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
Although clutch and flywheel selection seems like black magic science, it really is straightforward. When the right choices are made you get an unbeatable clutch and flywheel combo that will serve you well for years to come. In the long run, you will save a bunch of money with the right decision to begin with. And can you afford to make a bad decision? Think about it.
Karl Benz is generally recognized as the father of the automotive clutch, which was invented nearly a century ago. There have been many variations of the humble hard-working clutch in the years since. The clutch’s primary task is to transmit power from the engine’s crankshaft to the transmission smoothly and firmly without slippage. Some slippage is important for smooth engagement, yet without excessive slippage that will cause unnecessary wear and heat issues. You want reasonable pedal effort yet solid engagement, which is what you will get from our Superior clutches.
Clutch selection depends upon the kind of driving you intend to do. If you’re going cruising or doing the workday commute, you need a high-quality diaphragm style single disc organic clutch. Organic clutch frictions provide smooth engagement and longevity in street use. Weekend racing calls for a Kevlar/organic combination. And if you’re real serious about performance driving, a Kevlar high-performance clutch is what you’re going to need to meet the demand.
There are two basic types of clutches — diaphragm and Long style.
The Long Style three-finger clutch was original equipment in a lot of vintage applications. Because the three-finger clutch and pressure plate exert incredible amounts of pressure they also create tremendous amounts of clutch pedal effort. Back in the day a stiff clutch pedal was the norm, but not anymore. Clutch technology has since become such that you don’t have to put up with a stiff clutch pedal (or the resulting knee replacement surgery) anymore. We can’t think of any reason here at Modern Driveline why you should have to put up with a heavy-handed clutch pedal.
The Long Style clutch consists of:
Now that’s a lot of parts!
The Long style three-finger clutch exerts brute pressure on the disc via a series of coil springs between the cover and plate. Spring pressure is the same regardless of engine rpm. Pedal effort is also the same regardless of engine speed. The Long Style clutch is an archaic design at best. It doesn’t make sense to use one anymore.
Diaphragm style clutches, as the name implies, consist of a diaphragm spring that “oil cans” between engaged and disengaged. It is easy to depress yet it delivers whopping hold power. The advantage of a diaphragm clutch is greater clamping power than a conventional three-finger Long style clutch yet without high pedal effort. What’s more, as diaphragm clutch discs wear clamping power increases over the life of the clutch, which improves clutch performance. This is one of the great advantages of a diaphragm style clutch. Another advantage is pedal effort, which is considerably less than a Long style clutch.
Although there are a lot of diaphragm style clutches in the marketplace Modern Driveline brings you a better clutch and here’s why. Modern Driveline brings you greater selection with a complete line of clutches and flywheels from the most trusted names in the industry — Superior and McLeod. We enjoy an extraordinary relationship with Superior Clutch because each clutch is handcrafted for Modern Driveline. Each Superior clutch goes through rigorous testing throughout the manufacturing and quality assurance process resulting in high reliability and performance.
A lot of manufacturers talk about Kevlar clutch frictions, yet in truth their clutch discs contain a very modest amount of Kevlar — some 10-20-percent — which isn’t saying much. Superior Clutch’s disc are nearly 100-percent Kevlar, which can stand extreme punishment and come back for more as hard miles are driven. If you’re a civilized driver your Superior Kevlar clutch from Modern Driveline may well be the last clutch you ever buy. They’re that good.
We can get into a lot of clutch science and physics. However, here’s what you need to know in order to make an educated purchase. There are four Modern Driveline clutch discs from Superior Clutch for your consideration — Organic, Kevlar/Organic, Kevlar/Kevlar, and Kevlar/Metal.
The Superior Organic clutch from Modern Driveline is a basic stock replacement in either 10″ or 10.5″ and with either a standard pressure plate or the brute King Cobra pressure plate. The Superior clutch is economical and well suited to the daily/weekend driver or show car. The Superior Super King Cobra clutch from Modern Driveline features steel backed facings to prevent disc separation common with original King Cobra clutches and heavy abuse.
Modern Driveline’s Superior Kevlar/Organic clutch is a multi-friction type with Kevlar® and Organic surfaces designed with additional grip in mind and high heat tolerance from high performance applications. These clutches are available in 10″ or 10.5″. The Superior Kevlar/Organic clutch is optimum for street and track use.
Superior Clutch’s Kevlar/Kevlar clutch from Modern Driveline is designed for long life and minimal wear under extreme duty conditions. These heavy-duty clutches available in 10.5” are preferable for the high performance, high torque applications. What’s more, they’re preferable for lightweight vehicles prone to clutch chatter.
The Kevlar/Metal discs are designed for high horsepower, drag racing or street performance where “grab” is the name of the game. These are typically considered an “on/off” clutch. You cannot “feather” them for smooth street driving.
Fig. 1: Side by side are the two basic types of clutches — diaphragm (left) and Long style (right). By design three-finger Long style clutches yield a very stiff clutch pedal. Diaphragm style clutches provide a powerful grip without a stiff clutch pedal.
Fig. 2: Two organic clutch discs — coarse tooth (left) and fine tooth (right). Clutch discs are generally designed for application (size and transmission input shaft) the type of use they will experience. Organic clutch frictions are for mild street use.
Fig. 3/4 Here’s why the Long style three-finger clutch gives us such a stiff pedal — powerful springs that provide exceptional holding power. However, they also make it very hard on your left leg and mechanicals because pressure plate holding pressure is transmitted directly to the clutch pedal linkage. This really is an outdated clutch design when there’s a better choice.
Fig. 5 The diaphragm clutch is simply a better piece by design because it gives us great holding power without a stiff pedal. This is a small light-duty diaphragm pressure plate.
Fig. 6/7 Here’s another larger diaphragm clutch and disc for GM applications with fine-tooth clutch disc. .
Fig. 8 The beauty of the diaphragm clutch is simplicity — fewer parts. This link ties the pressure head to the backing plate.
Fig. 9 These coil springs act as shock absorbers in a clutch disc. This is what gives us a smoother engagement as the clutch disc and pressure plate take hold.
Fig. 10 Diaphragm clutch fully engaged with clutch disc compressed.
Fig. 11 Organic and Kevlar clutch discs back to back. On the left is the organic friction disc, which is thicker and heavier. On the right is the lighter and tougher Kevlar disc. Superior Kevlar discs are nearly 100-percent Kevlar designed to take the extremes of racing. Organic discs are strictly for street use.
Fig. 12 From left to right are metallic, Kevlar, and Organic discs
Fig. 13 Here’s the Superior diaphragm clutch with organic friction for street use. The organic disc will not stand up to the punishment associated with racing. This is a coarse tooth hub for vintage Ford applications.
Fig. 14 This is a Kevlar clutch friction in fine-tooth for GM applications. A fine tooth hub provides greater strength because there’s more shaft and hub surface area. Kevlar can be used for street and strip, however, it is primarily a racing clutch. It can take tremendous punishment and come back for more.
Fig. 15 Here’s a closer look at the Kevlar surface. All of our Superior Kevlar clutches are nearly 100-percent Kevlar for extraordinary durability and long life. Kevlar doesn’t look all that intimidating but don’t let looks deceive you. Kevlar is the toughest clutch material available. If you buy it for street use you will never have to buy another clutch.
Fig. 16/17 Here’s a metallic Superior clutch for all-out racing applications. This pressure plate is for GM products. As a rule most GM applications employ a fine-tooth input shaft.
Fig. 18 We also stock McLeod dual-friction diaphragm style clutches for performance applications. Choice depends upon how much power you have and what you want your vehicle to do.
Fig. 19 Here are the input shaft types you can expect to see for domestic applications. These are clutch alignment tools. Two from clutch kits and one copped from a Ford transmission. The steel input shaft works well as a clutch alignment tool. Our Superior clutches come with a clutch alignment tool for your convenience.
Fig. 20 Clutch pilot bushings (right) are typical for OEM installations. Compliment your Modern Driveline clutch installation with a pilot bearing (left) for precision function and smooth clutch operation. While you have your vehicle apart check the engine oil pan gasket, rear main seal, and transmission input shaft seal for leakage. Any leakage issues must be handled at this time or you wind up with clutch disc contamination.
Fig. 21 Clutch release bearing to diaphragm alignment must be spot on for proper function and longevity. Diaphragm style clutch release bearings are constant duty parts meaning they’re in contact with the diaphragm at all times.
Fig. 22 Compliment your new Superior clutch installation with a new flywheel from Modern Driveline. This is money well spent because every new clutch deserves a new flywheel surface. We stock steel, billet, and aluminum flywheels for nearly every application imaginable.
This is why you do business with Modern Driveline. We are not a large company with huge product development teams with bean counters and Wall Street investors looking over our shoulders. We are automotive enthusiasts bent on getting our products right before they’re ever sold to the public. We are also very determined to deliver great technical support after the sale. As a Modern Driveline customer, you will never be our guinea pig. We find problems before they get to you and solve them before they ever become a headache for you.
We don’t just find the fault — we find the fix at the prototype phase before products ever reach the mass production phase. And if you ever encounter a problem with any of our products, we invite your contact so we may solve it before it ever becomes a frustration for you. If there is a problem after the sale, we are here to provide lifetime technical support for any Modern Driveline product you buy.
If you’re looking for more information or a quote for your project send us your details and we’ll get back to you with a quote. Or call…