(This article first appeared in the September 2007 issue of SportsCar magazine)

By Philip Royle

It turns out, building an STS2 autocross car that can also Club race is tricky – so tricky, in fact, that Project Miata is now a C Street Prepared car. Why the class change? With roughly 80,000 miles on the stock clutch, launching the car was beginning to involve hefty, undesirable clutch slip. Following a couple interesting launches, it was obvious a clutch with a little more grab was needed – and since this car will ultimately be a Club racer, we were forced to install a heftier clutch than STS2 rules allow.

Choosing the right performance clutch isn’t difficult. Many clutch manufacturers have easy to follow guidelines on their Web sites, noting vehicle usage and modifications as deciding factors as to which product is ideal. Our search led us to a company called Fidanza. Fidanza’s roots are in SCCA Club Racing, and consequently, the company understands the needs of the racer.

Fidanza offers four clutch configurations, all made with varying clutch material. Fidanza clutches range from carbon Kevlar to Kevlar to ceramic to sintered iron, with each material offering unique holding qualities. Other clutch manufacturers offer clutches constructed of similar materials, while some offer clutches with a completely different construction. Regardless of manufacturer, though, the important thing is you use the right clutch for your application.

Getting enough clutch

Our project Miata would never see gobs of power, but the car itself would encounter plenty of abuse. As a result, we needed more than a stock replacement clutch, and it seemed Fidanza’s Kevlar or ceramic clutch would do the trick – ultimately, we opted for ceramic.

To be sure we were ordering the right unit, we called Bob Sheid, vice president of Fidanza, and he explained our options.

“The Kevlar clutch will hold about 60-percent more torque than the stock clutch, but it has a break-in period,” explains Scheid, noting that break-in periods may not be ideal for Club racers. “The material has to be heated and cooled a couple times, so we recommend a couple hundred miles of break-in. Once it’s broken in, you can abuse it because it won’t glaze.

“The ceramic is an aggressive material and will hold about 80-percent more torque than the factory clutch. It can be driven on the street, but you have to be forgiving.” Scheid noted that, unlike Kevlar, ceramic clutches have no break-in period. The downside is ceramic can glaze, although it is tough to do.

Sheid noted that installing a ceramic clutch will result in some clutch chatter. “It’s the nature of the beast,” says Scheid. “[Ceramic is] very good at holding power, [but] when you let off of the clutch pedal, this clutch is going to engage. It’s not going to slip for you.”

Since ceramic – as well as Kevlar and sintered iron, for that matter – can aggressively hold power, clutch surface area can be minimized. Consequently, material can be trimmed from the edge of the clutch to reduce weight and increase cooling. This trimming forms the pucks that many are familiar seeing on performance clutches. The ceramic clutch we chose is a six-puck clutch; it weighed 3lbs. 7.1oz. Interestingly, the stock clutch we removed weighed 2lbs. 8.8oz.

Scheid also pointed out the importance of a good pressure plate. The pressure plate is responsible for clamping the clutch against the flywheel, transferring the engine’s rotating power to the transmission.

“The things that are most critical on a pressure plate are the fingers in the middle, called the diaphragm,” explains Scheid. “The stronger those springs are, the more holding power the clutch is going to have. The downside is it usually takes more leg to press it.” To reduce the chance of a hard clutch pedal, Fidanza makes the pressure plate’s springs longer and uses leverage to keep the clutch pedal in check.

When we weighed the Fidanza pressure plate, it measured 9lbs. 2.8oz. The stock used pressure plate’s weight was about equal at 9lbs. 0.6oz.

You’re there anyway

Installing a clutch isn’t easy. For the Miata, it required removing a bulk of the underside of the car. Needless to say. doing this on jack stands is a pain and not something we wanted to tackle. Consequently, we headed to Advanced Performance Industries (API) in Cypress, Calif. API prides itself on working on a variety of vehicles, rather than specializing on a specific marquee. API is also an authorized Fidanza distributor.

API also suggested replacing the rear main seal. While the seal wasn’t leaking, the fact that it was a $25 part meant we should replace it anyway. API also noted that if you’re going to swap the exhaust, this is a great time to do so since it will be removed anyway.

Also, if the rules for your specific class allow, both Fidanza and API suggest installing a lighter flywheel while replacing the clutch – sadly, Improved Touring rules require the use of a stock, unmodified flywheel. But since the flywheel would be removed to replace the rear main seal, we chose to install a new stock flywheel.

Once API removed the stock clutch assembly and flywheel, we found the stock clutch had almost no signs of glazing, and the flywheel and pressure plate looked mostly fine. Considering the car has seen uncountable autocrosses and a number of weekend track days, not to mention daily commuting duty for the first seven years of its life, it’s fair to say the stock clutch is one sturdy unit.

On the road again

With the Fidanza 4.3 ceramic clutch installed, the car does experience some clutch chatter. We have found, however, revving the engine to 3000rpm or above nearly eliminates the chatter on slow starts. The engagement is quick, but since this clutch uses a marcel and a sprung hub, both of which smoothing clutch engagement, it is tolerable. And amazingly, the clutch pedal pressure is almost identical to the stock setup. On the Solo course, launches are a breeze with a very fast engagement, resulting in ample tire spin to put the car in motion.

Per Solo rules, however, changing the clutch material moves the car from STS2 to CSP. Since the car still sports the Hankook RS-2 street tires, we’re left – for the moment – with a very under prepared car.

Because the car is not daily driven, we could afford to install a heftier clutch than we need at the present time. Would we recommend someone install a ceramic clutch on a daily driven car? Probably not. Dealing with stop and go traffic with the ceramic clutch’s engagement characteristics would quickly drive you up the wall – although it could be done. Sadly, since Kevlar clutches require a break in period, the ceramic clutch is the obvious choice for a Club racer that isn’t ready for the sintered iron setup.

Now that the Miata is a CSP car, we can begin to strip out certain components and prep the car for the roll cage, seat and other components needed to obtain a Club Racing logbook. We are about to hit the point of no return with this project. On the plus side, though, disassembling a car is really fun.

SOURCES:
API, (714) 229-0880, http://www.advancedperformanceind.com

Fidanza, (440) 259-5656, http://www.fidanza.com