GMC BIG BLOCK V6 AND TOROFLOW DIESEL

Source: forums.carcraft.com (no longer a valid domain)

The DH478 was a GMC V-6 diesel engine. It was actually a “Converted” gas engine.

These were awful engines overall.  I worked for a couple of fleets that got the original DH478s on GMC 4000-series school bus chassis orders, and they were a bundle of trouble from Day 1. Failed injection pumps. fouled injectors, burned pistons, you name it, these things were maintenance pigs even on a GOOD day. For power they weren’t too good either, and needed a two speed rear end to keep them in the torque band for hill climbing.  First, you need to know where it came from.

In 1955, GM introduced its “new-look” truck series. GMC Truck and Coach had been producing its own L6 truck engines for decades, and, unlike Chevrolet Division, had no “native” V8 of their own…not a BAD thing, since the Chevy big block was a pig of a motor anyway, and the small block was a horrid truck engine.  Chevrolet was able to offer its 265 small block in its pickup line, so GMC had to go “borrowing” from other GM divisions for their own, since Ed Cole’s Chevy small block was considered to be unsuitable for truck use. The result was that all V8 GMC pickups from 1955 to 1959 had Pontiac Division V8s, available with GMC’s own “Truck Hydra-Matic” variant, which was much beefier than the car Hydra-Matics.  Chevy had their “Powerslush” garbage as the low-line automatic, and car Hydra-Matic (leftover production used in Oldses) was avaialable after 1955 due to poor performance of Powerglide in truck use.  Medium-duty GMC truck lines used the Olds Rocket and the Buick engine, both heavily modified for truck use.  They were marginally successful, at best.

Meanwhile, GMC Truck and Coach was developing its own line of superior truck engines that weren’t car derivatives.  Problems with the Olds 371 (GMC called it the 370) in medium-duty truck and bus use (overheating, cracked piston skirts, stuck lifters, low specific torque) proved that GMC’s use of car engines in trucks would only be a temporary “fix.”  What GMC came up with was a huge Y block 60° V6, initially offered as the 305B (medium duty, 4 rings per piston) and 305A (lighter duty, with vacuum advance for pickups), a 351, and a 401 cu. in. line of offerings that were real truck engines, with low specific BHP but HUGE amounts of torque.  Later, in ‘62, the 305C (medium-duty) and 305Ds (pickups with vacuum advance) were introduced, using Stromberg WW carburetors, whereas the original A and B 305s used single-barrel Holleys.  This boosted BHP somewhat.  Still later, the 351E was introduced that supplanted all previous 351 versions, and remained their best seller until V6 engine production ended around 1972.  Around 1964, GMC introuced its 478 version of this engine, and added 2 cylinders to the 478 to make a 637” 60° V8.  All GMC V6s and V8s had separate throws for each cylinder on forged crankshafts, so vee angle was of little consequence.  The V8s were shared with Chevrolet for their medium-duty line, since their car-based big blocks were notoriously weak in truck service.  GMC also “glued” two 401 V6 blocks together, making the 702” 60° V12, very popular in Canada and the Pacific Northwest in logging trucks until the early ’70s.  Also in the catalog were the “Magnum” versions of the 351 and 401, sold as the “351M and 401M”, which had even better breathing and bigger carbs, offering more BHP at 3400…and worse fuel economy.

The ToroFlow came about as a way to obviate the need to use the 53-series GM Diesel (later Detroit Diesel) in their medium-duty line.  Essentially the ToroFlow was a 351, 401 or 478 gas V6 (and later the 637 V8) with 17.5 compression ratio, indirect injection with a Roosamaster pump, and not too much else.  THe GMC V6 gasser was a VERY strong engine (as long as it wasn’t overheated or oversped) so logic from GM Corporate was to try this trick to provide fleets with a lighter, less costly alternative to the 53-series Detroit.  It was an abject failure, mostly due to the injection system and lack of care taking in the conversion to diesel.  Runaway pumps, fouled injectors, leaking plumbing, burned pistons and excessive smoke were all hallmarks of the “Toilet Flush.”  Many fleet operators, after spending thousands on rebuilding pumps, doing piston jobs and going through dozens of injectors, converted to 351 gas versions, which offered 400K mile reliability, which the ToroFlow couldn’t do.  Note that the basic block wasn’t at fault here; the clumsly conversion to diesel had mostly to do with the rinky-dink injection system.  Specific power output from the DH478 was about the same as a 305E, although the DH478 could turn in 12 MPG on #2 diesel while a 351 in the same chassis was good for 7.

GMC V6 gas engines were known as the “Million Milers”, and indeed many were known to go that far without complete rebuilding.  However, any overheating, or running them over 3400 RPM, would cause expensive cracked heads or thrown rods.  The ToroFlows, and later, still-unsucessful ToroFlow IIs, would barely make 50K miles before needing new injectors and/or pumps, with piston replacement usually needed around 150K, scarcely better than a car engine.  ToroFlow II production ended when the gas V6 ended, in 1972.  The DH478 ToroFlows’ redline was 2800 RPM, not ONE RPM faster.

My suggestion would be to get rid of this boat anchor, but in pickup truck service, it might last quite awhile. Injector servicing will be a constant headache, and parts for any ToroFlow are virtually “unobtainium” now. Parts for GMC gassers are also getting tough to get.  The last one I rebuilt was my 305D in 1989, and most parts were “special order” from the usual suppliers.  GM quit supplying all V6 parts decades ago.  The reason for the Big V6’s deminse were two…one, they couldn’t get it to smog very easily owing to the VERY oversquare cylinder dimensions, but the big reason was cost-per-unit.  Sources inside GM Truck and Coach told me that Woodward Ave. forced GMC to dump their own engines because they cost almost twice as much per unit to produce than the usual crappy Chevy V8.  The 351E was replaced in GMC’s medium truck and school bus lines by the 366 Chevy, which drove many fleet buyers to Ford in the mid-’70s.