with that. This is a simple test for any vehicle but please if you’re not completely sure of how to put the car on stands safely bring the car to a certified technician to perform the test.

Drivetrain Components