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By: Brad Ocock
Photography by: Brad Ocock
Reprint from Super Rod Magazine, December 2001
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You have only just begun to hear us expound on
the virtues of the Gear Vendors Under/Overdrive unit. No matter how you
slice it, this is a unit that can cure mechanical-ratio, tire-sizing and
displacement woes. We have numerous stories planned to help you
understand how the addition of this assembly can benefit just about any
application. Lowering engine rpm at highway cruising speeds via the use of an "overdrive" mechanism that is attached to the transmission is not new: Chrysler offered it in its Airflow in 1934. The benefits of an overdrive transmission are threefold: wear and tear is reduced, fuel economy is improved, and speed ratio between engine rpm and tire rotation is decreased. And, in the case of the GV unit, there's an added benefit: you can use standard or overdrive ratios in all gears to advantage the multiplication factor of the driven wheels. The principle behind an overdrive is essentially this: The traditional final drive in a non-overdriven transmission is 1:1, or "straight through," meaning that the output shaft of the transmission rotates once every time the engine's crankshaft rotates. An overdrive transmission increases the output shaft's speed in relation to the crank speed by adding an additional gear ratio after the final gear. For the sake of keeping it |
1 The Gear Vendors gear splitter, or Under/Overdrive unit kit, is complete. It includes your choice of switches to manually activate the unit, as well as the Autodrive unit-an electronic e-prom "brain"-to automatically engage the overdrive gear. simple, we'll say our example overdrive unit has a ratio of 0.75:1, meaning that for every 0.75 rotation of the engine's crankshaft, the output shaft rotates one full revolution. The result is that the overdrive in the transmission enables engine rpm to be 25 percent lower (at the same mph) compared to 1:1 as with a non-OD transmission. Bump up the rpm to match the 1:1 final drive, and the car with the overdrive is going to have a faster rpm, or quicker acceleration, everything else being equal. This is the mechanical advantage. |
Page Two
Getting an automatic overdrive transmission for
your super rod is obviously no problem. But there are a number of
situations where buying a factory overdrive may not be practical. For
instance, say you spent a couple of grand having a Turbo 400 built just
a year or two ago. If you're a Ford or Mopar fan, then you know that
their modern overdriven offerings have small block-only bell housings. An
adapter plate or an aftermarket SFI-style bell housing can be added to
make the combo work with a big block, but that can be a can or worms. And
then there's the question of durability behind large amounts of power, and
whether the modern offerings are able to hold up. We're not going to offer
our opinion on that matter right now, but late-model transmission failures
have become commonplace. There's also the question of the off-brands. What if you're running a 727 TorqueFlite for an AMC? How about the Buick/Olds/Pontiac versions of the Turbo 400? Don't forget the real bastard children, such as the 401/425 Buick Nailhead's ST400 (a Turbo 400 with a unique bell-housing mounting pattern). For these instances, an aftermarket overdrive unit is the answer. And we know of no better product than the tried and proven Gear Vendors Under/Overdrive. The GV unit is marketed as a gear splitter, meaning quite simply, that it can be engaged anywhere throughout the shifting pattern, not just as an auxiliary gear after 1:1 final drive. In essence, you can turn your three- speed auto into a six-speed auto, because you are able to engage it between the 1-2 shift, the 2-3 shift, and overdrive. Once you are good at it, you can keep the engine in its prime rpm band by shifting between gears rather than at the "end" of the gear. For instance, a typical transmission has about a 40 percent difference in gearing between first and second gear, which causes the engine rpm to drop substantially. With the Gear Vendors unit installed, you have a gear between first and second that is only 22 percent different than first and 24 percent different than second, so you can keep the engine in its powerband rather than having to climb back up to it. It's the same with third. The GV unit is a separate planetary-style gear with a final-drive ratio of 0.78:1 (see "Splitting Gears," SUPER ROD, Sept. '01, p 156). It's totally encased in its own housing and has its own fluid supply and its own actuation circuit to engage the unit. You install the unit by removing the tailshaft of the transmission and installing an adapter in its place, to which the GV planetary gear attaches. The output shaft of the transmission become the output shaft of the GV unit, coupling to the front driveshaft yoke. The unit is electrically controlled, actuated manually with either a foot switch in the floor (like a headlight-dimmer switch) or a button on a T-handle, or automatically through the system's Autodrive control box, which engages the |
overdrive at a pre-set mph (about 45), with the car's original speedo cable
running through an adapter (called an impulse generator) and driven off the
back of the GV unit. You can run it manually or automatically, it's your
choice, and you can switch back and forth between them two, depending on
the driving situation. GV Under/Overdrive units are mostly thought of as being installed behind two- and three- speed automatic transmissions. However, they are also available for most factory overdrive automatics from Ford, Chrysler and GM manual transmission, and even special applications such as enclosed (torque tube) drivelines. Additionally, out of the box, a GV unit is capable of handling up to 1,200 hp, and can be beefed up at the factory to handle up to 2,000 hp. The GV overdrive unit is touted as being an easy bolt-in, Saturday project. The only problem might be getting it to clear the stock floorpan in certain installations. The GV unit isn't particularly big, but when compared to the tailshaft of a transmission, it is generally larger in diameter. As a result, the case of the GV unit can interfere with the transmission tunnel. Conveniently, the GV unit typically sits under or right in front of the front seat of most vehicles, so any floorpan modification is both easy and easily hidden. You can get around having to clearance the trans tunnel by building a lower transmission crossmember, but the U-joint angle must be double-checked. Time Machines in Hudson, Florida was installing a GV unit on a Hemi-powered Dodge Challenger, backed by a 727 TorqueFlite. Connecting the unit to the trans and hooking up the controls took a day. While the underside of the Challenger is as detailed as the top, the crew ran into the clearance issues discussed, but solved the problem with a lowered trans mount and 5-pound persuader. For many installations, some kind of minor floorpan surgery may be required, but if you know that going in, there's no reason installing a Gear Vendors unit should take more than a weekend. SR
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2 We followed along as Time Machines in Hudson,
Florida, installed a Gear Vendors unit behind a 727 TorqueFlite. This installation
is fairly representative of what you'll have to do to install one behind most
other transmissions.
3a-3b The installation begins by removing the
trans crossmember and draining the fluid. It should go without saying that the
trans needs to be supported, but we'll say it anyway.
4a-4d On the 727, the tailshaft is removed by
unbolting it from the transmission's main case and disengaging a snap ring from
the support bearing inside the housing. Other transmissions will vary, so follow
a factory shop manual's procedure for removing the tailshaft. After the snap ring
was overcome, the tailshaft housing slid right off.
5 The tailshaft adapter for the GV unit is a little
longer than the stock tailshaft housing. They're generally the same length for
many transmissions. The Mopar's tailshaft housing is long, so the GV unit must
be long. The stock tailshaft for a Turbo 400 or a Ford C6 is much shorter, so
the GV adapter is sized accordingly. Notice the heavy strengthening ribs cast
into the GV tail housing.
6a-6d Before installing the GV tail housing, the
parking pawl assembly must be transferred from the stock housing to the GV
unit. The actual parking pawl is replaced with a much better piece (left). Removing
the spring and remaining pieces from the stock tailshaft housing and installing
them in the new unit is easier with an extra set of hands. But other than
getting everything to fit is relatively straightforward.
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7a-7b After the parking pawl is installed, the
GV tailshaft slides into place. To get it all the way on, two things have
to align into place. The snap ring on the support bearing and the splines
at the end of the tailshaft must mesh into the coupler in the GV housing.
This coupler is splined to fit over the tailshaft just like the original
output yoke. The overdrive planetary then connects to the other side of the
coupler.
8a-8c A quick tip to get the splines lined up:
The input shaft of the GV planetary unit is removable, so pull it out and
use it like a clutch alignment tool to line up the splines. Once aligned, the
tail housing slips right into place and can be bolted to the transmission case.
9 Here is the GV planetary installed on the
back of the 727. It does add considerable length to the back of a
transmission. Obviously, the driveshaft needs to be shortened a corresponding
amount. The boss for the crossmember mount is in the same place, but it's not
simply a matter of re-installing it, as you'll see in a moment.
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Page Five
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10 The body of the Gear Vendors planetary case
isn't particularly big. However, the spot in the trans tunnel it occupies was
designed for the driveshaft. As a result, some interference may occur. In
this case, it was tighter than some. Sometimes you're able to get away with
a lower trans crossmember, which will move the end of the housing away from
the floor. However, this can lead to driveshaft-angle problems. A less
sophisticated solution is to break out the air hammer or a 5-pount mall and
star whacking away at the floor for clearance. At least in most cars, the
planetary sits about under the front seats, so cutting the floorpan or opening
up this area will almost always go undetected. Opening up the trans tunnel is
by far the best solution, as it gives you the clearance and maintains
factory-spec driveshaft angles.
11 The GV unit has its own fluid supply, and
is completely divorced the transmission fluid. It calls for either
GM PN 12346190 or Mopar PN 04874459 synthetic gear oil, which is not
supplied with the kit. The instructions say that petroleum-based gear oil
can also be used, but results in a firmer shift. The GV planetary is just
like the tailshaft of any transmission-have the yoke in place before
filling it up. Duh!
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Page Six
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12a-12b After the hardware is installed, hook
up the electrical side of the GV unit. The gear splitter is activated
electronically, and can be done automatically through the Autodrive control
unit (which engages the planetary at about 45 mph), or manually via a floor-mounted
dimmer switch or a Hurst T-handle button, both supplied in the kit.
13 Follow the instructions for the wiring;
they're excellent. Off the back of the GV unit is the speedo drive, just like
stock, as well as the solenoid that engages the unit. You have to get these to
the front of the car, but the speedo cable doesn't make a tight bend, and
space is limited in the trans tunnel. A hole was cut to pass the cable into
the passenger compartment. From there, it hooks into the impulse generator
(which sends the signal to the Autodrive box), and the stock speedo cable
plugs into the other side of the impulse generator. These will be hidden
under the carpet, and when next to the console in this E-Body, you'll never
know they are there.
14 Mounting and connecting the Autodrive box
and the dash-mounted control switch is typical of any under-dash electronic
situation. The instructions provide an excellent wiring diagram. The
dash-mounted auto-switch was mounted under the dash. Time Machines went
with a T-handle actuation button on top of the gearshift lever. All in all,
it's a clean installation, and one most anybody can accomplish with a
minimum of hassle. Just expect clearance issues with the floor, and the
installation will go just fine.
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