Why were the SD engines discontinued?
Were the SD engines offered in medium duties or strictly heavy duties?
That's a two part answer. 1). Ford replaced the SD engine family with the 429 (the same family as the 460) in the early 1980's. The 429 gave more bang for the buck (or milage for the gallon) than the SD. 2). The biggest reason was diesel. When the Super duty was first introduced in the late 1950's diesel didn't have the popularity it has today. Infact, The 534's performance matched that of the Cummins diesels Ford offered in the N-series trucks offered during the early 1960's. As time went by, diesel's reliability and performance improved making it the engine of choice in large trucks. Anyone who has ever driven a SD powered truck will tell you they are large, heavy, thirsty, low reving engines which have no business in any vehicle other than a class 7 or 8 truck.
The 534 twin turbo & intercooled engines that came out of a 46' motoryacht I'm restoring weighed 1800#, as much as a Cat 3208 turbodiesel. I'm told that when the throttles were planted forward, there was an enormous sucking sound, the rear of the boat would squat and the nose went for the sky. Within a second or two, the nose would slap down and that beast would take off--it was a 40kt boat, which is extremely fast for a vessel of this size especially when you consider that this upgrade was done in 1972. The downside, of course, is that it would consume three gallons per mile at full speed whereas a Cat of similar power output might consume only one gallon.
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