Introducing the Cummins Westport ISX12 G natural gas engine - a new, larger-displacement natural gas engine.  The ISX12 G suits a variety of heavy-duty vehicles, including regional-haul truck/tractor, vocational, and refuse applications.  With a displacement of 11.9 liters and up to 400 hp and 1450 lb-ft of torque, the ISX12 G is the natural choice when considering alternative fuel for demanding applications.   The ISX12 G operates on 100% natural gas which can be carried on the vehicle in either compressed (CNG) or liquefied (LNG) form. The ISL G can also run on renewable natural gas (RNG).

Featuring the same proven Stoichiometric cooled Exhaust Gas Recirculation (SEGR) combustion technology, spark ignition and simple Three-Way Catalyst (TWC) aftertreatment as the Cummins Westport ISL G engine, the ISX12 G delivers strong pulling power and heavy-duty durability and performance that customers can depend on.

For best reliability and durability, the recommended maximum GVW for line-haul applications is 80,000 lb (36,287 kg). Recommended gearing is 1400 rpm to 1500 rpm at cruise speed, to optimize fuel economy.  Learn more about intended use guidelines here.

The ISX12 G meets 2013 U.S. Environmental Protection Agency (EPA) and California Air Resource Board (ARB) emission standards as well as the 2014 EPA and U.S. Department of Transportation fuel economy and greenhouse gases regulations.

Watch Videos
ISX12 G Walk-Around
ISX12 G Driver Familiarization

Download Brochures:
ISX12 G brochure
Natural Gas Driver Tips Card
Intended Use Guidelines

Advertised Horsepower 400 hp 298 kW
Peak Torque 1450 lb-ft 1966 N•m
Governed Speed 2,100 rpm
Clutch Engagement Torque 700 lb-ft 949 N•m
Type 4-cycle, spark-ignited, inline 6-cylinder, turbocharged, CAC
Displacement 726.2 cu. in. 11.9 L
Bore and Stroke 5.11 in x 5.91 in 130 mm x 150 mm
Operating Cycles
Oil System Capacity 12 U.S. gal 45.4 L
Coolant Capacity 26.5 U.S. qts 25.1 L
Net Weight w/ Std. Accessories, Dry 2,650 lbs 1.202 kg
Aftertreatment Three-Way Catalyst (TWC)
Fuel Types CNG / LNG / Biomethane
Methane number 75 or greater
Availability  2013

Cummins Westport engines can accept a wide range of natural gas fuel compositions.
Use our Fuel Performance Specification Calculator to ensure compatibility.

Engine Model Advertised Horsepower
Peak Torque 
lb-ft (N•m) @ rpm
ISX12 G 400 400 (298) 1450 (1966) @ 1200 2100 rpm
ISX12 G 385  385 (287) 1350 (1830) @ 1200 2100 rpm
ISX12 G 350* 350 (261) 1450 (1966) @ 1200 2100 rpm
ISX12 G 350* 350 (261) 1350 (1830) @1200 2100 rpm
ISX12 G 330* 330 (246) 1250 (1695) @ 1200 2100 rpm
ISX12 G 320* 320 (239) 1150 (1559) @ 1200 2100 rpm

 *Refuse/Vocational Truck Application Ratings

Overall, maintenance for the ISX12 G is similar to that of diesel engines, but there are a few key differences: 

  • The spark plugs will need to be replaced per the maintenance schedule at 1,500 hours (45,000 miles/72,000 km).
  • Motor oil specifically formulated for natural gas engines is required. Natural gas engine oil should be CES 20074-approved. Do not use diesel engine oil in a natural gas engine. If diesel engine oil is used, valve torching, piston scuffing and reduction in spark-plug life will occur.

ISX12 G Maintenance Intervals

Maintenance Item Distance Hours Months
Spin-on Fuel Filter Daily Check
Oil And Filter*^ 25,000 mi (40,000 km) 500 6
Coolant Filter ** 67,500 mi (108,500 km) 1,500 12
Spin-on Fuel Filter* 45,000 mi (72,300 km) 1,000 12
Spark Plugs* 67,500 mi (108,500 km) 1,500 12
Overhead Adjustment* 67,500 mi (108,500 km) 1,500 12
Engine Brake Adjustment 270,000 mi (435,000 km) 6,000 24
Standard Coolant 270,000 mi (435,000 km) 6,000 24
Air cleaner / element Follow vehicle manufacturers published recommendations

* Assuming normal duty cycle/based on 45 mph (72.4 kph) average speed. Spark plug interval and overhead adjustment must be reduced for slower speed applications.
**Do not change if SCA above 3.
^Requires natural gas engine oil (CES20074).  Click here for more information.

  • Intervals for normal duty - must be reduced for lower average speeds/duty cycles. Default interval is the hours stated. Interval is whichever comes first – hours, miles or months. 
  • Refer to QuiickServe® Online or Owners Manual for complete details on maintenance intervals.
  • Factory-Built, Dedicated Natural Gas Engine – Manufactured on the same assembly line as Cummins ISX diesels, the ISX12 G shares many components and parts with Cummins ISX12 Series diesels.
  • Cooled-EGR Combustion Technology – Takes a measured quantity of exhaust gas and passes it through a cooler to reduce the temperature before mixing it with fuel and the incoming air charge to the cylinder. The use of cooled EGR in combination with stoichiometric combustion improves power density and fuel economy versus lean-burn or traditional stoichiometric natural gas engines.
  • Three-Way Catalyst Aftertreatment – Three-Way Catalysts (TWCs) are effective, simple devices, packaged as part of the muffler, that provide consistent emissions control performance and are maintenance-free.  No Diesel Particulate Filter (DPF) or Selective Catalytic Reduction (SCR) systems are required.
  • Advanced Air Handling – Electronic control for precise air handling. Wastegate turbocharger with water-cooled bearing housing for durability. 
  • Fuel Capability – Natural gas can be stored on the vehicle in either compressed (CNG) or liquefied (LNG) form. The ISX12 G can also operate on renewable natural gas or biomethane that meets fuel-quality specifications.