Dale Amy
October 1, 2003

Horse Sense:
The laws of gearhead economics dictate that a wise man will always have a clutch that is just a tad weaker than his transmission, since it's far cheaper to replace and should act as a driveline fuse.

V-8 Mustangs and manual gearboxes go together like peanut butter and jelly, pizza and beer, bureaucrats and taxes. There's just something about a successful up- or downshift's orchestrated ballet of hand and foot that is deeply satisfying to the enthusiast's soul.

Pleasure aside, all the horsepower in the world is meaningless without some way of getting it to the wheels in efficient fashion, so it's important to understand the basics of power flow from the crankshaft to the rear-end. And, as with so many aspects of Mustang ownership, we're bombarded with options when it comes to upgrading clutches and cables, bellhousings, shifters, driveshafts, and of course, the transmissions. With all these thoughts spinning and meshing in our heads, join us now for a beginner's basic primer on stick-shift hardware and how it all ties together.

But first, we must give credit where credit's due. It seems that every time we want the straight skinny on manual gearboxes, we turn to Don Walsh Sr. at D&D Performance, both for his seemingly bottomless well of knowledge on the subject, and for his guru-like patience in dealing with our often-infantile questions and time-consuming photography. Gearhead, Mustang drag racer, and all-round gentleman, before retiring in mid-2000 to devote full time to D&D Performance's rapidly expanding business, Don was senior drivetrain and chassis engineer at Ford Motorsport SVO (now Ford Racing Performance Parts). This man genuinely seems to know at least as much about drivetrains as most people know about breathing. Don spent the majority of his corporate engineering career with Ford, but he also spent a few quality years with both Rocketdyne and TRW, working on propulsion systems for the Apollo space program. So, you see, he literally is a rocket scientist, and that's good enough for us.

No discussion of Mustang manual gearboxes would be complete without a quick mention of the manufacturer(s) involved. Back in the day, Borg-Warner and Tremec were competing entities, each building its own line of five-speed transmissions. Nowadays the two product lines are merged under the ownership of Transmission Technologies Corporation (TTC), so it's all one big happy family. For simplicity and familiarity, we may occasionally slip back to the old corporate names. Hopefully, TTC will forgive us.

First, the Obvious Since we're talking basics here, let's get right down to them. Without a gearbox, even the most powerful Mustang's acceleration would be comparable to that of an Allegheny coal train pointed uphill. Yet the five or six forward ratios of the modern manual transmission are there to not only mul-tiply torque for getting underway, but also to reduce engine rpm and consequently noise, engine wear, and fuel consumption, while in Overdrive. That way you can have your 450 hp and still get 20 mpg.

Because it employs a direct coupling to the flywheel via the clutch, rather than the fluid coupling of an automatic, a manual-tranny drivetrain is more mechanically efficient but is also potentially subject to more shock and abuse. The clutch's primary task is a tough one-to provide sufficient friction to feed every bit of the engine's power on to the transmission without slippage. It's second, equally important, role is to completely and cleanly disengage the transmission while the vehicle is at rest. Naturally, both the tranny and the clutch assembly must be sufficiently robust to handle whatever torque is thrown at them. In our horsepower-mad circles, that's where the problem comes in. Since we're only happy once, we double, triple, or quadruple our pony's factory output. Luckily, for most sane endeavors and power levels, the aftermarket has us covered.

Go With the Flow
Before moving on to the individual bits, let's confirm how they work together to transmit power. The logical starting point is the pilot bearing, which is pressed into the end of the crankshaft and supports the nose of the transmission input shaft. This immediately brings up a point worth noting. If converting a car from an automatic to a manual gearbox, remember there are no pilot bearings on automatics. You'll need to obtain one, otherwise the front of the transmission input shaft will be unsupported and can be quickly damaged.

Moving on, the flywheel is bolted to the crank, and the pressure plate, or clutch cover, is bolted to the flywheel. Both therefore have no choice but to spin at crank speed. The link to the gearbox is the clutch disc, sandwiched in between the flywheel and pressure plate via friction material on each of its sides, and splined to the transmission input shaft. The clutch disc is clearly marked to indicate the side facing the engine, as there is a recess cut out of the flywheel to clear the springs in the disc hub. If you manage to reverse this, sparks will fly and your language will quickly deteriorate to rap-music levels.

The pressure-plate's diaphragm spring must exert sufficient pressure to clamp the friction surfaces of the clutch disc firmly between the pressure plate and flywheel, thereby synchronizing crankshaft and tranny input shaft speeds. With the clutch thus engaged, power routes through the tranny's input shaft to the countershaft, spins whatever gear is selected on the output shaft, and goes on to feed the driveshaft via the output shaft's splined end.

When you depress the clutch pedal, the connected cable pulls the clutch fork forward, pushing the throwout-or release-bearing against the fingers of the diaphragm spring and releasing the pressure plate's squeeze on the clutch disc, so that the engine can spin happily away while the transmission input shaft remains stationary. Whew, that's tiring, but it about sums up the process. Now let's take a closer look at some of the major players in this rotating relay race.

The Flywheel
Flywheels vary greatly between pushrod and modular applications. The externally balanced 5.0 Mustang uses a 157-tooth flywheel that is offset weighted. The imbalance weight is 28.2 ounces on pre-'81 engines, and 50 ounces on those '81 and newer. In contrast, the reciprocating/rotating assembly on modular engines is internally balanced, so there is no offset weight on their 164-tooth flywheels. In a further distinction, Romeo-built 4.6 modular engines use six bolts to secure the flywheel to the crank, while Windsor-built SOHCs and all DOHCs have an eight-bolt attachment.

With the exception of '96-and-newer Cobras, production Mustang flywheels are cast iron. Modular Cobra flywheels, however, are constructed of nodular iron to survive higher rpm. Aside from cast and nodular iron, stronger replacements can be had in billet steel and billet aluminum. As an example, D&D's billet steel flywheel has been tested by the manufacturer up to 13,000 rpm (if you ever achieve that rpm without splattering your engine, let us know). Stock iron flywheels can shatter. Don considers the essentially indestructible billet flywheel as "cheap insurance," especially if you're not able to run a steel scattershield.

Steel flywheels are heavy and therefore have a lot of inertia. Traditional thinking is this makes them ideal for drag-race applications, especially on slicks where you drop the clutch with the engine already thoroughly wound up. On the other hand, billet-aluminum flywheels have comparatively low inertia, allowing an engine to accel-erate more quickly, offering a distinct advantage on a road course. This same lack of inertia, though, can sometimes make aluminum flywheel-equipped vehicles a bit jerky on the street. Interestingly and somewhat unexpectedly, in the development of the '00 Cobra R, SVT found aluminum flywheels to be quicker on both the road course and the dragstrip. Regardless, billet steel is still the flywheel of choice for most drag racers on slicks. By the way, the term "aluminum flywheel" is something of a misnomer, as the actual friction surface area consists of a replaceable steel insert.

Whatever your application, Don adamantly recommends replacing the flywheel with any clutch swap, rather than trying to have it resurfaced. In his experience, a resurfaced flywheel is an invitation to clutch chatter and other problems.

The Clutch
In terms of diameter, the 5.0 cars started off with 10-inch clutches, switching to 10.5 inches in 1986. Factory clamp loads on these, says Don, ranged from about 1,600 to 1,900 pounds. This diameter continued right through the first couple years of the modular-Two- and Four-Valve-before being upped to 11 inches for the '99 Cobra. The GT followed suit sometime in 2000 or 2001. This 11-inch "Cobra" clutch provides about 2,200 pounds of clamp load, but it requires the modular's larger 164-tooth flywheel, which, in case you're getting ideas, will not fit within a factory 5.0's aluminum bellhousing. This doesn't mean you can't put an 11-inch clutch behind your 5.0 pushrod, but doing so will require swapping to a steel bellhousing for the larger flywheel. It should be fairly obvious that the larger the clutch diameter, the more its surface area, and, all else being equal, the higher its torque-handling capacity.

The factory 11-inch setup is more than adequate for production-line power levels and simple bolt-ons, but it approaches its limits once a blower is added, especially to a Cobra. D&D Performance has developed a Super Heavy Duty clutch kit featuring a disc with grippy Kevlar pads on the fly-wheel side combined with more typical organic lining material on the pressure-plate side, for perfect street manners. This disc was spin-tested to an insane 9,500 rpm to make sure it would stay together. Kevlar has a much higher coefficient of friction than the factory material and therefore provides similarly elevated torque handling capacity.

The other deciding factor in torque handling is the clamping force-or spring pressure-of the pressure plate. Though it's possible to "mix and match" clutch discs and pressure plates, it is imperative that the disc be of the correct thickness. For instance, the Cobra 11-inch pressure plate is designed for use with a 0.270-inch-thick disc, whereas there are many discs on the market having 0.305-inch thickness. Using a too-thick disc will reduce the clamping force of the clutch by placing the diaphragm spring at the wrong point in its curve.

Production 5.0 pressure plates were of cast-iron construction. Given the higher rpm capability of the cammer, all modular Mustangs instead got a tougher nodular-iron pressure plate. For any sort of elevated power levels, when replacing your stock 5.0 clutch, be sure to opt for one with a nodular-iron pressure plate.

The Bellhousing
Factory Mustangs are equipped with cast-aluminum bellhousings that are light but not strong enough to contain internal parts in the case of catastrophic flywheel or clutch failure. The 5.0's stock T5 transmission has a separate bolt-on bellhousing that can be easily replaced by an SFI-approved steel "scattershield," as do the popular replacement TTC (Tremec) 3550 and TKO. Drag sanctioning bodies require such a steel bellhousing for e.t.'s of 11.99 seconds or quicker. However, on modular Mustangs using the five-speed T45 and 3650 gearboxes, the bellhousing is an integral part of the transmission front cover for which no steel replacement is available. For peace of mind, Don recommends these cars be fitted with a billet flywheel (they already have a nodular-iron pressure plate).

An additional incentive for 5.0 owners to go to a steel bellhousing is that there will then be room to fit an 11-inch clutch package in place of their factory 10.5-inch components.

The Throwout Bearing
There's not a lot to say about the throwout bearing, an inexpensive but important part that plays a pivotal role in the quiet, smooth operation of a manual drivetrain. Though this bearing is just about as reliable as rocks, it still makes sense to replace this sub-$20 part with any clutch change or work. D&D includes one with each clutch kit.

The Clutch Cable and Quadrant
Most pushrod and all modular Mustangs utilize cable-actuated clutches having an ingenious but somewhat fragile self-adjusting plastic quadrant bellcrank arrangement. The quadrant's spring-loaded, ratcheting configuration automatically compensates for clutch wear and cable stretch over time, but it doesn't hold up well under the ravages of hard use, or the higher pedal pressure hat sometimes accompanies an aftermarket clutch.

A common aftermarket upgrade is a swap to a manually adjusted cable complete with a much stronger aluminum quadrant. Most are adjusted by a double-nut arrangement down at the cable's junction with the clutch fork, while some, including the Steeda unit carried by D&D, can also be adjusted at the firewall.

Don stresses that aftermarket clutch cables must be adjusted upon installation to mimic the factory 20-pound preload of the throwout bearing against the pressure-plate's diaphragm spring. Failure to do so may result in less than complete clutch disengagement, with subsequent damage to synchronizers and other gearbox internals.

The Shifter
Given the variety of transmissions installed during the past 20 years or so in Mustangs, it should come as no surprise that a similar diversity of factory shifters has been fitted. All have been specifically engineered to transmit little or no gear noise into the cabin, and most have been passably short of throw. So why, then, do we all insist on replacing these perfectly adequate devices with one of the multitude of aftermarket versions? Perhaps because-thanks to the modern manual's internal shift-rail construction-it's a fairly simple, inexpensive, and satisfying project that can be accomplished in short order in the garage or driveway. Most aftermarket shifters forego the factory's rubber isolation, so the price for this additional security is putting up with a bit of gear noise and vibration.

The Driveshaft Steel is the material of choice for production driveshafts, as it has superior noise-isolation properties. But steel is also heavy and, in drag racing, anything standing still in the drivetrain at the moment you launch should be as light as possible for minimum inertia. Steel shafts also have a lower "speed limit" before they go into resonance with potentially disastrous effect on the rest of the driveline. Thus, the popularity of aluminum driveshafts for the Mustang. Aluminum also provides more torsional "give" to somewhat cushion the blow to rearend components, and tends to filter out driveline vibration to a considerable extent.

Beyond aluminum in both price and strength are metal-matrix, chrome-moly, and carbon-fiber driveshafts, but these are also beyond the scope of this discussion.

The Transmissions From 1984 on, 5.0 Mustangs used the ubiquitous Borg-Warner T5 five-speed transmission (the earliest Fox 5.0s had the SROD three-speed-plus-Overdrive, and if you come across one of these, run away screaming). As the 5.0 progressively received more horsepower, the T5 gained progressive enhancements to handle it. In other words, late T5s are much stronger than earlier ones, yet this tranny still suffers a somewhat undeserved reputation for fragility. We lack the room here to cover the scores of continual engineering upgrades and improvements, but suffice it to say that a properly equipped T5 can handle a surprising amount of power if treated with some common sense.

What do we mean by properly equipped? Well, the strongest form of T5 is one equipped with D&D's 2.95 Super Alloy gearset, a kit that replaces the factory countershaft, or cluster; the input shaft; and First, Second, and Third gears with ones of revised ratios and stronger, double-moly alloy. The "2.95" refers to the kit's First-gear ratio; factory T5s had a 3.35:1 First that was fine with the original 2.73:1 axle ratios, but is really too steep once a decent ring-and-pinion ratio is installed.

Along the way, it became trendy to swap out the T5 for the heavier, tougher, but notably notchier Tremec 3550, or the even-tougher TKO. Placing a 3550 behind a 5.0 requires a new bellhousing because of a different bolt pattern. Opting for the TKO also requires a new clutch disc and driveshaft slip yoke, owing to a different spline count on the TKO's larger input and output shafts. D&D furnishes a 31-tooth slip yoke with their TKOs, to mate the larger output shaft with the existing driveshaft.

With the advent of the modular-powered Mustang in 1996, the top-loader T5 bowed out in favor of the T45, an end-loading gearbox that, early on (say '96 through '98), suffered from weak internal shift linkage, notably the forks and especially the Z-links. These problems were all but licked by 1999 (Don tells us there were no less than 104 engineering improvements made between 1996 and 1999), and the late-production T45s that remained in use until the '01 model year were fairly tough units.

In mid-2001 a switch was made from the T45 to the somewhat heavier and presumably stronger TTC 3650. Interestingly, Don suggests Ford's reason for the change was not so much due to strength issues, but because the 3650 has a fully synchronized Reverse gear. This was in response to complaints that T45-equipped Mustangs were sometimes (often?) difficult to get into Reverse. Not much is yet known about the eventual durability of the 3650, but, at this time, the only place to get parts for one is through your Ford dealer.

The first regular-production Mustang (outside the ultra-rare '00 R-model) to benefit from a six-speed is the '03 Cobra, with its strong and smooth T56. Versions of this admittedly pricey but oh-so-beefy box are now available to retrofit to virtually any pushrod or modular Mustang. D&D even offers a gorilla-tough variant originally engineered for the torque-rageous Dodge Viper. How tough is the Viper box? "In the 3 1/2 years we've been selling these," Don says, "we have yet to have a single gear failure." This has to be the ultimate manual transmission for any Mustang that sees more than just a quarter-mile.

And that's a wrap for now. Thanks for investing the time to read this not-so-brief overview of what your right hand and left foot command on a stick-shift 'Stang. Though we've no doubt raised more questions than we've answered, at least we have you thinking about what goes on beneath the floorboards every time you bang a gear. 5.0