5.0 Mustang & Super FordsHow To Drivetrain
Tremec TKO II Gearbox - Driving Driveline
Stepping Up Our Open-Track GT's Clutch, Transmission, And Driveshaft
It's difficult to remember our "stripe car" is already seven years old. While that's middle-age for a street car, it's the prime of life for a track-oriented machine where the modifications come quickly and no one is worried about warrantees or a minor scratch in the paint.
We've been extremely pleased with our open-track project car. Given a thorough chassis and suspension makeover by Maximum Motorsports, Baer Braking Systems, and Nitto Tires, the GT often vies for fastest-at-the-track honors. It also offers air-conditioned comfort on the street. And if you appreciate stone-chip chic, it even looks cool when hanging out with gussied-up street sissies.
In fact, sitting in our Kirkey seats and pedaling our 4.6 GT with its Ford Racing Performance Parts cylinder heads and intake, we'd be hard-pressed to say there was anything our project car really needs. It's fast and sort of hard-core-fun and furious-all at the same time.
It's a machine any enthusiast could duplicate given a bit of time and money, and then enjoy for years and years.
But leaving it alone is not the way of project cars, and naturally there's a Santa's list of things we should do to this track toy before calling it finished. For one thing, it's heavy, so anything that loses weight, or takes some weight off the front end, would be a good thing. And while the power is already fun at the track, it doesn't have the big-power hit that's so satisfying on the street or awe-inspiring on the track.
So that's why we've changed the flywheel, clutch, transmission, and driveshaft. Huh, you say? Those parts don't make any power. In fact, word is they might cause a weight gain. OK, that's almost all true as we found out, but not all true as we'll show in a minute. But above all, let's not forget the TKO Tremec can handle torque the stock T45 gearbox runs away from. And now that you know what direction we're headed, let's begin with what we've been running.
Making the Move
Let's be perfectly clear. The driveline we've been running has been dead-reliable and ideally suited for our car's current performance level. That driveline was a stock flywheel and clutch (or so we thought), as well as a T45 gearbox and driveshaft. The tranny was making a bit of noise-you would too if you'd been driven this hard this long-but it was a light, slick shifter. It offered a high-ratio Fifth gear for relaxed freeway driving when headed a couple hundred miles to the track. And, according to street wisdom, it is the lightest transmission possible. On the downside, Fifth was high-geared at the track (but not impossibly so), and all the parts had plenty of hard miles on them.
To move the driveline up the performance ladder, we needed a higher torque capacity in the transmission. And everything could stand to lose weight, both static and rotating. An aluminum flywheel was thus indicated. Along the same line, we'd heard nice things about Centerforce's Light Metal Clutch (LMC) in drag applications and thought it worth a go in a road racer. The transmission choice was easy. Tremec offers the T-3550 TKO II with much beefier internals (wider shaft spacing for larger gears). The driveshaft was another easy pick, what with FRPP's aluminum unit available from hundreds of sources around the country.
Getting into the details of our swap, we found few (read almost no) aluminum flywheels for six-bolt-crankshaft modular GT engines. There are plenty of eight-bolt modular V-8 flywheels available thanks to the Cobra trade, but six-bolt units are a special-order item. Luckily, Brothers Performance Warehouse, which had kindly offered to do the swapping for us, does offer the necessary six-bolt flywheel on a custom basis, so we were OK there.
Centerforce doesn't list its LMC as a modular engine part (it's just for 5.0s), but as the bolt pattern, pressure-plate height, spline count, and overall size are the same between 5.0 and 4.6 engines with TKOs, the LMC will work in a 4.6. Featuring an aluminum pressure plate with a riveted-on iron face, the LMC offers better-than-stock clamping force along with less weight.
As for the Tremec TKO II gearbox, going into the job we knew it had 475 lb-ft of torque capacity to handle our up-coming high-power engine (the T45 is supposedly done at 325 lb-ft), and the road-racing version's 0.82 Fifth gear is spaced to be usable on the track. Two of our regular haunts, Willow Springs and the Fontana road course, are fast enough to require Fifth gear given our 3.73 rear-axle ratio (we hit 140 mph in Turn 2 at Fontana last time we were there), so the Tremec looked to be a real performance gain. Offsetting this high-speed track advantage, the TKO II was a big, heavy thing according to conventional wisdom, and the lower Fifth gear didn't exactly promise any increases in our freeway cruising speed or fuel economy.
As for the driveshaft, the FRPP aluminum unit is readily available and easy enough on price. These shafts once had a rap for mediocre balance, but whatever issues regarding this have evaporated through the years, as everyone we spoke to liked their FRPP driveshafts. The only caution was the FRPP driveshaft is made from material too thin for local shops to work with in case lengthening or shortening is required. That's fine with us, as we didn't have to modify the driveshaft for this installation and we have no plans on changing to another gearbox or rear axle. And, the thin shaft likely means less weight.
Highlights of installing these parts are shown in the photos, but there are a few important points to emphasize here. If nothing else, all this work is under the car, so it's a great time to have access to a lift. For us, that meant once again bumming from Brothers Performance Warehouse. Manager Tim Gilpin is a NASA American Iron competitor, so he has a soft spot for our similarly equipped open-tracker.
Nice as it is to have a lift, all this work can be done in a home garage. The job is little more than removing and replacing a stock transmission as the Tremec bolted right to our 4.6, and, thanks to good quality control, Brothers says it doesn't even bother measuring the flatness or alignment of the Tremec bellhousing and gearbox anymore as they're always fine.
The TKO II uses a different spline count on its output shaft than the T45, so the stock driveshaft won't work. However, the TKO II comes with a new driveshaft yoke-all you need to do is swap yokes. The TKO is also 17 mm (5/8 inch, approximately) shorter than the T45 (it is the old 5.0 Mustang T5 length), so the driveshaft doesn't fully engage the transmission output shaft splines. You can certainly drive around on what engagement you do get, but all-out driving isn't recommended. The cure is a simple 5/8-inch driveshaft spacer. We got ours from D&D Performance for $59.
There are slight variations in the rear transmission mount between T45 and TKO II boxes. Sometimes the transmission crossmember requires a bit of fabrication to work, other times no. Ours was easy enough. The stock rear transmission mount would not fit, but an Energy Suspension urethane mount did, so we used it. The slots in the transmission crossmember also required light filing to extend the slots slightly.
If you have a round file, you're set on that bit. You'll also want a whiz wheel or other high-speed cutter to enlarge a notch on the TKO II tailshaft when fitting the rear mount.
The biggest surprise for us, however, was when we weighed everything. The supposedly heavy TKO II turned out barely fleshier than the stock T45. Therefore, not counting the clutch, the system weights (flywheel, transmission, and driveshaft) were 156.5 pounds for the stock gear and 143.5 pounds for the aluminum intensive driveline. That's a 13-pound weight savings, not to men-tion the benefits of the reduced rotating mass-so much for the Tremec's supposed weight penalty. It's only 5 pounds more than the T45 we pulled out, and that was made up by the lighter flywheel and driveshaft.
Another surprise was we already had a Centerforce Dual Friction clutch in the car (this car came to us with a few unknown modifications). We had figured we were driving around in a stocker, so that goes to show you how light the pedal pressure is with these designs. This also means we didn't have a stock clutch handy to weigh, but you can figure the LMC Centerforce saves several pounds of rotating mass.
By our deadline we'd driven the new flywheel, clutch, trans, and driveshaft installation 50 miles home from Brothers' shop and little more. Still, the major personality traits of the combination are clear. Clutch effort is the same as before-essentially stock. Shift effort is heavier and notchier than with a T45, as expected, but it's well within acceptable limits (Tremec has been improving these gearboxes-years ago they were a bear to shift, but not now). The gearbox also whines at high pitch during deceleration, but we were cautioned by reliable sources that they all do this at first and the noise will soon go away. So too will some of the notchiness, as this is a brand-new gearbox, after all.
There is a vibration that's quite apparent when taking off in First gear and less noticeable when accelerating after each shift. We believe it is due to the urethane rear transmission mount. Pinion angle could also be an issue, but the vibration is transitory, which points us at the firm transmission mount. The stock mount is a mushy design, so no doubt the hard mount transmits more shake to the cockpit.
The lighter driveline requires a touch more technique to launch and shift. It's no big deal, but you find yourself paying a bit more attention to matching rpm and clutch release speed when shifting. Launching from a dead stop is more on-off than with the heavy stock flywheel, so a hair more clutch slip is required.
The engine may free-rev more quickly-we haven't discerned any freer acceleration, however. The stopwatch will be where these improvements show up.
The biggest difference is Fifth gear. If the jump from Third to Fourth gear in a stock T45 transmission is one step, then the spacing from Fourth to Fifth is one-and-a-half steps. With the road-race Tremec, the Three-Four gap is the same, but the Four-Five distance is just one step wide-not one and a half. With 3.73 rear gears this means you can get in Fifth gear fairly quickly-such as around town-and the car definitely has more urge in Fifth on the freeway than before. It's simply the next gear up, and we're eager to give it a go on the faster tracks where it ought to pull like gangbusters from 100 mph or so on up.
On the freeway, however, you must really want to hustle if you are to pace fast traffic. There are long stretches of 80-plus-mph freeway traffic around the country these days, and hitting 80 mph requires 3,305 rpm with our 3.73 gears and 24.875-inch-tall 275/40-17 Nitto 555 R tires. That's a fairly noisy, fuel-consuming way to cross country, but as the only time we go long distances in this car is for track driving, it's a workable trade-off. You may think otherwise should you daily drive your track car. It's one of those judgment calls each owner must make alone.
To give a better idea of the rpm and speeds involved, we developed the following chart for rpm at road and track speeds in Fifth gear. We used our 3.73 final gearing and 24.875-inch-tall tires in our figuring. All calculations are for Fifth gear. Note that the stock T45 uses a 0.68:1 Fifth gear, and that this taller ratio would be Sixth gear in a T56.
|(mph)||(0.82 Fifth)||(0.68 Fifth)|
Obviously the 0.82 Fifth gear takes more rpm, especially as road speed increases. For example, the T45 combination turns about 2,741 rpm at 80 mph, while the TKO II is delivering only 66 mph at the same rpm. That's 14 mph you'll really notice.
We also have to think the TKO's shorter Fifth gear is going to make a big difference on the track. Look how lazily the T45 turns the engine at 100 mph-just 3,400 rpm. With a pushrod 5.0 and a blower, this is just starting to make boost, but with a naturally aspirated, higher-revving modular GT engine such as ours, that's still a ways from where the power picks up. Having an extra 705 rpm thanks to the 0.82 Fifth gear will really come in handy. We're looking forward to trying it at the track.
|T45||115.0 lbs||As removed, with mounts, oil, bellhousing|
|T-3550 TKO II||100.0 lbs||Out of the box, no fluid, no bellhousing|
|T-3550 TKO II||113.5 lbs||As above, w/bellhousing, add 0.5 lb for bolts|
|T-3550 TKO II||120.0 lbs est.||Estimate w/bellhousing, fluid, mount, bolts|
|Driveshaft, Stock Steel||20.5 lbs||With yoke|
|Driveshaft, Aluminum||13.0 lbs||With yoke|
|Flywheel, Iron||21.0 lbs|
|Flywheel, Aluminum||12.5 lbs|
|LMC Pressure Plate||14.5 lbs|
|LMC Clutch Disc||4.5 lbs|
|Clutch Fork Weight||2.0 lbs||Removed to clear Tremec bellhousing|
TKO II vs. T56
It's inevitable when considering a higher-performance transmission such as the TKO II to look even further ahead and eye the six-speed T56 box, also built by Tremec. As with anything, there are pros and cons to the six-speed. The obvious plus is the Sixth gear. Tremec uses the same closely spaced first four gears in the TKO II and T56, with Fourth the standard 1.00:1 ratio, and then in the T56 two Overdrives as Fifth and Sixth gears. These two ratios are the TKO II's 0.82 Fifth gear and the T45's Fifth gear 0.68 ratio as the T56's Sixth gear. Thus, this gives you both the racy 0.82 overdrive in Fifth and the relaxed cruising of a 0.68 Sixth gear in the T56. That makes Sixth in the T56 the same as Fifth in a Mustang's stock T5 or T45, so freeway rpm will not change in case you were searching for a super-overdrive or something.
The downsides include the inevitable money-the T56 is another $400 over a regular 3550 and roughly double what a T-5 costs-and the T56 is another 15-29 pounds heavier than a 3550 or TKO II depending on the exact model. And get this-the T56 has 25 lb-ft less torque capacity depending on the model (Dodge Vipers use a T56, so you know there are versions that'll take the heat). So, a 3550 looks good, until you really want both overdrive top gears.