Will a higher capacity clutch last longer because it can hold way more torque than my engine makes?
Replacing a clutch isn’t easy, and it can be a fairly expensive repair if your clutch fails. So the expected life of a clutch is something that often becomes a factor when you are picking a new one. One common thought that comes up is getting a clutch that has a much higher rating than you would ever need. The idea being that if the clutch can hold way more torque, then it should never slip, so it should last much longer than a clutch that has only a slightly higher capacity than you would need.
While this may seem like sound logic, it doesn’t quite work this way in practice. The factor here that is missed is how much of a priority a clutch manufacturer puts into long life compared to performance for a given clutch kit.
The clutch that comes in a car from the manufacturer will typically last at least 100,000 miles or more. So when clutch manufacturers make an Original Equipment (OE) replacement clutch, they will typically do their best to use a friction material for the disk that will deliver a similar amount of use. But as the torque capacity of a clutch goes up, generally the ability of a clutch to hold more and more torque becomes the priority, and the clutch's ability to last tens of thousands of miles becomes secondary to that.
The difference between a street brake pad and a track brake pad is a fitting analogy here. In both cases, a friction material that grabs more, and therefore has a higher torque capacity, also tends to have a shorter life.
So if you want to get as much life out of a clutch as possible, it is generally best to simply get a clutch that is designed to work for your application, and not to overshoot your torque capacity.
Do you need a lightweight flywheel?
A flywheel is the component that bolts directly to your engines’ crankshaft, and it is typically constructed of a fairly large, and fairly thick piece of solid metal. So factory flywheels are usually fairly heavy. If you are going in to replace your clutch, one other part that you might consider is a lightweight flywheel as a way to save weight, and to improve performance.
While it is fairly easy to realize the benefits of a lightweight flywheel in a 2-wheel drive car, it is fairly easy to run into the drawbacks of a lightweight flywheel in an all-wheel drive car. Where this typically becomes a problem is when you have to get the car moving from a dead stop. The reason is all of the traction that you have in an all-wheel drive car makes it much easier to stall the car from a stop. A heavy flywheel has more mass and therefore more inertia as well. That helps to make it easier to get the car moving. I will also mention that lightweight flywheels are more prone to juddering as you are starting to engage them as well for the same reason.
As frustrating as that can be, there are a couple of things that you would naturally do to prevent this in a car with this kind of a flywheel; let the clutch our more slowly and hold the engine at a slightly higher rpm as you are doing so. What this amounts to is slipping the clutch more, which can dramatically reduce its life.
In short, in a Subaru that is mostly driven on the street, a stock flywheel will have the best manners. Especially if you typically find yourself in a lot of stop and go traffic. If you do go lighter, the STI flywheel at 18 lbs is just about at the sweet spot for a balance between weight savings and performance, so if you can find a flywheel that is in the 15 lb range, that is about as low as we recommend going unless you have a car that is mostly used on track.
Can you resurface a Subaru flywheel?
If you have a car with a clutch that is slipping and needs to be replaced, or if you are replacing your clutch and decide you want to stick with a stock flywheel rather than go with a new lightweight one, you may consider having your stock flywheel resurfaced so you can simply re-use it.
In the case of a turbocharged Subaru that has a fully hydraulic clutch system, this is not recommended. The reason for this is that the hydraulic clutch on a Subaru has very little range to adjust the engagement of the clutch, and it is possible to remove enough material in a standard
re-surfacing process to make it such that you can’t properly adjust the clutch with the resurfaced flywheel. If this happens, the result is that you are going to have highly accelerated clutch wear, which can result in needing to go in and replace your clutch again much sooner than you would expect.
Our recommendation is to always use a new flywheel when your original one can’t be re-used to avoid this problem.