Your vehicle's clutch is one of the most important parts of the drivetrain because it connects and transmits power from the engine to the transmission. However, choosing the right performance clutch can be a daunting task. To help you make an informed decision, read on as we provide more information to address common questions and provide a more comprehensive understanding of performance clutches.
Your vehicle's clutch plays a pivotal role in the drivetrain, linking and transmitting power from the engine to the transmission. Selecting the right performance clutch is crucial but can be a complex decision. Let's delve into the specifics to aid your decision-making process and provide a comprehensive understanding of performance clutches.
A performance clutch is a critical component that efficiently transmits engine power to the transmission, enhancing the vehicle's performance. It offers better engagement, durability, and power handling compared to standard factory clutches.
Differentiating between performance clutch kits based on stages is common. Manufacturers specify the recommended horsepower range for each stage, which involves varying clutch disc materials, pressure plate clamping force, and power handling. However, selecting the right stage should also consider factors such as vehicle use, weight, traction, and RPM range for optimal performance.
Explore the diverse range of performance clutches available, including single-disc, multi-disc, and puck-style clutches. Discover the advantages and disadvantages of each type to determine which best suits your application, whether it's drag racing, road racing, or street performance.
Understand how your driving style influences the clutch selection. Factors like quick engagement and high torque capacity are essential for drag racing, while street performance requires a balance between performance and daily drivability.
Delve into the characteristics of various clutch disc materials such as ceramic, carbon, Kevlar, and organic. Learn about their heat resistance, durability, and engagement feel to align with your specific preferences and requirements.
Recognize the significance of the flywheel in clutch selection. Factors like weight, material, and surface finish impact clutch engagement, response, and overall drivability. Ensure compatibility between your clutch and flywheel for optimal performance.
Clutch Disc Clutch Pressure Plate Throw-Out Bearing Pilot Bearing Clutch Pilot ToolPerformance clutches employ stronger pressure plate springs and friction materials like ceramic, carbon, or Kevlar, providing enhanced grip against the engine flywheel for higher horsepower applications. Properly matched kits are crucial to prevent slippage or parts seizing from excessive heat.
A racing or competition clutch is a high-performance clutch designed for racing applications, labeled differently by manufacturers targeting the automotive performance community.
Receive practical advice on installing a performance clutch, understanding the importance of a break-in period, adjusting the clutch pedal, and essential maintenance to extend the clutch's lifespan.
Our aim is to equip you with comprehensive knowledge to facilitate an informed decision based on your specific requirements. At JEGS, we offer a wide array of top-quality performance clutches from industry-leading brands. Contact our knowledgeable tech support team for further assistance in selecting the perfect performance clutch for your vehicle.
It is normal through the life of a car to have to replace the clutch once, or maybe twice. But if you have a project car or race car that is constantly changing and evolving, you may find that the time to find a new clutch comes up more often. This is especially true if you make a big jump in power, if you have the engine or transmission out for a rebuild, and so on.
There are now more options for a new clutch than ever before, and with all of the options of disk types, disk material, and number of disks for that matter, it can be hard to know that you are getting a clutch that will work the way that you want it to. For this blog, I wanted to go through as many of those options as possible to give you the information that you need to help pick your next clutch.
But before we get into the specifics of the clutches themselves, there are a couple of pieces of information that you will need to know about your car first.
Is Horsepower rating or Torque rating more important when choosing a clutch?
Most of the time, when someone calls in looking for a clutch, they want to make sure that the clutch that they pick will hold the power that their car makes. While this is a valid thing to consider, it turns out that looking at a clutch’s torque capacity is generally a better approach than looking at horsepower. The reason for this is that torque is a measure of the actual twisting force that your engine is developing, and therefore a more accurate measure of the amount of twisting force that your clutch will need to be able to hold.
Beyond that, horsepower is a measure of work that is calculated with torque and engine rpm. We have a blog that goes into specific detail about this relationship, but the important part here is that there are a number of different combinations of torque and rpm that will produce the same amount of horsepower. Because the twisting force of that torque will always be what the clutch has to hold, that is a better basis to use to make sure that you have a clutch with enough capacity.
Click here for our blog on the difference between torque and horsepower
How much of a safety factor should you build into that torque capacity?
There is a good chance that between you and the shop/tuner you are working with, you have a good idea of how much torque your engine does or should make. Let’s say that you should be making 400 lb-ft of peak torque. It would be safe to assume that you need a clutch that will hold just a bit more torque than your engine makes at a minimum so that you wouldn’t be taking the clutch right up to the point where it would start to slip every time you make that peak torque.
But it turns out that the number or dyno sheet that you have with that peak torque value on it already has a safety factor built into it. That is the dyno correction factor.
Unless your dyno tuner gave you a fully uncorrected sheet for your car (which is pretty unusual unless you specifically ask for it), your dyno is giving you a reading with a correction factor built into it so that your cars output is standardized to optimal running conditions to make it more useful to compare to different cars, on different dynos, run in different days in different conditions.
In short, even if you have a dyno sheet saying your car makes 400 lb-ft of torque, it is actually making a little less than that. Generally speaking a dyno correction factor is typically somewhere between 15% and 25% depending on the elevation of the dyno, and the conditions.And yes, a correction factor is still applied if the dyno is at sea-level. We have a blog on what a Dyno Correction factor is and why it is used if you would like to read more about that as well.
So if you have a dyno sheet showing 400 lb-ft of torque, your engine is actually making between 340 lb-ft of torque with a 15% dyno correction factor, and 300 lb-ft of torque with a 25% correction factor. In either case, a clutch that has a capacity of 400 lb-ft of torque would be perfectly fine for this application.
Click here for our blog, what is a Dyno correction factor?
But is that torque at the wheels? Or torque at the flywheel?
Because your clutch bolts directly to the crankshaft of your engine, it has to be able to hold whatever amount of twisting force the engine can create. But if you are measuring your car’s output on a chassis dynamometer, you are getting your measurement at the wheels after any losses from the drivetrain, etc. would have happened. This is a case where units do matter because a rating of flywheel torque is much different than a rating of wheel torque (wtq).
At this time, the vast majority of tuners are using chassis dynos to measure a cars output, and clutch manufacturers are aware of that. And it is for this reason, that you will most typically find their clutches rated for wheel torque. In some cases, as with Exedy, they will give you the ratings for a clutch in both flywheel torque and wheel torque.
If for some reason the clutch you are looking at doesn’t tell you which unit they are using for their rating, you would want to get that clarification. And if you only know your wheel torque rating, and the clutch is only rated in flywheel torque, that is a fairly easy conversion. If you look at the engine power that your car made from the factory, and compare that to what a stock car makes on a dyno, you will have a measure of your drivetrain loss. For instance, on a typical Subaru WRX or STI, that drivetrain loss will be about 70 hp and 70 lb-ft of torque. If you just add that to your wtq rating, you have a reasonable estimate for your flywheel torque as well.
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.
The main benefit from saving weight in a flywheel is that you are reducing the mass that the engine has to turn as it accelerates. So in theory, saving weight here makes your engine more responsive in a similar way to a lightweight crank pulley, which is especially noticeable when blipping the throttle for downshifts.
Often times there is a lot more weight that can potentially be saved in a flywheel compared to crank pulley. For example, a 2.0 liter WRX flywheel weighs in at about 28 lbs. An OEM STI flywheel weighs in at about 18 lbs. And in either case, there are lightweight aftermarket options that weigh in between 12 lbs and 15 lbs, and at an extreme, some that weigh even less.
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.
Let’s talk about clutch disks
The clutch disk sits in between the flywheel and the pressure plate, and it holds the friction material that largely determines how much torque the clutch will hold, and how the clutch will feel to engage.
The most commonly used clutch disk (which is used for the vast majority of OEM clutches) is referred to as a full-face, organic disk. This means that it is made up using an organic friction material which is fairly thick, and it sits in a complete ring or circle on the clutch disk. The two main reasons that this type of disk is so widely used is because first, the organic friction material offers a lot of options in terms of friction characteristics to be able to be tuned to hold a given amount of torque, and because the friction material is a complete ring, it has a very easy to control linear engagement. In other words, as you start to let the pedal off the floor, the clutch will start to engage. This ability to control the clutch results in a very easy to use clutch that also typically will last a long time.
The other common type of friction material is ceramic. Ceramic grabs the flywheel and pressure plate much more aggressively than the organic material, so it is fairly common for the ceramic material to be broken up into small areas on the clutch disk. These areas are referred to as “pucks” and most ceramic clutch disks are either 3, 4, or 6 pucks.
One important characteristic of a ceramic clutch disk is that they do not slip like an organic disk does. Generally speaking, a ceramic clutch will have one very small section of the clutches travel where the clutch will slip, and anywhere else in the clutch pedals travel, the clutch is either fully engaged, or fully disengaged. You could think of a ceramic clutch as an on/off switch, compared to an organic clutch which functions more like a dimmer.
The advantage of a ceramic clutch disk is that this quick engagement makes the clutch feel very responsive as you are switching through the gears once you are moving. But it does also make it harder to get the car moving from a stop. This is especially true with an all-wheel drive car.
The other friction material that is becoming more common in clutches now is Kevlar. The best way to think of a Kevlar clutch is that it falls in between the Organic and Ceramic material. It grabs a bit more aggressively than the Organic, but it will last a bit longer than a ceramic clutch.
Sprung clutch disks vs. unsprung clutch disks
If you look at the center of the clutch disk, you will see a set of springs there (typically 4). Virtually every OEM clutch disk will have a sprung center section of the clutch disk because those springs dampen the engagement of the clutch, making it feel more smooth, and they also can serve to reduce noise as well.
If you are looking at a clutch disk designed for a racing application, it will usually be a solid design that doesn’t use any springs, or is an unsprung disk. An un-sprung disk has a more immediate response when you are engaging the clutch, but it also makes that engagement more harsh (similar to the ceramic friction material). Generally, this you would only want to consider an unsprung clutch disk for a race application where you would virtually never do any stop-and-go driving with the car, and you have a very strong drivetrain that isn’t easily damaged from harsh clutch engagement.
Pressure plates and Pressure Plate Springs
The last key component of the clutch is the pressure plate. This is the part of the clutch that lets you engage and disengage the clutch, and it is this part that you are moving when you press in the clutch pedal. What allows the pressure plate to move is a spring in the center that allows you to apply and release pressure on the clutch disk.
Ultimately, the amount of pressure that the clutch can use to hold or squeeze the clutch disk to prevent it from moving is the pressure plate spring. This makes the pressure plate spring one of the key parts to adjust to increase the torque capacity of a clutch. But the trade-off made by making the pressure plate stiffer is that as the capacity of the clutch goes up, the pedal also gets stiffer, or harder to move.
Exedy and ACT both have heavy duty pressure plates that use a stiffer pressure plate spring to increase capacity of the clutch, and ACT even goes one step further with an Extreme Duty pressure plate to increase torque capacity even more. If you are looking at a clutch that has one of these pressure plates, you want to factor in the fact that the pedal is going to get harder to push down. If you are concerned about increasing pedal pressure, you would want to look at a clutch that uses a more aggressive friction material in the disk, or a clutch that uses multiple clutch disks to increase the capacity instead of the pressure plate spring by itself.
When would you want to consider a twin or triple disk clutch?
As we have discussed above, the two main methods that you would use to increase a clutch’s torque capacity is with a more aggressive friction material in the disk, or a stronger spring in the pressure plate. Both of those approaches can have a potential down-side for some applications, and there is a limit to how much capacity you can gain from those two options without making the clutch very hard to use.
This is where multi-plate clutches came in originally. If you need to increase the amount of friction that a clutch disk can create, rather than changing the material, adding a second disk is a very simple way to do just that and this kind of design can have some advantages over a high capacity single plate clutch.
The biggest advantage is that you can increase torque capacity without needing a stiff pressure plate spring. Now that there are more and more options available, a multi-plate clutch is definitely worth considering for a high-power car that is driven on the street because you can have a clutch that feels like stock, but can hold a lot more power. This is especially true of a twin disk clutch that uses 2 organic clutch disks. There are a couple of manufacturers that make this clutch now for the STI (Exedy, ACT, and Competition Clutch).
For motorsport use, another characteristic of a multi-plate is that you can increase capacity, but use a smaller and lighter clutch disk. Saving weight in the clutch helps to reduce rotating mass in the drivetrain, and so does using a smaller disk that keeps the weight closer and closer to the center of the clutch. This makes the clutch feel more responsive and lets the engine spin up or down much more easily as well making it feel more responsive too.
Lastly, higher-end motorsports multi-plate clutches are designed with fully floating clutch disks. In these clutches, the clutch disks sit in a basket such that when pressure is removed, there is no connection at all between the input shaft from the engine, and the transmission. This helps the clutch work reliably for long periods of time, and handle heat better than a standard single disk clutch. It also means that the clutches will make a fair amount of noise when the clutch is pressed in, so this really is something that you would only want for a race application.
When it comes to picking a clutch, it can simply come down to making sure that it will work with your car and engine. But as you start making power, or start building the car for more of a performance application, there can be more to consider than the clutch’s capacity alone. Hopefully this will give you more information to find the clutch that you need, and as always, if you have any questions at all, please reach out to us and we will do our best to help.
- Jon Cooley