Written by Larry Gay
John Deere introduced three new models of tractors in 1970 for the 1971 model year. The 4320 and 4620 were 2-wheel-drive, row-crop tractors, rated at 115 and 135 PTO horsepower, respectively. The 7020 was a 4-wheel-drive tractor with a 145 PTO horsepower rating. All three tractors were powered by a 6-cylinder, 404-cubic-inch John Deere turbocharged diesel engine. The naturally-aspirated version of the 404-cubic-inch diesel engine produced 95 PTO horsepower for the John Deere 4020 tractor.
However, the 4620 and 7020 tractors were industry trendsetters, because their engines were also intercooled. An intake manifold intercooler cooled the air after it left the turbocharger, making it more dense and permitting more fuel and air to be taken into the combustion chamber of the engine. The additional fuel resulted in an increase in the power delivered by the engine.
The 4620 was available with the 8-speed Syncro-Range transmission or the Power Shift transmission with eight forward speeds. Optional equipment included dual rear tires, a front-wheel assist powered by hydraulic motors, and a Roll-Gard Cab with a ROPS frame inside the cab. The 7020 also featured the 8-speed Syncro-Range transmission and the optional hi-low transmission doubled the number of forward speeds to 16. The Roll-Gard Cab was standard equipment for the 7020 and dual tires were optional.
The John Deere 7020 was the first tractor with a turbocharged and intercooled diesel engine to be tested at Nebraska. During its test in April-May 1971, the 7020 exceeded its initial rating by a small amount by developing 146.1 PTO horsepower. The Nebraska tests of the 4620 followed in May and June. The Power Shift version produced 135.6 PTO horsepower and the tractor with the Syncro-Range transmission was measured at a nearly identical 135.7 PTO horsepower.
The use of an intercooler soon led to a new industry trend. Tractor engineers started developing a family of three models with three levels of horsepower, but with the same engine displacement. One model had a naturally aspirated engine, the second was built with a turbocharged engine, and the engine for the third model was turbocharged and intercooled.
Larry Gay is the author of four farm tractor books published by the American Society of Agricultural and Biological Engineers, including Farm Tractors 1975-1995. This book may be obtained from ASABE at 800-695-2723 or asabe.org, click publications and then click history books.