Drivetrain Power Loss - The Brutal Truth

Where's It Going?The basic fact is that any drivetrain uses power to transmit power between the flywheel and the rear tires. The biggest culprit in this power-loss chain is the automatic transmission. An automatic will consume power in two ways-slippage in the torque converter and line pressure. Slippage is an inherent function of any torque converter (excluding lockup converters) with a slippage factor of between 5 and 8 percent. This does not mean you lose 5 percent power, but anytime that converter gets hot, that is flywheel power that is lost to slippage. In addition, all automatics must drive a pump to create the hydraulic pressure required to apply the clutches and bands. This eats power. Heavier transmissions, like the GM TH400 or Ford C-6, for example, also require additional power to spin those heavy clutch drums. The quicker you accelerate, the more power these units eat.

But let's not forget the manual transmission or the rearend. If a gearbox gets hot, that's energy that's lost before reaching the rear tires. Gear multiplication (meshing of two gears) will always require power to complete because of the friction it generates. High gear in a four-speed, for example, is the most efficient gear since it merely connects the input and output shafts together. It is this 1:1 relationship that eliminates the slippage of a torque converter and makes a manual trans more efficient than an automatic.

Rear axles also eat power because of the sliding action of the hypoid gear design. The Ford 9-inch consumes more power than a GM 12-bolt because the 9-inch's pinion is placed lower on the ring gear for better gear contact. This makes it stronger than the 12-bolt, but it also requires more power to turn the gears because of increased gear overlap.

These are the main sources of power loss. The more attention you pay to these details, the more power will end up twisting the rear tires.