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
Difference between Hp and Torque
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?
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.