STi Clutch Upgrade

Changing the clutch on a car is something that every manual-transmission car owner has or will need to experience at least once. It’s not for the faint-of-heart, since to gain access to the clutch requires removing either the engine or transmission. There are methods to dramatically shorten the time and labor required, but that is not covered in this write-up.

At about 60k miles, the clutch on my 2005 STi began to have trouble holding torque during full-throttle runs. As I floored the gas, the RPMs would shoot up while the car would not accelerate. I could still get the car to move but I could not apply full throttle anymore. I had an idea this day was coming due to the constant and unrelenting abuse I gave while racing the car at the drag strip. During my earlier days at the drag strip, I had done a lot learning on how to launch the car, which in turn also significantly shortened the life-span of the original factory clutch.

The factory clutch has a light effort, wide friction point, and engages relatively late at the end of the pedal stroke. With a factory clutch and flywheel, you can get the car started from a standstill by poking the gas and catching the clutch as the RPMs came down.

During the research phase, I learned about the different types of clutches on the market and what their applications were. One important thing to note is that clutches hold torque not horsepower – you will need to determine your goals on torque for your car, driveability characteristics, and spending budget and find an appropriate clutch that matches what you want.

The factory clutch in the 2004-2007 STi is manufactured by Exedy. It is a single plate full-faced sprung organic clutch mounted to a relatively light pressure-plate with a 18.9lb flywheel. Additionally, all 2004-2007 STis use pull-type hydraulic clutches.

A clutch “kit” has several components:

• Friction plate
• Pressure plate
• Throw-out bearing
• Pilot bearing

Aftermarket clutches come in the following configurations:

• Full Faced
• Windowed
• Pucked
• Multi-plate

Full faced clutches have the entire surface of the clutch covered in clutch friction material. They are designed to be easy to modulate, have a wide friction point, and be very easy to drive on the street. Almost all factory clutches will be a full faced clutch because it offers the best streetability.

Windowed clutches are full faced clutches with “windows” cut out of the material. They are basically like an inverted version of the pucked clutch, and are designed to be a compromise between a full faced clutch and pucked clutch with a small weight savings. These are usually also very streetable, with higher holding torque.

Pucked clutches use either 6, 5, 4, or 3 “pucks” mounted on a hub. The friction material is on the pucks only. These types of clutches can be very difficult to drive on the street because of their sudden and abrupt friction point. The tradeoff is a very light weight, and extremely high holding torque. As a general rule of thumb, the less “pucks” the clutch has, the higher the holding torque. They are also able to cool off very quickly and provide more consistent performance under heavy usage.

Multi-plate clutches are designed to compromise between full faced clutches and pucked clutches. By separating the friction surface from one friction plate into several friction plates, they can use a full faced friction surface for extremely streetable performance but hold a lot of torque for race applications. The downside to a multi-plate clutch is their extremely high cost.

Of those clutch types, they can also be either sprung or unsprung. Clutches with a focus on being “streetable” – that is, meant to be easier to tolerate driving on a public road or in traffic will usually be sprung. This means there are springs installed inside the friction plate that absorbs some of the shock load of engaging and disengaging the clutch. Clutches designed for racing will usually have no springs to allow for quicker and more solid shifts and gear changes to reduce transition time, but the tradeoff is harsh engagement and more shock load to the transmission.

The pressure plate is what presses the friction plate into the flywheel, with the friction plate sandwiched in between. The engine is connected to the pressure plate and flywheel, and the transmission is connected to the friction plate. The throw-out bearing is what engages and dis-enagages the pressure plate from the clutch. The pressure-plate is what determines how stiff the clutch pedal will be (along with other factors like bleeding the clutch line like you bleed a brake line)

After a few months of research, I decided on an ACT 6 puck sprung clutch with the Xtreme pressure plate. This setup will give a very stiff clutch pedal while being slightly streetable. I also added a Streetlight flywheel to increase engine responsiveness.

There are several methods to doing a clutch job on an STi. For people with access to a vehicle lift and transmission lift, the easiest way is by removing the transmission. However, for the weekend mechanic with no access to a lift, removing the engine is the easiest way to do it. Our boxer engines are very small and longitudinally mounted, which makes them very easy to remove.

---

Did you find any of the information useful? Have I helped you save a bit of money / time / frustration? I've spent a lot of time taking pictures and meticulously documenting everything to post it here. I'd really appreciate it if you could send me some money via Paypal! This helps defray the cost of running this website and also provides me with beer to continue writing these articles. Thanks!