The 4L85-E SuperMatic is a 4L85-E was based on the 4L80-E, but it enjoys internal modifica
Lower engine rpm at highway speeds produces gains in fuel economy beyond the obvious slower turning of the crankshaft and less firing events. In any engine, there are three areas of loss that don't allow the complete transfer of energy from the fuel to the crankshaft. (Energy is measured in British thermal units [BTU] and is considered a caloric value.) The internal combustion engine suffers from thermal, frictional, and pumping losses: the heat from combustion that goes into the exhaust port, the internal friction to run the engine and move the parts, and the work required to pump the air in and out of the cylinder bores.
At minimal throttle-plate opening (either with a carburetor or a throttle body), the pumping losses are increased, since the engine is trying to breathe through a severe restriction. To put this in practical terms, think of how difficult it is to breathe when you have a stuffy nose. The throttle restriction makes the engine work harder to try and fill the cylinders, which is defined as volumetric efficiency (VE).
When an engine is running through an overdrive gear set, even though the crankshaft speed is lower, the throttle plate is open more. This is due to the higher load than at the same road speed when in a direct-drive gear. The additional throttle-plate opening reduces the pumping losses and allows more of the gasoline's chemical energy to be converted to work, thus providing better fuel economy.
For the 4L80-E, 4L85-E, and SuperMatic, electronic solenoids control fluid flow instead of
Old Meets New
A while back, HPP documented the buildup of an SD-455 in a '74 Trans-Am owned by Melvin Benzaquen. The Pontiac mill was tweaked to the tune of over 500 hp, and was run with both a four-barrel carburetor and the Mass-Flo EFI system. The engine provided stunning performance and driveability, even though the rearend gearing wasn't dragstrip-friendly and the torque converter was on the tight side for the cammed mill. Since this Pontiac already was a melding of old and new engine technology, when the opportunity arose to upgrade the transmission to a modern overdrive, it was taken.
GM Performance Parts (www.gmperformanceparts.com) offers a complete THM 4L85-E SuperMatic unit with all the controls that allow installation in an older Pontiac, gaining the referenced attributes in driveability and performance. The installation of this transmission, along with the tuning of the electronic controller, will be covered in an upcoming issue of HPP. The task at hand now is to provide an overview of the THM 4L85-E SuperMatic and a brief history of GM's other automatic overdrive transmissions.
One of the upgraded components that make the 4L85-E and SuperMatic stronger is the five-pi
A Numbers Game
When GM introduced its first overdrive transmission for '81 models, it was identified as the THM 200-4R and had gear ratios of 2.74 First, 1.57 Second, 1.00 Third, and 0.067 Fourth. This unit was basically a modified TMH 200 three-speed, which was one of the first lightweight designs produced in response to the downsized vehicle models and government-mandated fuel-economy standards of the late '70s.
The THM 200-4R was created by adding a third planetary unit (gear set) in the front of the geartrain. This created the fourth (overdrive) ratio. It was a fully automatic transmission with a three-element lock-up torque converter and a compound planetary unit (due to the added third member). Five multiple-disc clutches, two roller clutches, and a band provided the friction elements required to obtain the desired function of the compound planetary gear set and the overdrive unit. This trans was used through the '90 model year in various Big-Cars.