The typical tech call for a clutch recommendation starts like this: “How much power will your RAM xxx clutch hold?”  Customers have been conditioned by other manufacturers to select their clutch by ‘stage’ or ‘level’ or ‘rating’.  While the horsepower and torque are certainly important, they are not the end all of clutch selection.  We have seen plenty of 400 horsepower applications that can slip a 650 ‘rated’ clutch.

I am much more interested in hearing about the load factors that the clutch system is going to see.  These are more important in the initial recommendation.  So what are the biggest load factors to consider?

• Vehicle weight – this is perhaps the most important load factor as well as selection factor in determining the clutch needed.  Simply put, the heavier the car, the more work the clutch has to do to hold without slipping.
• Vehicle gearing – This applies to both rear gear and low gear in the transmission.  Higher rear gearing (lower numerically) requires more slippage on takeoff to make a transition, and puts a much bigger load on the clutch with a harder launch or acceleration.  Likewise, a higher low gear in the transmission could be like starting out in second gear in terms of the load the clutch will see.
• Rear tire used – as you might imagine, a larger, stickier tire or slick that is more likely to hook up and not spin is going to put a much harder load on the clutch.  So what tire will you be running?
• Use of the vehicle – If you plan to spend time at the race track where traction is good compared to the street, this is going to load the clutch system harder on launch and through the gears, whereas on the street it might be more likely that the tires will spin.
• Horsepower/torque – Yes, this is certainly a selection factor to consider as it will affect the load the clutch sees.

Now consider this worst case scenario – a potential customer has a 4000 pound, 450 horsepower muscle car that they are converting from an automatic to a manual.  They have meticulously acquired all of the parts to do this conversion including pedals, linkage, etc.  They install a Muncie transmission with a 2.49 low gear and leave the 2.90 rear gear in the car that it had with the automatic.  What is going to happen when they let out the clutch on this combination?  LOAD LOAD LOAD!  Not to mention, they will have to slip the clutch for an eighth of a mile just to get it rolling and engaged!

Bottom line?  Get all of your ducks in line when selecting and setting up your clutch system.  Give yourself the best chance for success by making smart decisions not just with the clutch, but also with your gearing.  And have realistic expectations for what you will do with the vehicle and how the clutch is going to perform based on the load factors you introduce into the combination.  Understanding these load factors will help you make the best selection of both clutch and flywheel, which we will tackle in another blog post.

So here you are.  Clutch feels like it is starting to slip.  Or it feels mushy.  Or it isn’t disengaging completely.  Time to pick a new one, but how do you know what to choose?  This title says it all, picking a clutch is like picking a suit.  No one size or style is going to be right for everybody.  You need make your selection based on YOUR car and YOUR desired ‘feel’ and ‘performance’.

Here is how NOT to pick a clutch.  Put a post on your favorite social media platform and ask for advice.  You will get 532 different answers and some of them will be from people who don’t even have a clutch in their car, or just advocate for a specific brand because their ‘buddy’ has one in their car (or did in 1980).  Instead, consider these more personal questions:

What are my expectations for a new clutch system? 

Do I want it to drive smoothly and have a light pedal like stock?  Do I need extra holding power for engine upgrades, now or in the future?  Am I okay with sacrificing some driveability (i.e. smoothness of engagement) in order to have a higher holding capacity?

What sort of load will the new clutch system see in my car? 

Load is the most important factor in selecting a clutch, and we’ll talk more in depth about that in a future column.  For now, keep in mind that this means any factors that will contribute to more load on the clutch system.  These include, but are not limited to, Tire size and type, rear gearing, low gear in the transmission, and perhaps most importantly, the weight or mass of the vehicle.  Any time you are adding load to the clutch system, you must compensate in kind with your selection.

What kind of driver am I, and where is the car going to be used? 

This is where you have to level with yourself!  If your intended use is to drive your car every other weekend to car shows and back home to the garage, you probably don’t need an aggressive clutch that is capable of massive holding power.  Conversely, if you are a ‘spirited’ driver, or if you ‘beat the crap out of’ your car, or go the track every other weekend, you need to compensate properly in your clutch selection with something capable of holding more power or withstanding higher loads.

Don’t get hung up on horsepower ratings. 

The most important factors in your clutch selection are those listed above.  Sure, the horsepower is a consideration, but it’s far from the only one.

When I get a call asking for a clutch recommendation, these are some of the most important questions I ask.  And the manufacturer you choose should be asking the same questions.  If they are not, find one that does and truly cares about getting you in the right clutch for you, not just selling you the ‘one size fits all’ cookie cutter suit.