83GMCK2500
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If we're picking fly **** out of pepper...the "Heavy Half" was the marketing name for the GMC, "Big 10" is the Chevrolet equivalent, and it was offered in '76***-'80.
*** - The F44 package debuted on halftons in 1975 but neither marketing name was applied until 1976.
Some Big 10 history:
The Big 10 was promoted as a heavy duty two wheel drive half ton for the 1975 through 1980 model years. Although it offered truck buyers somewhat more load carrying ability than the standard C10 half tons, its real advantage- -and the motive behind its conception- -was that it provided buyers with the opportunity to purchase a half ton truck that was unencumbered by the dreaded catalytic converter, which was first introduced to a skeptical American car buying public just in time for the 1975 model year.
For several years prior to that time, all trucks with gross vehicle weight ratings of 6,000 lbs and below were forced to comply with the strict light duty emission standards that also affected passenger cars. The EPA's selection of the 6,000 lb. threshold likely reflected its awareness that that number had become the traditional de facto dividing line between half ton and three quarter ton trucks. And since the vast majority of trucks purchased by consumers were half tons, limiting the emission constraints to them probably seemed to be a reasonable compromise to the government, while avoiding potential protests about cost and implementation by the truck manufacturers- -and their commercial customers- -that might have occurred had the laws been expanded to the heavier vehicles.
Before the advent of the catalytic converter, the distinction between light- and heavy- duty emission controlled trucks was largely ignored by consumers. But the converter's introduction and it's nearly universal application in 1975 light duty emission vehicles changed things dramatically. Unlike the EGR and evaporative canister devices that preceded it, the converter had an exclusive appetite for expensive unleaded fuel which outraged truck buyers- -and especially fleet purchasers- -in a nation that still had access to relatively cheaper leaded fuel.
Recognizing a new marketing opportunity, or necessity, light truck makers made relatively minor spring, tire, and brake modifications to their existing half ton models to push GVWRs just over the 6,000 lb. threshold. Ford lead the way by introducing the heavy half ton F150 as an alternative to their traditional half ton F100, while Chevrolet introduced the F44 Heavy Duty Chassis package as an option for their C10 pickup.
These new-for-1975 models moved into the heavy duty emission classification enjoyed by three quarter ton and heavier rated models. This allowed the F44 equipped C10, later dubbed the Big 10 for market visibility, to comply with government emissions regulations using only a PCV valve, heat stove, and relatively loose controlled combustion system (CCS) tuning. In contrast, the 1975 light duty emission certified standard capacity C10 required a PCV valve, heat stove, stricter controlled combustion system (CCS) tuning, EGR, evaporative canisters, a vacuum activated early fuel evaporation (EFE heat riser) valve, special outside air ducting to the air cleaner, and a catalytic converter. But by 1979 the EPA- -following legislation enacted in California a year earlier- -raised the light duty/heavy duty emission dividing line from 6,000 to 8,500 lbs. GVWR which brought all heavy duty half ton and almost all three quarter ton pickups into the light duty emission fold. Following this legislation, consumer appreciation for the heavy duty half ton concept- -so gelded- -largely faded away.
A look at the chassis component differences that distinguish C10s, Big 10s, and C20s:
What does this have to do with hauling around cement blocks? Well, let's redirect the discussion to some component specifications for standard half ton C10s, heavy duty half ton Big 10s and three quarter ton C20s. Note that the following pertains only to two wheel drive vehicles.
A careful study of the 1975 Chevrolet Light Truck Data Book provides some insight here. Leading into 1975 the regular (i.e. not heavy duty) C10 pickup offered several different GVWR packages. Those ratings were 4,900, 5,300, 5,400, 5,600, and 6,000 lbs. Beyond this, the new F44 Heavy Duty Chassis package provided a 6,200 lb. GVWR. Mid way through the model year, Chevy broadened the F44's choices by adding a 6,050 rating option to the existing 6,200 lb package. This late-availability 6,050 pound option replaced the C10's 6,000 lb. package, which was cancelled.
All seven of these C10/ Big 10 GVWR packages used the same basic frame with a side rail width, depth, and thickness of 2.30", 5.92", and .156" respectively. The frame section modulus was 3.14. Also, all of these packages utilized the GM 12 bolt semi floating axle with a capacity of 3,750 lbs., and used a standard 15"x6" five lug wheel rim. Differences in GVWR were attributable to choices in spring and tire capacities, and brake system components. Some engine and transmission option recommendations/restrictions also accompanied different GVWR offerings.
