Chevy Gear-Splitting Unit - Rev It Up And Go

Underdrive And Overdrive Your Factory Transmission With Some Help From Gear Vendors

Mike Harrington Jun 1, 2008 0 Comment(s)
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This isn't the first time Super Chevy magazine has examined and installed a Gear Vendors under/overdrive gear-splitting unit. However, it's been a few years since the last install article. With freshly cut government checks in many hands and the price of oil more than $100 a barrel, this might be a way to upgrade our Bow Ties with bolt-on economy and performance.

What exactly is a Gear Vendors unit, and how does it work? No doubt a majority of Chevy owners are already equipped with factory transmissions such as the Powerglide, TH350, TH400, 700-R4, 4L60E, M21, M22, and T-10. Simply put, a Gear Vendors unit replaces the existing tailshaft housing on any of these transmissions and adds an extra Overdrive gear of 0.78:1 and the ability to split the already existing gears in the transmission. A three-speed automatic transmission will now become a six-speed, a four-speed automatic will become an eight-speed, and the same applies to the GM manual transmissions as well.

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A worm's eye view of a '56 Bel Air prior to the assembly of the Gear Vendors unit.

There's an added fuel economy gain with one of these units (if you can keep the throttle off the floor). For example, a 3.73 differential at highway cruising speed will experience a 22 percent rpm drop and feel more like a 2.91. Virtually all three-speed automatics (like the Turbo 350 that's in this '56 Chevy) have the same ratios within a few hundredths of each other: 2.5 first, 1.5 second, 1.0 high. This means a transmission takes 2.5 engine revs to turn the driveshaft once in First gear, 1.5 in Second, and is direct drive 1:1 in high gear.

Gears are multipliers of torque, and since the math of horsepower is torque x rpm / 5,252 = hp, putting a 2.0 gear between First and Second is going to keep the torque multiplication and engine rpm both high, which means more horsepower getting to the rear wheels. If we can then put a 1.2 gear between Second and Third and an Overdrive gear beyond Third, we now have a close-ratio six-speed overdrive trans with four underdrive power gears, a direct-drive high gear, and an Overdrive cruise gear.

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First things first: Remove the old driveshaft. The driveshaft will eventually have to be shortened approximately 13 inches on this '56 due to the length of the Gear Vendors unit.

Gear Vendors' under/overdrive unit can be used to double the underdrive performance ratios and provide overdrive cruising economies. Take a look at the tables on the next page for further explanation of transmission and differential ratios.

By now many may be thinking, "Sure, it's nice, but if I want an overdrive, I'll just find a 700-R4 at the local Super Chevy swap meet, thank you." True, a 700-R4 swapped into a vehicle would be good, but it really comes down to how you drive your vehicle. If your ride idles off the trailer and into a parking spot at the local show, then idles back onto the trailer a 10th of a mile away, installing a Gear Vendors unit on your vehicle will serve no benefit other than bragging rights. If you drive your vehicle like the 30 True Street participants at the recent Baton Rouge Super Chevy show, a Gear Vendors unit could greatly benefit your highway cruise and quarter-mile times.

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The next step is to remove the tailshaft housing.

Put the dash switch in manual mode, put the shifter in First, hit the gas, and the overdrive will shift while accelerating and First gear begins to wind out. The vehicle accelerates through First and First-over and is now 28 percent farther down the track before you have to move the shifter up to Second. When the shifter is moved to Second, you can leave the overdrive engaged or hit the button right when Second grabs and split the gear just the same way First gear was split. All this keeps the engine revving in the sweet rpm range while torque and horsepower are dumped out to the rear tires.

With Auto Launch and M21 or 22, the vehicle will leave the line in First and shift automatically without a clutch to First-over, which is only 4/100 different than if you shifted to Second. You can further exploit the timeslip by shifting to Third and then clutchless to Third-over, which is only 1/100 different than Fourth, and do the entire dragstrip run with just one clutch depression.

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