Maximum Overdrive
Jim Veale, Wayne NJ: I am a longtime subscriber and really appreciate the work you do at the magazine. I had a question regarding the Gear Vendors overdrive. I have a '67 Nova that has a 434ci small-block and a Powerglide. The car runs 10-flat at 133 mph as a stripped-down bracket car with a 5.00 rear gear. It seems the easiest way to convert the car to street use is with a Gear Vendors overdrive unit. The company claims the car will be at 2,800 rpm at 65 mph with 30-inch-tall tires.
I have not seen anything in your magazine about this product or any actual testing. Do you have any insight into how reliable the product is in a drag-racing application? I am also curious about the effects of driving a car at low-rpm highway speeds below the converter lockup speed. For example, I have a 4,500-stall, and I don't know the effect of driving it around at say 3,000 rpm. Thanks for years of informative reading.
Jeff Smith: The Gear Vendors overdrive is an excellent unit. The add-on overdrive employs a planetary unit that provides 22 percent overdrive at the push of a button. The unit is incredibly strong using the planetary design, which is most often used in automatic transmissions. Small gears spin around a central or sun gear and are in constant mesh, unlike a manual transmission, where two gears have to be engaged. This is what makes the Gear Vendors unit strong.
The unit is designed into a new extension housing that mates to the output shaft of the transmission. This adds both length and weight to the transmission, sometimes requiring a relocated crossmember. Obviously, this will also require a shorter driveshaft. It's also important to consider floor clearance. Your early Nova chassis does not offer a very large driveshaft tunnel, and according to the folks at National Nostalgic Nova, the Gear Vendors case size will require some floorpan surgery. The best plan is to call the company and ask for its experience.
As for the overdrive, with a 0.78 ratio, it offers a 22 percent decrease in engine rpm for the same vehicle speed. In your car, with a 5.00 gear, shifting into Overdrive will effectively reduce that ratio to a 3.90 gear ratio. Your 30-inch tires are pretty tall, but assuming that tire height, your freeway cruising speed in Overdrive at 2,800 rpm would be 64 mph, so the company's estimate is pretty close. What this does not take into account is converter slippage. Our calculation is based on a locked-up converter. Typically, a loose-performance converter slips about 200 rpm at highway speeds, mainly because highway cruising requires very little power, which minimizes the slippage.
However, you bring up an excellent point concerning your high-stall-speed converter. The 4,500-rpm spec you mentioned is the stall speed for your engine at maximum load at very close to zero vehicle speed. As the speed increases, the converter approaches the point of lockup, which is really the minimum amount of slippage that will occur at wide-open throttle (WOT). While a 4,500-rpm converter is certainly loose, at cruise rpm with very little torque being transferred through the converter at around 3,000 rpm, the slippage is minimal. This may still be around 5 percent, which equals 150 rpm at 3,000 rpm engine speed. It's why the OEMs long ago went to lockup converters to eliminate this efficiency loss to help gas mileage. Slippage also creates heat in the converter, which you will need to address by placing a sufficient-sized cooler in a high-pressure area, such as right ahead of the radiator. The other thing is that your driveshaft and ring-and-pinion are buzzing 22 percent faster. For short trips, that's not a big concern, but it's worth knowing.