7.8L Duramax LG4, LF8

The 7.8L Duramax diesel, also referred to as the Duramax 7800 and sometimes stylized as the "DuraMax" 7800, is a re-branded variant of the Isuzu 6HK1 turbodiesel found in Chevrolet and GMC medium duty truck applications beginning in 2003 and ending after the 2009 model year when General Motors began its hiatus from the medium duty segment. GM utilized the RPO code "LG4" through the 2007 model year and began using the RPO code "LF8" for 2008 when the 7.8L Duramax adopted a diesel particulate filter. The designations "7.8L Duramax" and "Isuzu 6H" are used synonymously in various GM advertisements, brochures, and spec sheets relevant to applications that offered the Duramax 7800.

7.8L Duramax Engine Specs, History, & Information

Unlike the 6.6L Duramax engine family, the 7.8L Duramax is not a result of the Isuzu-GM joint venture, DMAX. The Isuzu built engine was offered in GM applications as part of a separate partnership, General Motors Isuzu Commercial Truck LLC, which was forged in the early 2000's. This joint venture sought to consolidate GM and Isuzu's commercial vehicle lines. Although Isuzu had a strong presence in the segment, one could argue that GM's family of brands were more identifiable in the U.S. markets. Joining forces allowed GM to tap into Isuzu's impressive line of diesel engines, which widely carried positive reputations with regard to reliability and longevity. Furthermore, Isuzu would have an opportunity to deliver more trucks, particularly cabovers, with the help of the Chevrolet and GMC brands. Usage of the Duramax namesake, with regards to the 7.8L Isuzu 6H, was little more than a marketing tactic intent on bolstering sales and increasing the scope of the flagship Duramax diesel brand.

The 7.8L Duramax LG4 was offered in one of several tiered output ratings ranging from 195 to 300 horsepower and 441 to 860 lb-ft of torque. Note that the available ratings varied between applications and there are at least 16 different configurations, although only 11 of the combinations were available in Kodiak and TopKick applications, ranging between 200 and 300 horsepower and 520 and 860 lb-ft of torque. The 7.8L Duramax was offered alongside GM's 8.1L Vortec V-8 (RPO code L18) and the Caterpillar C7 (RPO code LG5). The Cat C7 was offered in similar power configurations with a maximum torque output of 800 lb-ft.

When the 7.8L Duramax adopted a diesel particulate filter for the 2008 model year, the RPO code was changed to "LF8" and GM began referring to the engine as the "Duramax" less and the "Isuzu 6H" designation became more common in owners manual and spec sheet references. The engine features a high pressure common rail injection system with solenoid-type fuel injectors and a Denso injection pump. A Garrett variable geometry turbocharger was employed for minimal turbocharger lag and favorable response across the operating range.

7.8L Duramax LG4 & LF8 Specs (Isuzu 6H)

Engine Family Duramax 7800, Isuzu 6HK1 (Isuzu 6H)
RPO Code LG4 (2003 - 2007), LF8 (2008 - 2009)
Applications/Production Years 2003 - 2009 Chevrolet Kodiak C6500, C7500, C8500 medium duty trucks
2003 - 2009 GMC TopKick C6500, C7500, C8500 medium duty trucks
2003 - 2009 Chevrolet/GMC T-Series cabover WT5500, T6500, T7500, T8500
Displacement 7.79 liters (7.8 liters nominal), 475.2 CID (475 CID nominal)
Configuration Inline 6 cylinder
B10 Life 410,000 miles
B50 Life Not specified
Bore 4.527 inches (115.0 mm)
Stroke 4.921 inches (125.0 mm)
Bore/Stroke Ratio 0.92 (undersquare)
Compression Ratio [1]: 16.0 : 1, 17.5 : 1
Firing Order 1-5-3-6-2-4
Cylinder Numbers 7.8 Duramax cylinder number locations
Engine Block Material Cast gray iron, dry sleeved cylinders
Cylinder Head Material Cast iron
Injection System Direct injection
Denso high pressure common rail
Solenoid type fuel injectors, 7 hole nozzles
Aspiration Turbocharged, intercooled (air-to-air intercooler)
Garrett GT3788LVA variable geometry turbocharger
Valvetrain Single overhead camshaft (SOHC) , 4 valves per cylinder (24 valve)
Valve clearance (lash) Intake 0.016 in (0.4 mm), engine cold
Exhaust 0.016 in (0.4 mm), engine cold
Engine Oil Capacity 23.8 quarts with oil filter change
Additional engine oil required in vehicles equipped with an auxiliary oil filtration system
Lube Oil Filter P/N ACDelco PF2203
Fuel #2, #1 diesel fuel (includes winterized blends)
Horsepower 195 hp - 300 hp @ 2,200 - 2,400 rpm
Torque 441 - 860 lb-ft @ 1,450 rpm
Idle Speed ~ 600 - 630 rpm
Governed Speed 2,200 - 2,500 rpm (electronically governed) [2]
Engine Weight 1,180 - 1,250 lbs (dry)
Engine Dimensions Length 54.61 inches
Width 36.18 inches
Height 46.46 inches

[1] - There are some discrepancies in the listed compression ratio for the 7.8L Duramax; the original GM spec sheet from 2003 lists the compression ratio at 16.0 to 1 while the 2003 Isuzu 6HK1 workshop manual lists the compression ratio as 17.5 to 1. Compression ratio may differ with horsepower ratings.
[2] - Full load governed engine speed dependent on model year and horsepower/torque configuration.

7.8L Duramax Horsepower & Torque by RPO Code

Note that the availability of power configurations varied with model year and application; some power options are listed as RPO codes in sales material, but could not be connected to any particular model or model year.

RPO Code Horsepower
(hp @ rpm)
(lb-ft @ rpm)
LPS [1] 195 @ 2,200 441 @ 1,450
200 @ 2,200 441 @ 1,500
LQB 200 @ 2,400 520 @ 1,450
LPU 215 @ 2,200 "low torque"
LPT 215 @ 2,200 "high torque"
LPE 215 @ 2,400 605 @ 1,450
LQC 230 @ 2,400 660 @ 1,450
LPC 250 @ 2,400 660 @ 1,450
LQS 250 @ 2,400 800 @ 1,450
LYA 230 @ 2,400 "low torque"
LYB 230 @ 2,400 "high torque"
LYC 260 @ 2,400 "low torque"
LYD 260 @ 2,400 "high torque"
LQH 275 @ 2,400 800 @ 1,450
LPK 275 @ 2,400 860 @ 1,450
LQF 300 @ 2,200 860 @ 1,450

[1] - RPO code LPS denotes a 195 horsepower power option for the 7.8L Duramax, however this power rating was never advertised for Kodiak, TopKick, or T-Series cabover truck models. The 200 horsepower, 441 lb-ft variant of the engine was standard on the 2003 model year WT5500 cabover, but was never offered in the Chevrolet Kodiak nor GMC TopKick.

2003 GMC TopKick box truck
2005 Chevrolet T-Series cabover
2003 GMC TopKick (top) and 2005 Chevrolet T-Series cabover (bottom)
The 7.8L Duramax (Isuzu 6H) was offered in both models beginning in 2003 and ending in 2009