A modern and efficient 4.1 liter V8, the HT4100 has been the an exciting path for Cadillac’s propulsion needs in the early ’80s. The engine got hot from a highly questionable cylinder deactivation experiment, V8-6-4. Unfortunately, much like the cylinder sets before and the Northstar after, the HT was plagued with issues that took years to resolve. The HT in its name stood for High Technology but could have stood for Halfway There. Let’s go back to the 70s and talk about cylinders.
The period leading up to the release of the HT power plant was difficult for domestic automakers. Their queues were filled with huge, inefficient vehicles made from cheap parts. They were loosely assembled by workers who could be drunk depending on the day of the week. American consumers were becoming wiser with the arrival of efficient, higher-quality Japanese cars, and the government was stirring the pot with fuel regulations.
These regulations were the result of the 1973 Arab Oil Embargo, a crisis that prompted Gerald Ford’s administration to take steps to improve market-wide fuel economy of cars sold in the United States. United. Cadillac would need a new V8 soon enough, but the HT was still just an idea. GM compromised and decided that Cadillac’s V8 would receive cylinder deactivation.
The idea of cylinder deactivation was not new and was first tested on multi-cylinder engines during World War II. Cadillac sought to modernize the technology, which it marketed as the V8-6-4. GM engineers worked with the Vehicle Division of Eaton Corporation to develop the electronically controlled deactivation system. What they created was a first, but is now found on every car: an engine control unit, or ECU.
Said ECU determined the power demand at any given time by running the GM program called Modulated Displacement. The system promised to switch the engine to eight, six or four cylinders appropriately and automatically. Three modes of operation were usable as the ECU fired opposing cylinder pairs. It was “like three engines in one”, according to the advertisements. Part of the elaborate ECU included an on-board diagnostic program that displayed engine trouble codes on the HVAC screen. Very advanced for its time, the diagnostic system stored all fault codes and meant that mechanics did not need to use a separate scan tool. A digital number was also displayed in the new MPG Sentinel. The Sentinel showed the number of cylinders in operation, the average or instantaneous fuel consumption and an estimate of the range of the car empty.
Able to run on as few as four cylinders, the technology promised to save a lot of fuel for Cadillac customers. Cadillac couldn’t really use smaller engines at the time, because that wasn’t acceptable to the American luxury car buyer. The feature was made standard on 1981 Cadillacs and was exclusive to the brand. Seville was excluded from the standard feature, as it used a diesel V8 as its base engine. This engine was also a mess, but the Seville diesel base model was the first American car sold as standard with a diesel engine.
The V8-6-4 was added to Cadillac’s L62 V8. This engine was new for 1980 (as the L61) and featured the always desirable throttle body fuel injection. With 6.0 liters of displacement, the 368 cubic inch engine was a bored out version of the 1968 472 V8 and was designed specifically to comply with CAFE fuel economy requirements. In its transition from L61 to L62, power went from 145 horsepower and 270 lb-ft of torque to 140 horsepower and 265 lb-ft of torque.
The new V8-6-4 was immediately hailed as a technology landmark, but the praise didn’t last long. Almost immediately, Cadillac owners began complaining that their new cars weren’t running properly. The engine issues were extensive, and the main culprit was the brand new ECU. Limited in its processing speed, programming, and overall computing power, the ECU could not effectively handle the heavy task of modulated travel. There were simply too many variables; the ECU could never keep up.
Additionally, the L62 required a different type of EGR valve than the normal-port variety. The engineers chose a positive back pressure type EGR (no electronic EGR at present). While this setup would have worked in a normal engine, with the V8-6-4 it caused the engine to ping. On four cylinders, the engine produced less exhaust than normal and had less pressure to run the EGR. General Motors had to act, so they brought the engine to PROM.
PROM in this case meant programmable read only memory. GM has released engine control module chip PROM updates, 13 of them in total. But customers didn’t feel too interested in continually returning to the dealership for PROM updates or even saving money on fuel. As a result, the service department often shut down the system. It was easy enough to turn off the modulated shift feature and you had to pull a wire from the transmission. When completed, the L62 returned to eight-cylinder business as usual. It is incredibly rare to find a current year Cadillac with a working cylinder deactivation system.
GM was very sure that they no longer wanted to offer the V8-6-4 the following model year, so it was dropped for 1982. GM went further and removed the 368 engine (L61 and L62 standard) from almost all Cadillac. Holdout for cylinder deactivation was a singular (oddly enough) Fleetwood limo until 1984. The L61 368 remained in use with a carburetor in the commercial Cadillac chassis for hearses and the like, also until 1984.
The 368 died in infamy, but was the last big cast-iron pushrod V8 offered in a production car. Its big-block brethren had died out much earlier and were finished in 1978. Something else died alongside the 368, the use of the legendary THM400 in GM factory cars. The heavy-duty three-speed was last used with the remaining 368 cars from 1984.
As a result of the V8-6-4 mess, Cadillac was forced to take decisive action for a replacement. Initially, the HT engine was slated for a 1983 model year introduction, on new front-wheel-drive Cadillac offerings. But the company’s current line of cars couldn’t wait that long, so HT was rushed into development, pushed into production, and debuted in 1982 on Cadillac’s rear-drive models. And it went so well! More on this in Part II.
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