Diesel World - - Contents -

The ’70s TV talk show host Tom Sny­der was known for ask­ing, “What goes through their minds?” when con­fronted with in­ex­pli­ca­ble hu­man be­hav­ior. That might ap­ply when pon­der­ing why GMC ex­ec­u­tives di­rected the de­vel­op­ment of the Toro-flow diesel. Given the ar­ray of diesels the GMC truck line had to choose from, most no­tably those built by GM it­self, it seems an odd way to spend money. Well, GMC ex­ecs saw a low-cost diesel gap in the medium-duty truck mar­ket. Yes, the GM two-strokes were widely avail­able, but they were ex­pen­sive and noisy. GMC was not averse to go­ing out­side the com­pany for an en­gine, but none of the choices quite met the low-cost cri­te­ria ei­ther. In­tro­duced in 1964, the Toro-flow was the an­swer, but it had some bag­gage.

The de­vel­op­ment process for a four-stroke, low-cost diesel be­gan in 1953 and was even­tu­ally syn­chro­nized with the de­vel­op­ment of a new 60-de­gree V6 gaso­line en­gine to be in­tro­duced for 1960. This is not to say the diesels would be “con­verted gas en­gines.” They weren’t. It’s more ac­cu­rate to say the gas and diesel V6s were con­cur­rent de­vel­op­ments to be built with sim­i­lar ar­chi­tec­ture and on sim­i­lar tool­ing. There was very lit­tle ac­tual parts changeover.

The well-re­garded GMC V6 gassers were ini­tially of­fered in 305, 351 and 401 ci, but by ’62 had grown to 478 ci and later 379 and 432 ci ver­sions were of­fered. They were one of the first V6 en­gines of­fered in the United States, and the 305-pow­ered GMC fleet of light trucks, a de­lib­er­ate snub of the Bowtie Stove­bolt in­line. But enough gasser drivel!

The Toro-flow diesels de­buted in 1964 in two dis­place­ments, 351 ci (the D351, 130 hp) and 478 ci (D478, 150 hp). A high-out­put model was also of­fered, the DH478, crank­ing out 170 hp. It’s been re­ported that GMC toyed with the idea of a diesel 305 V6 in the light trucks but it isn’t clear how far the idea went. Prob­a­bly

 A fully dressed D478 V6 from a 1964 show where the en­gine was first be­ing in­tro­duced. Note the twin air fil­ters. The air fil­ter ar­range­ment var­ied from th­ese twins to ducted, cen­trally mounted fil­ters, de­pend­ing on un­der­hood clear­ance. The GMC orange paint is seen on many ’64 and ’65 en­gines but grad­u­ally seems to change to yel­low af­ter that.

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