Cooked Your Clutch To Perfection? Exedy Has An Option

    15 JAN 2024 - Jeff Willis

    BEST CLUTCH UPGRADES FOR YOUR STI

     

    Since the Subaru STI hit US markets nearly 20 years ago, it has become one of the most desired sport compact AWD automobiles among car enthusiasts. The unique flat-four turbocharged EJ engine that is combined with Subaru’s track-focused all-wheel-drive system is powered by a beefy 6-speed manual transmission. The full mechanical torque split will divide the power 50/50 to each axle so that each wheel will receive the same amount of torque. This is why Subaru has campaigned a very successful race program throughout many World Rally Championship events, and continues to dominate many grassroots racing style events for the weekend track lovers. And one of the key reasons is that the Subaru STI is highly receptive to upgrades throughout the entire embodiment of the vehicle. From brakes, suspension, chassis, but mainly the engine.

     

    Whether if its Subarus, or other high-performance platforms, the clutch must support all the power created by the engine and transfer the power and torque to the wheels. This is especially a larger task for all-wheel vehicles, and even greater for those with modified all-wheel drive examples as the clutch will absorb more energy to send power to the 4 wheels as opposed to a rear-wheel drive or front wheel drive vehicle.

     

    HOW DOES A CLUTCH WORK?

    In a nutshell, a clutch is like a giant brake setup between your engine and transmission. You have a flywheel that is bolted onto your engine. And your clutch kit is bolted onto the flywheel. You have your clutch pressure plate and your clutch disc. The surface of the flywheel and the surface of the pressure plate are like a brake rotor in a sense. The clutch disc that is sandwiched between the pressure plate and the flywheel is designed with friction material, much like a brake pad. The flywheel, clutch disc, and pressure plate spin at the same time when the engine is running. When you depress your clutch pedal, your clutch fork will press against a throw-out bearing that pushes down on your pressure plate. The pressure plate will have a high clamping load, and when you depress your clutch pedal, the throw-out bearing pushes down on your pressure plate to release the clamping load or tension between the disc and pressure plate, allowing the clutch disc to freely spin. When the clutch disc can freely spin, there is no friction between the clutch and flywheel, allowing you to change gears. Overall, it is the clutch disc that must support the power and of the engine before transferring that energy to the wheels through the drivetrain.  

     

    WHAT HAPPENS WHEN YOUR CLUTCH WEARS OUT?

    For most of the time when your clutch fails, it’s going to be the clutch disc. The clutch disc is a wear and tear item and will wear out like a brake pad. Clutch discs are designed with built-in friction pads, and they will wear down with mileage, or hard driving conditions. When those friction pads on your clutch disc can no longer create friction between your flywheel and pressure plate, you can no longer change gears, and your vehicle can no longer move because the disc will freely spin between your flywheel and pressure plate.

     

    WHAT IS CLUTCH SLIPPAGE? HOW DO I KNOW WHEN MY CLUTCH IS WEARING OUT?

    Clutch slippage is when you try to accelerate, and your RPMs shoot up high in range before your wheels start to move. The best way to detect early clutch slippage is to be in 5th or 6th gear on the highway, then do a short acceleration pull. If your RPMs shoot up and your vehicle doesn’t accelerate in sync with the RPMs, then you have a slipping clutch disc. It would be hard to tell in lower gears because the higher the gear, the higher the torque load. Once your clutch starts slipping, then you’re on borrowed time.

     

    BUT I JUST INSTALLED A CLUTCH IN MY STI A FEW MONTHS AGO AND ITS BROKE WHAT HAPPENED?

    Here are some reasons why you may have cooked your clutch. It doesn’t matter if you installed your clutch an hour ago. Here are some ways you can kill your clutch quickly.

     

    • IMPROPER DRIVING TECHNIQUES: This is a killer of clutches for sure.  If you over-slip your clutch pedal while driving, you wouldn’t completely kill your clutch off the bat, but it could certainly reduce your clutch life. Now if you are new to driving a manual transmission, this is almost normal as you are figuring out how everything works. But 9 times out of 10 it will be at the expense of your clutch. Over-slipping your clutch is when you rev your engine to 5k RPM or above, just to get the car moving. What you are doing here is overheating the disc. The higher your engine spins, the more torque and power the disc must run into once contacting the flywheel. Now if you are constantly doing this, you risk overheating your clutch disc which can glaze over the flywheel much like brake fade, eliminating any friction between the disc and the flywheel.
    • FREQUENT AWD LAUNCHES: Bouncing off the rev limiter and dumping the clutch is fun occasionally. Here is what your clutch is experiencing. Almost like slipping your clutch but this time is personal. When you are doing a clutch dump, you are sending all the power to your engine as you bounce off the rev-limiter. When you release the clutch pedal, the flywheel is already spinning at 7K RPMs. Your disc has the daunting task of absorbing all that power and torque while sending all of that energy to the axles and wheels. Because the disc essentially gets slammed on both sides from the flywheel and the pressure plate, disc material will burn off at a high rate. Most importantly, if your clutch is not rated to suit your power levels, it will cook. Which brings us to our next key point.
    • NOT HAVING THE RIGHT CLUTCH TO MATCH YOUR POWER LEVELS: The more modifications your engine has, the faster and more powerful it will react. Your clutch needs to keep up. This means that if you decide to get a Stage 3 Power Package that is supposed to make 400WHP+ and you have an OEM clutch that is rated to 255 ft/lbs of torque, the disc material will overheat quickly, and you will have surpassed the limits of what your clutch can handle. Exedy has a great variety of clutch kits to match the power levels your Subaru is making.

     

    HOW CAN I MAKE MY CLUTCH LAST?

    A proper break-in is required for many clutch kits. A break-in period is where you do 300 miles worth of regular driving without getting into boost. So, 300 miles of driving around town mainly, some highway driving, as well as stop-and go driving is required. This is like a bedding-in process for the friction pads on your clutch disc. Additionally this will help create a proper contact surface area between your disc and flywheel. And don’t slip the clutch too much or start off in 2nd gear where it increases slippage from a stop. At this point you should know your STI enough to where you know how it drives. At the same time whether if you have changed a clutch out yourself on your STI or have paid a mechanic to do so you know how labor-intensive it is and can understand the costs associated with replacing a clutch.

     

    HOW DOES AN OEM CLUTCH FEEL VS A RACE CLUTCH?

    Obviously, an OEM-style clutch will feel just like, or close to the clutch that came equipped in your Subaru. Now race-clutches, or clutches designed to endure more power can vary in terms of how they feel. It’s not so much about the pedal pressure, but more so the engagement. If you look at OEM-style clutch discs or even upgraded clutch discs, you will see integrated springs. Those springs are designed to absorb both shock and torque when the clutch pedal is released. This provides easier feedback and allows you to smoothly roll into motion. A clutch with a race disc or a clutch kit designed to handle a substantial amount of power will have an abrupt engagement. This is because of the materials used on the friction pads, the pads are larger for more grip, and are more difficult and uncomfortable to drive in daily situations.

    Exedy does have a solution and a good middle ground for an aggressive clutch that can support larger power levels but can sustain daily driving comfortability. And that is a multi-plate clutch setup. Multi-plate clutch kits use (2) and up to (4) friction discs to transmit higher torque loads while maintaining smoother gear shifts under faster shifting conditions. Multi-plate clutch kits will also usually have their own flywheel that has been balanced to work with the provided discs.

     

     

     

    WHAT IS THE BEST EXEDY CLUTCH KIT FOR MY STI?

    Again, this is dependent on what upgrades you have, because if you are making more power than your clutch can support, you will have a short-lived clutch life. You also must consider your driving style. Most Subaru enthusiasts like to use their cars as daily drivers while enjoying the weekend autocross or track event. Consider the modifications you have, and may perform in the future, then choose the clutch kit that best suits your driving style. Below are some examples that are available for various Subaru STI models.

     

    EXEDY OEM REPLACEMENT CLUTCH – SUBARU STI 2004+

    As the title suggests, this clutch has been designed to meet or exceed OEM build quality with the same torque levels. This is the right clutch for those of you with a bone stock STI and intent on keeping it that way.

     

    EXEDY STAGE 1 ORGANIC DISC CLUTCH KIT – SUBARU STI 2004+

    This clutch is perfect for those of you who have mild bolt-ons such as a cat-back exhaust or an intake. This clutch is rated to 321 ft/lbs of torque and provides comfortable daily-driving manners for those of you who want to stay relatively stock but want a clutch kit that is a little stronger than the OEM unit.

     

    EXEDY STAGE 1 HEAVY-DUTY ORGANIC DISC CLUTCH KIT – SUBARU STI 2004+

    This particular clutch kit is the heavy-duty version of Exedy’s Stage 1 Organic Clutch Kit. Except this can handle just a little more torque at 332 ft/lbs. OEM-like street manners as all Exedy’s stage 1 kits will provide. Ideal for those of you who want to stay below the 350 WHP range.

     

    EXEDY STAGE 2 CERAMETALLIC DISC CLUTCH KIT – SUBARU STI 2004+

    Now that we have reached stage 2, this one is going to start getting a little aggressive. With a unique disc design in a triangle with (3) large pads to grab the flywheel, it tries to compensate with (6) springs to absorb the torque. This clutch kit is rated at just under 370 ft/lbs, and it is ideal for those with a tune, full exhaust, and intake who want to stay below those 400WHP power levels which is considered by many to be the STI’s sweet spot in terms of having a rewarding driving experience.

     

    EXEDY STAGE 2 HD CERAMETALLIC DISC CLUTCH KIT – SUBARU STI 2004+

    The Stage 2 Heavy-Duty Cerametallic Disc Clutch Kit gets a major jump in torque capacity. This clutch is rated at 548 ft/lbs at the wheels, and this is ideal for those of you who may have full exhaust, tune, injectors, intake, and want a clutch that can handle the abuse of weekend track events that handle shifts that live in the upper-RPM ranges. It is also ideal for those of you in the 500 WHP range and want a clutch that can sustain the occasional spirited pull without the risk of cooking an OEM-like clutch on moderate bolt-ons.

     

    EXEDY STAGE 3 HYPER SINGLE CLUTCH AND FLYWHEEL KIT – SUBARU STI 2004+

    Unlike staged clutch kits, this is a complete package that includes a balanced flywheel specifically made to use in conjunction with this clutch. This clutch is rated to just under 380 ft/lbs of torque, and the purpose is to provide you with a comfortable driving experience with long-lasting performance, better shifts, and includes Exedy’s forged aluminum purples pressure plate that offers the most comfortable of driving conditions.

     

    EXEDY HYPER TWIN SERIES ORGANIC CLUTCH AND FLYWHEEL KIT – Subaru STI 2004+

    This is one of Exedy’s multi-plate clutch kits designed to provide optimal comfort and drivability for Subaru STI models that want to stay below the rating of 500 ft/lbs of torque. The multi-disc design provides a very strong clamping force instead of a single disc design with a compensating yet really stiff and heavy-duty pressure plate. For those of you with moderate upgrades such as a built engine, and a larger turbocharger setup that wants to stay at the 500 ft/lbs torque rating without sacrificing comfort for daily use, this would be the option.