If you already have or are planning to install an overdrive transmission, there are a few things you should know. For instance, while better gas mileage is a fine idea, it's possible to slow the engine to the point that vibrations occur at highway speeds. Read on to see why this happens and what you can do about it.
OEM overdrive transmissions typically slow engine rpms by 30 percent. If you can cruise at 2,000 rpm in high gear with your Turbo 350, you will be cruising at 1,400 with a 700-R4 or other GM overdrive transmission. Such low engine rpms open a can of worms for the car because of the increased torque required to maintain the same road speed. On top of this, the torque converter locks up at some number of rpm to effectively connect the crankshaft to the ground with no slippage anywhere in the system. The resulting driveshaft vibration will be rhythmic or pulsating and will cause mirrors, gearshift levers, and miscellaneous car parts to vibrate at different times.
The vibration happens because the power coming out of the engine is not smooth. Every time a cylinder fires, the crankshaft is accelerated. The accelerations are called torsionals and there are four of them per crank revolution in a V-8. As engine rpm declines and the required torque goes up, so does the strength of the torsional spikes. Overdrives reduce the four crankshaft pulses to 2.8 pulses per revolution of the transmission output shaft. Downstream, the driveshaft is already changing speed twice per revolution because of U-joint angles, making it very sensitive to these lower frequency torsional disturbances.
For street rods the magic number for smooth running in overdrive seems to be a minimum of 1,800 engine rpm with 2,100-2,300 rpm at 55-65 mph a good goal. The longer the converter stays locked up below 1,800 rpm, the greater the torque-induced vibrations will be. Think of it as pulling up to a stop sign with a manual transmission car while forgetting to put the clutch down and you'll get the picture.
It used to be that a sure fire way to cure torsional problems was to re-gear the rearend. In some cases, just changing the tire size may have been enough. The computer-controlled engine has put an end to such straightforward solutions, but final drive ratio is still a governing figure. Street and Performance has published a dandy chart showing the trade offs between gear ratio and tire size. This chart is based on the formula in Figure 2 which you can use to figure your own car's situation and the effect of different changes. This formula ought to be used before you shop for a rearend or decide on tires and wheels when building a new car.
If your car is carbureted, your 700-R4 won't lock up at all. With slippage in the converter you will not experience torsional disturbance, but you will wear out the transmission due to reduced lubrication while running in Fourth gear. For the carbureted set there are many kits available to get the transmission talking to the converter. While these kits differ in design and function, all will enable you to adjust lockup points.
A problem arises with the injected engine because it is told what to do by a computer. This computer left the factory hooked up to a whole car. The computer knew where it was and what it had for neighbors in the transmission, rearend, and tires, among other parts. When you put one of these engines in your street rod you must tell it about its new neighbors or it will not associate with them gracefully.
Say, for example, you buy an engine with a 4L60-E out of a wreck. These two are still together, so you suppose you have done the right thing. But then, the car they came from may have had a 2:73 rearend in it and you want a 3:42 rearend. This change will move the shift points and the converter lockup down by 20 percent in mph. The result will be that the computer, which was avoiding torsional problems by a narrow margin, will both shift and lockup early. More torque will be required to move the slower car resulting in stronger torsional spikes that will cause the driveshaft to vibrate.
Fortunately, there is a clear, simple, and reasonably priced way to correct this kind of computer generated trouble. You just have to tell the computer about its new circumstances. GM offers chips in many combinations of tire size and rearend ratio or you can choose to go with a modified prom. ALL GM computers can be accessed and modified to operate in perfect harmony with the other components. Some such components may need a little help. For instance, a 200-4R, a 700-R4, or a 4L60 (no "E"), may require re-calibration to correct shift points and may also require an external converter lockup kit. The point is that with commonly available parts your car can be made to operate smoothly.
Torsionally induced driveshaft vibration has been a nightmare in overdrive vehicles for years. These vibrations originate outside the driveshaft and are independent of its quality. Knowing their source and having a means to modify that source will save untold hours of frustration and many uselessly spent dollars.