Let's Split

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  • Control cable that engages the gear splitter. - 2
  • The Gear Vendors Under/Overdrive is fairly compact, and will fit underneath just about any muscle car. - 3
  • For a manual transmission, the switch for activating the unit mounts on the gearshift lever. For automatics, a foot-operated switch is used on the left side of the driver’s footwell. - 4
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by Steve Temple  More from Author

Modernizing a Muscle Car Transmission With a Gear Splitter

Just everybody who’s ever manhandled a muscle car has experienced the following scenario: You settle into a nice cruise down the highway, but the rearend ratio, while low enough for a good launch off the line, keeps that V8 buzzing, even in top gear. At first the exhaust note sounds kinda cool, but after a while, the drone can drive you batty (not to mention run up your gas bill).

Obviously an overdrive gear would come in handy, but we’re not going to recommend a late-model, 5- or 6-speed transmission. Most of the older musclecar trannies are basically bulletproof, and you wouldn’t want to lose their feel and look, which adds to the intrinsic value of the car. Why replace the good four gears you’ve already got for a newer trans that merely adds one gear, but doesn’t shift as well, nor handle as much power and abuse?

Instead, check out this’65 GTO, tastefully “restified” by Larry Weiner of Performance West Group. Among many other upgrades, he modified the Muncie M20 with a Gear Vendors Under/Overdrive. Also called a gear splitter, the name comes from its ability to provide an increased number of underdrive gear ratios (those that lower the overall gear ratios) and overdrive gear ratios (those that make the ratios higher). Putting it more simply, this particular unit adds four more gear selections, divvying up the powerband more precisely.

Gear Vendors’ modern yet essentially invisible upgrade enables the GTO to have lightning-quick acceleration. Couple that with an overdrive, and you get great long-legged performance on the highway as well.

Getting specific with the numbers, the factory ratios on the M20 are 2.52, 1.88, 1.46, and 1.00. The gear splitter adds overdrive that is 28.6 percent faster than 1.00. What that means on the road is that now the car is able to go 71 mph at the same rpm as at 55 mph without overdrive.

How about under hard acceleration? That’s where the clutchless shift function comes in real handy. With a Gear Vendors behind the M20 you can hit the button on the Hurst shifter before you ever leave the line (or traffic light). Then drop the clutch in 1st and the car will rev through 1st gear and then automatically (or you can hit it manually) shift to 1st-over, all without touching the clutch pedal.

The 2.52 ratio of 1st then becomes 2.52x.78 or 1.96. Since second is a 1.88 gear, this is only .08 different (imperceptible) than your true 2nd but gives you a much faster, tire-barking shift. This full-throttle shift ability is what really separates a Gear Vendors equipped car from any other.

Typically when you want to show off or are trying to lay down a good run at the strip, the 1-2 shift is where the automatic guys blow on by. That’s because by the time your tires hook up and you get the car driving down the lane, you immediately have to clutch and that gets the chassis all unloaded and squirrelly again. With the Gear Vendors unit, though, you are 28.6 percent farther down the track and all the way through 2.52 and 1.96 ratios before you have to clutch.

At this point you have two choices. You can shift to 2nd gear in the main box (which will be just like you shifted to 3rd, since 1.88 x .778 is 1.46 exactly). Most street guys will do exactly that (grabbing the clutchless OD gear whenever they accelerate in any gear and then just leaving it on a move-up through the gearbox).

Or the strip guys will actually take another clutchless advantage by flipping the side-cover shift lever (the little lever your linkage fits in to) and flip it upside down so they can grab 3rd gear in the 4th gear gate. This lets them go 1st then clutchless to 1st over, then pull straight back while punching out the Gear Vendors button into third and then clutchless to 3rd over. Sound a bit confusing? Putting it simpler, that means they can do the entire run with just one clutch depression and two clutchless shifts. Try that against any 5- or 6-speed and you can see why these Gear Vendors units are popular.

That 3rd over gear with the M20 solves another problem. It brings the same close-ratio shift to the M20 that the M21 and M22 are known for. M21 and M22’s have a closer ratio on the 3-4 shift than does the M20. When you use a Gear Vendors on an M21/22, 3rd over is exactly 1.00 which is the ratio of 4th. On the M20, because of its lower 1st gear, Muncie’s engineers had to put more spread somewhere, and that was between the 3-4. All this means you have the taken the M20’s only fault (a wide 3-4) and now fixed it so that all your quarter-mile and street acceleration will benefit from a closer and clutchless ratio spread.

As for other types of muscle-car trannies, Gear Vendors makes kits to fit its planetary overdrive on a number of other makes as well. In addition to bolting right up to the Muncie M20, M21 and M22, it also goes on the BorgWarner T10, Mopar 833, and Ford Top Loader, along with a number of racing transmissions such as the Gforce GF5R, Jerico, Tex Racing, and Lenco units.

The Gear Vendors Under/Overdrives are found in a number of applications besides muscle cars, including certain types of passenger cars, diesel pickups, delivery trucks, RVs, and even modern cars such as the Callaway Corvette. The unit has been in numerous record-setting competition vehicles as well, from land speed to drag racers. So the technology is a proven and reliable approach to improving performance.

The unit consists basically of a self-contained auxiliary transmission with planetary gears like those found in modern automatic transmissions. It fits in line between the transmission and the driveline. Since the unit has a pressurized lubrication system driven by an internal pump, it doesn’t share any of the fluids of the vehicle’s OE components.

Upon activating the unit with a switch on the shifter or dashboard, a solenoid closes a valve, changing a path of oil to hydraulically hold a clutch pack that drives the planetary gears. These gears reduce the drive ratio 22 percent (0.78:1), so you have intermediate ratios to maintain engine rpm at the optimum level.

Installation of a gear splitter at Gear Vendors typically takes about a half-day or so, and is fairly simple in most applications. You first bolt on the Gear Vendors replacement housing which has an eight-hole pattern to receive the overdrive. Then tighten the eight nuts and take the driveshaft with the new yoke (included) to the local driveshaft shop to be shortened (as per the instructions). One of the most important aspects to keep in mind is precise alignment of the driveline angles to ensure vibration-free operation. Plug in the electronics and fill the overdrive with the recommended lube, and you’re ready to roll.

The electronic control box is installed inside the cockpit, under the carpeting at the toe-board. For a manual transmission, the switch for activating the unit mounts on the gearshift lever. For automatics, a foot-operated switch is used on the left side of the driver’s footwell (like the old headlight dimmer switch).

The electronic module controls the splitter, so that it engages at the proper speed, and turns off automatically (at about 20 mph), in case you forget. The module also locks and unlocks the factory torque converter in an automatic. Overall, the gear splitter integrates seamlessly with the factory driveline, and is easy to operate when it comes time to make your move.

On this particular vehicle, a highway speeds without the gear splitter engaged, the tach shows 3500 rpm in Fourth. Engage the gear splitter, and the tach drops to 2800 rpm. That’s a big drop in revs, and the gear splitter provides better acceleration as well, but without having to replace that great vintage tranny. So what’s not to like? Then let’s get ready to split!



1717 North Magnolia Avenue,
El Cajon, CA 92020


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