7.8L Duramax I-6

Duramax 7800 Specs & Information

The 7.8L Duramax or 7800 Series Duramax is an inline 6 cylinder turbodiesel designed by Isuzu. The engine is identical to the Isuzu 6H series platform, and only assumes the "Duramax" name in medium duty GM truck applications. The engine is found in GM TopKick and Kodiak trucks with RPO code "LG4" and various Isuzu trucks with RPO code "LF8" (referred to only as the 6H series). General Motors has maintained a close relationship with Isuzu in the medium duty market and, at least in the past, has maintained a considerably interest in the company's diesel engine operations. In General Motor's own technical publications, no distinction is made between the 7.8L Duramax and Isuzu 6HK1 other than their respective applications.

Isuzu 6H Diesel Power Meets Medium Duty TopKick, Kodiak

There is no evidence to suggest that 7.8L Duramax has any connection with DMAX, the joint venture between GM and Isuzu. The engine is in fact the Isuzu 6HK1 - General Motors activity in the medium duty market has included careful collaboration with Isuzu's medium truck division. In fact, GM's technical data sheets and up-fitter guides from the 2000's cover the Chevrolet Kodiak, GMC TopKick, and the equivalently sized Isuzu applications.

In understanding the relationship between Isuzu and General Motors, it is important to note that GM had obtained a 49% share in Isuzu by 1999 (shortly after the creation of DMAX). Late in 2002, Isuzu re-purchased these shares from GM, reducing their ownership to nothing. However, part of this buy-back also included General Motors full acquisition of DMAX and access to all of Isuzu's diesel engine designs. Interestingly enough, GM's website as of 2015 claims that ownership of DMAX is split 60% General Motors and 40% Isuzu Diesel Services of America.

The 7.8L Duramax was offered in many power ratings ranging from 200 - 300 horsepower and 441 - 860 lb-ft of torque. During the model years in which the 7.8 was offered, the 7.2L Caterpillar I-6 was also optional (RPO code LG5) in both the TopKick and Kodiak. Optional power ratings for the 7.8L Duramax include 200, 215, 250, 275, and 300 horsepower variations. After the 2009 model year, General Motors retracted from the medium duty market, retiring both the Kodiak and TopKick. Isuzu continues to manufacturer the the 6H series turbodiesel engine for commercial and industrial applications.

7.8L Duramax Specs


7.8L Duramax 7800 (Isuzu 6H-Series), inline 6 cylinder turbodiesel

Years Produced:

2003 - 2009 for Chevrolet Kodiak and GMC TopKick


475 cubic inches, 7.8 liters

Head/Block Material:

Cast iron cylinder heads, cast iron engine block

Compression Ratio:

16.0 : 1 standard, 17.5 : 1 for 300 hp model






Turbocharged & intercooled (air-to-air)


Direct injection, high pressure common rail


Single overhead camshaft (SOHC) , 4 valves per cylinder

Engine Weight:

Approx. 1,400 lbs dry

Max Engine Speed:

2,400 rpm

Peak Horsepower*:

200 - 300 hp @ 2,200 - 2,400 rpm

Peak Torque*:

441 - 860 lb-ft @ 1,450 rpm

*Multiple rating combinations offered.