With a 625 horsepower Coyote V8 up front, our Factory Five Racing Challenge Car needed serious drivetrain to manage the stampede. Power is no good if your can’t transfer it reliably to the wheels. Therefore we turned to some of the best in specialty high-performance power coupling and transfer systems. When building a no-comprise track car like the FFR Challenge Car there are more considerations one must take into account.
Custom dimensions mean custom driveline components. Gear ratios must be tailored to the powerband of a screaming modular motor like the Coyote, a clutch capable of holding the the V8 in check, and a driveshaft compact enough to fit in the tightly packaged chassis. At Turnology we are devoted to the art of driving. The tactile interface of driving is important. How a car feels, responds, and answers to your inputs are not only a mark of performance, but passion. Steering is the first sensory stimulus enthusiasts of performance driving appreciate, but often overlooked is shifting. Positive gear engagement lends a sensation of confidence and control. For this, a unique shifter was a mandatory addition to the cockpit of our Challenge Car.
In this installment of the FFR Challenge Car build we will reveal our decisions, and share the reasoning that lead us to the drivetrain we chose. We will cover the installation of our Tilton Clutch, Jegs FRPP transmission, JE Reel Driveshaft, Ringbrothers Shifter, and Powermaster starter. Though few will ever see the internal workings of these components in the completed car, you will know the extent of our attention to detail.
Driving around at track speeds is hard on a clutch. When you factor-in a vehicle with 625 hp, and sticky tires, like our Proxes R888 the clutch will have to withstand a lot of abuse. Aggressive shifting would induce clutch slippage in an inferior combination of pressure plate and disc. Raw acceleration will fry a clutch the same as a set of street tires on a dragster. To ensure we have more than enough clamping force we turned to Tilton.
Working with the specialists at Tilton we decided on a 7.25-inch, triple-plate clutch assembly. Tilton produces multiple clutch disc compounds for different applications, and we elected to use the OT II design for a high-performing clutch that will still be forgiving enough, should this monster see any street driving. We spoke with Kirk Skanful at Tilton to get the inside scoop on the engineering involved with designing a clutch system.
Clamping Power For Our FFR Challenge Car
For our pressure plates Skaufel explained, “Traditionally we’d use a high-quality steel because you needed the strength, and although steel may have the strength but it doesn’t have the properties of iron. Most brake rotors are an iron base because they resist warpage, but they don’t have the strength. CGI (compacted graphite iron) has the strength we need for applications it’s being put into, but it has some of those iron properties to resist the warpage.”
Friction Is You Friend
In regards to clutch material, Skaufel mentioned, “We have OTII, OTIII — ‘OT’ has stood for ‘open type’ because traditionally in the ’80s most 7.25 racing clutches had a closed ring which didn’t allow debris and material to escape — didn’t let the clutches cool as well.” Our OTII metallic clutches are offered in single-plate to four-plate clutches. You ended up with a triple and within each of those variations we have six different diaphragm springs we can put into the clutch for different clamp loads and torque capacities. Just Because a clutch says it will hold 1,000 lb-ft of toque and you’re only making 500 lb-ft you don’t to go buy the 1,000 lb-ft clutch if you don’t have to because your release load is going to be higher. The amount of thrust load it puts on the engine crank is going to be higher. With the variety of springs we offer we can get the torque capacity you need, but keep the release load down as much as possible.”
Skaufel continued, “In the cintered metallic, we have one blend of friction material but we have a few iterations of the disc. We have a full circle disc with a six-rivet hub, and an eight-rivet hub. With more rivets you have more attachment strength, so for extreme applications we’ll use the eight-rivet version of the clutch. Lastly, within that same family of cintered-metallic clutches, we offer a paddle-type disc, so instead of being that full circle it’s a four-paddle.”
The Right Parts For The Right Job
Addressing the reasoning behind selecting the clutch we did, Skaufel explained, “We chose a 7.25 over a 5.5 because a 5.5 is going to get you more all-out performance while you lower the inertia quite a bit. If you bring the weight closer to the center of the crankshaft, the faster the engine is going to accelerate and decelerate. But, the down side of it is you really don’t have much modulation because it’s such a small clutch. If a car is going to be driven at all on the street it’s not going to have the mass or heat capacity to manage the heat when you slip the clutch, it will warp quickly.”
Release Bearing For our Clutch System
Justifying our hydraulic release bearing, Skaufel detailed the importance of creating a package of parts that work together, “It’s got to be looked at as a system, the other parts you supply to it maximize and optimize everything. We recommend a release bearing that has 44 mm contact diameter bearing, so we supplied one of our 6000 series hydraulic release bearings that had that appropriate contact diameter bearing. That bearing will contact right on the tips of the diaphragm spring fingers which gives you the most motion out of the spring for the best modulation. It then gets you the most leverage on the spring for the lowest effort.”
The clutch installation required some fine measurements. With a pair of calipers, and a straight-edge we measured the distance from the mounting face of the bell housing to the tips of the pressure plate fingers. Taking care to accommodate for the thickness of the ruler, we were able to get an accurate measurement. With this information and the overall extended length of the HRB (hydraulic release bearing) we mounted the bearing to the transmission output shaft with the appropriate free-play. Tilton prescribes .125 inch air gap between the release bearing (with the piston completely compressed) and the face of the transmission.
Jumping ahead a few steps, after the transmission and clutch assembly were in the car, we came to the final step of the clutch installation. Bleeding the HRB is an important part of the process to protect the investment of these high-end components, and refine pedal feel for the driver.
Slowly depressing the clutch pedal, we found the exact location at which the clutch disengages, and the wheels could rotate freely. Holding the clutch pedal in this location we set the bolt stop, preventing over-deflection of the clutch, and by extension the pressure plate fingers. Per Tilton’s instructions we added and additional 1/4 inch of pedal movement past the fully disengaged mark.
Jegs FRPP Transmission
The insides of a transmission may be intimidating, but we were in good hands with the people at Jegs. In order to make sure the power is geared into useable ratios we employed a Ford Racing transmission. We chose to go with a T56 Magnum with a 2.95 first gear. We made this decision over a TKO 600 for a few reasons. With a modular motor powering this Challenge Car, RPMs will be higher than a conventional pushrod V8. The T56 will be able to better handle RPMs above 6,000 than the 600.
While the 600 would have been over two inches shorter in length, the trade off for having a sixth gear is worth a little chassis modification. In order to fit the T56, the FFR rear transmission mount had to be modified. With an ultimate negative one degree of down angle, our transmission was mounted securely in place with polyurethane mounts.
JE Reel Driveshaft
The longer T56 transmission meant that the distance between the output and input of the differential was compressed to 8.5 inches. This cramped distance demanded a custom length driveshaft. We turned to JE Reel for an aluminum driveshaft that would fit the confines of our drivetrain. Jim Reel of JE Reel Driveline oversaw the custom fabrication of our aluminum driveshaft.
We spoke with Jim Reel about some considerations that go into a performance driveshaft. He explained, “The aluminum shaft is all 6061-T6, the yokes are forged, and the transmission yoke we used is a chromoly yoke. The process is pressing it together, then straightening it, welding it, straightening again, and balancing. After you weld it you cool it and re-straighten it again because of warpage.”
We asked Reel about the decision to go with an aluminum driveshaft in a performance application like ours, “Going with an aluminum on such a short vehicle — when there’s consideration for the offset in the U-joints, and being so short, there’s less critical mass in the middle. The chances of vibration are reduced, because you’re working with such a small tight area. If all the angles are set right, [the U-joints] are supposed to cancel each other and stop the vibrations. The shorter you get, the tighter it gets, and the more severe the angles are — even if they’re just one or two degrees, the aluminum would be something [that would lend to preventing vibration and improving engine acceleration]” he recalled.
Ringbrothers produce shifters in a variety of permutations to fit the style and ergonomics you desire. For this track car we find ourselves in between classical form, and modern function. Ringbrothers allows customers to build a shifter in a product configuration package. Options between heights, angles, anodizing, and knobs. allow the buyer to tailor their gear change controls.
We opted for the longer 10-inch length shift linkage for a taller profile. A shorter eight-inch is an available option if you have exceptionally lengthy arms. A throwback kick in the shift linkage brings the knob into hand for a classically informed musclecar aesthetic. Both raw aluminum and black anodized finishes are available, but we decided the gleam of raw machined aluminum would best compliment the cockpit of this retro-racer.
To make sure our Coyote V8 has sufficient cranking power we ordered a Powermaster starter. For our application we opted to go with the XS Torque. This 200 lb-ft starter features a 4.4:1 gear reduction for ample cranking in engines up to 18:1 compression ratio. Weighing in at only 8.5 pounds, this compact starter is tailored to fit in compact spaces, and is unphased by heat soak.
When selecting a starter for your application there are several considerations to take into account. According to Powermaster, “Starters, like engines, have different powerbands. Some have a maximum power point at a relatively high RPM with little torque, whereas others produce more torque and yet lower RPM.” The performance of a starter can be equated to that of an engine almost directly, demonstrating this, Powermaster supplies a dyno sheet with each of their starters so you know the performance range of your part.
That’s it for this installment of our FFR Challenge Car build. With these race-quality drivetrain parts, this Shelby replica will surely keep power moving to the wheels. If you would like more information about the application of any of these products, or need something for your build, have a look at our source bar below. The suppliers of these parts are more than willing to help you select the best drivetrain for your project. Stay tuned for more updates as we wrap up the final stages of our SEMA-bound FFR Challenge Car.