How do these aftermarket companies, let alone, anyone, rate clutches in terms of power and/or torque? For example, Brand X clutch is rated at 300-400 hp. Does that mean that clutch cannot hold more than 400 hp? Maybe clutches are more about torque capacity than power capacity. I'm not sure. Powertrain/drivetrain is not my specialty, unless you want me to crunch numbers .

 

drivetrain + crunching = please don't.

i believe that it has nothing to do with the power the clutches can hold or not, but the rotational force they have to counteract associated with an approximate level of horsepower. hmmmm... god, it's late. let's wait for someone who doesn't have to figure it out.

 

The clutch companies probably would rather express the clutch ratings in terms of clamping force(old school method) but people are more familiar with h.p. numbers. Remember, h.p. figures are calculated by multiplying torque x rpm. How the two compare, I have no idea. However, I figured that the factory clutch probably wouldn't withstand a few minor mods and a good dose of nitrous so I replaced it with an ACT clutch and pressure plate while I was doing the flywheel. To be safe, any mods that add more than 50, or so, h.p. probably need a new plate. The stock clutch disc is interesting on the 8 because it is the same one sent by SR Motorsports with the ACT Extreme Duty kit. Perhaps we could save a few bucks by upgrading the pressure plate only. I still wouldn't chance it, though.

Charles

 

Interesting. Well, I am hoping to make 360+ rwhp through forced induction, time and money permitted . You are right though. Clamping force is a vague parameter. If I remember correctly, hp is

[(torque)(rpm)]/5252

I can't remember where the constant '5252' comes from, and for conceptual purposes, it doesn't really matter. Stock clutches can take some serious abuse; they're designed that way. I am going to wait until more companies come out with a clutch that will match my car's inevitable output. ORC, Ogura Racing, has a clutch for the RX-8 as well. I can't remember their power rating, but my guess is about 50% over the stock. Feel free to quote me. I just woke up :o . It's a full-faced clutch, which is what I'm looking for. The only drawback is that no one sells it here. In addition, it's a JDM piece. Not that there is anything wrong with the quality of Japanese build, JDM roughly translated means expensive. I am aware that ACT has a full-face "street disc" for the RX-8, but last I heard it was still a prototype.

Anyways, I find it hard to trust clutch ratings. Some use power, others use torque capacity. Which is the right one? If anything, either is acceptable, since torque is work and power is the time derivative of work dW/dt.

 

The two comparisons I am not sure about are clamping force vs. horsepower limits. The number 5252 comes from the mathematical calculation involving torque levels and the way a unit of horsepower is defined. Anyway, Hot Rod magazine did a great article on the matter a few months ago. I can try to find the issue in my stack of mags, if you like. As an aside, a friend of mine builds and dynos the Honda Indy engines and he recently told me they are using off-the-shelf genuine Honda clutch discs and plates. Not even racing stuff. His point was that there is very little need for a friction zone with such a lightweight car. In my 8 I am using an Exedy disc, much like the stock unit, along with an ACT pressure plate #MZ-010-X.

Charles

 

Yes. I remember that 1 hp is equal to 550 (ft-lbf)/s, well at least in English units. I hate metric units . Next person who uses pressure in bar gets kicked. I'll find out where the 5252 comes from. I don't read Hot Rod Magazine. They don't know what they're talking about. I'd like to see them explain transient part of the energy balance...

Anyways, how does the Exedy clutch feel Charles? I'm looking to buy a full-face clutch myself in the not-to-distant future .

BTW, your friend who works for Honda is one lucky...:D

 

Hey S.M., The strange thing about that Exedy disc is that it "appears" 99% identical to the factory installed disc. They are both full-face, sprung, and similar plate thicknesses. In real-world performance the thing is a little grabby when cold but when it warms up it feels real smooth. The clutch pedal feel has now begun changing with regard to friction zone and pedal height depending on both ambient and clutch temp. I have ragged the clutch a few times and it handles the abuse quite well. The pressure plate is much heavier than stock but I am now used to it. I replaced the clutch early because I knew I would be installing nitrous pretty soon and that begins next week.

Charles

 

Thank you for the info Charles . I was tooling around in Turbo Magazine's webpage, and I found a decent article for dummies about clutches. The gist of it was that clutch ratings are more dependent on clamping force which was explained as

T=PFNR

where -

T=Torque, in ft-lbf
P=Clamping force, in lbf
F=The coefficient of friction, unitless
N=The number of surfaces
R=Radius of gyration, the mean radius of the friction surface

Hmm. This looks like,

M=Fd

where -

M=Moment, in ft-lbf, not Btu (stupid TI-89 )
F=Force, in lbf
d=distance

I really hate the word torque, so I use the moment about the axis of rotation. There seems to be some truth to this. Here's the link if anyone is interested .

http://www.turbomagazine.com/tech/0407tur_clutch/

I came to the conclusion that it would be difficult to rate a clutch based on power alone. Why? Torque cannot be translated into power; torque is NOT energy. Additionally, power is the time derivative of energy dW/dt. I hope this clarifies anything .

 

awesome, i wasn't completely wrong.

and metric units rock the retarded endless lists of conversion multipliers that Impirical units need (pound of force, which you somehow use to measure mass, and inertia which is some weird physical concept, using fractions of an inch, etc).

 

I'll keep the English unit system. The hardest concept was g[c] :o.

 

Hey S.M., the formula(s) you indicated are very close to the ones that appeared in the Hot Rod Mag article. I know that popular mags don't delve as deep into certain issues as they sometimes should but the article I mentioned earlier was quite good. After 20 years of bench-racing and such, the matter of h.p. vs. torque was clarified enough for me to explain it to my kids. I'll give the author of said article for at least that much. Besides, if you take a cross-section of those in the musclecar or import tuning world they typically don't know as much as they think they do.

Charles