The diesel truck wars are back! The rumors were indeed true! GM debuts a monstrous new engine for their updated 2027 Chevy Silverado HD and 2027 GMC Sierra HD pickup truck lineup. It’s the all-new 8.3L Duramax V8 turbo-diesel. GM refers to it as the new L6P engine. GM gives it a nickname – the “Mongoose”. We give it another nickname – the “Megamax”! We have all of the details about this engine is designed and how it was tested. Let’s dig in!
Although, most headlines are about the new 8.3L turbo-diesel V8, the 2027 GM HD pickup trucks also get a new e-boosted rack-and-pinion steering system, a new e-boost braking system with bigger brakes, and new suspension for their top of the line ZR2 and AT4X off-road versions. The 6.6L gas V8 is also updated with more power. More on all this later.
The New 8.3L Duramax V8
Think of the new 8.3 Duramax V8 as an enlarged and improved 6.6L Duramax. The L6P layout and architecture is very similar to that of the L5P. There is a large single turbocharger in the valley of V8. It’s a “cold V” architecture. The exhaust headers are on the outside of the engine. This the exhaust is routed behind an up toward the turbocharger that sits on the top of the engine.
The official 8.3L Duramax V8 power rating is: 555 hp @ 2,500 rpm and 1,230 lb-ft of torque @ 1,400 rpm. This is now an absolute class leader in terms of HD horsepower and HD torque. The L6P makes 85 horsepower more than the L5P, and 255 lb-ft more torque.
| Power | Torque | Max Towing | |
| 6.6L L5P Duramax V8 | 470 hp @ 2,800 | 975 lb-ft @ 1,500 | 36,000 lbs |
| 8.3L L6P Duramax V8 | 555 hp @ 2,500 | 1,230 lb-ft @ 1,600 | 38,000 lbs |
The new engine makes a lot more power and torque and does it at a lower rpm than before. GM says the new engine acts more like a traditional large commercial diesel engine. It’s able to operate and make massive power at lower rpm ranges. GM says it matches perfectly with the newly upgraded 10L1000 Allison-branded 10-speed automatic transmission. We currently don’t have the details of how the 10-speed was upgraded.
All of the L6P’s components are massive. The engine uses a larger turbocharger for additional boost. There is a new water-to-air cooling system that lowers the temperature of the compressed air that’s coming into the engine. There is a larger Exhaust Gas Recirculation (EGR) cooler. There is a larger DEF tank to allow for longer driving range between refills.
GM engineers had several goals with this new Mongoose turbo-diesel. Naturally, it’s to offer their customers more power, torque, and trailer towing capability. The new 2027 GM HD trucks are now rated to tow up to 38,000 lbs. The other goals include improving engine emissions and efficiency, while maintaining durability and longevity. If you expected this new Duramax to be “factory deleted”, it is not. It has a DPF and SCR systems that are very similar to the current trucks.
We do not have official confirmation yet, but an insider source told us the new 8.3L Duramax V8 can deliver a noticeably better MPG than the current 6.6L Duramax V8 when towing a heavy trailer. This is great news when the current price of diesel fuel is a record high levels. The current national diesel fuel average is $6.527 per gallon. The average price of a gallon of diesel in California is $8.437 (according to Gasprices.AAA.com).
New Diesel Fuel Pump
This new 8.3L engine uses what GM refers to a DPF7 high-pressure diesel fuel pump. This pump is supplied by Delphi. The DPF7 replaces the HP4 by Denso high-pressure pump that L5P uses.
Diesel Competition?
Ford’s current high-output 2027 model year 6.7L Power Stroke V8 is rated at 500 hp and 1,200 lb-ft of torque. Ram’s new high-output 2027 model year 6.7L Cummins I6 is rated at 430 hp and 1,075 lb-ft of torque.
There are many rumors that Ford is currently working on a 8.0L Power Stroke V8 that will offer much more power, torque, and improved emissions. The latest rumor is that the new Power Stroke will be rated at close 600 hp and 1,450 lb-ft of torque. It’s just a rumor.
We hear rumblings that Ram is also working on a higher-powered straight-six engine with Cummins. However, we do not have any good guesses on new power or torque figures. Personally speaking, I think Ram and Cummins could get a higher torque output without a lot of trouble. However, I don’t expect the Cummins horsepower number to jump much higher to where it is now.
| Power | Torque | Max Towing | |
| 8.3L Duramax V8 | 555 hp @ 2,500 | 1,230 lb-ft @ 1,400 | 38,000 lbs |
| 6.7L Power Stroke V8 | 500 hp @ 2,600 | 1,200 lb-ft @ 1,600 | 40,000 lbs* |
| 6.7L Cummins I6 | 430 hp @ 2,800 | 1,075 lb-ft @ 1,800 | 37,020 lbs |
- *the current Ford F-350 dually has a maximum tow rating of 38,000 lbs, and a consumer version of the F-450 1-tonne dually has a rating of 40,000 lbs.
The New Steering System
Until now, every heavy-duty pickup truck has had a recirculating ball steering system. The new 2027 GM HD pickup trucks introduce a segment first heavy-duty rack and pinion steering system with electric assist (EPS). GM says this steering system has the durability, redundancy, and capability to withstand all the demands of a fully-loaded HD pickup truck. It should also have other benefits of more steering assist (up to 30% more assist than current system), more precision, more confident handling, and a possibility of adding a Super Cruise hands-free driving capability. GM did not provide any comment about the availability of Super Cruise in their HD pickup trucks, but this rack and pinion system could make it possible.
We will have to wait until we drive these new trucks to truly evaluate the new steering system.
The New Braking System
The 2027 GM HD trucks introduce another segment first – electrically boosted braking system. All current heavy-duty pickup trucks rely on a Hydro-Boost hydraulically assisted braking system. The 2027 Silverado HD and Sierra HDs will use a new eBoost system that is combined with new larger 14.6-inch front brake rotors. GM says the new braking system will provide stronger and more consistent braking power. This system system supports the latest Adaptive Cruise Control and Automated Emergency Braking features. Still, no word about Super Cruise.
The new braking system enables more driving modes.
New Off-Road Suspension & Drive Modes
The 2027 GM HD trucks will offer a new Terrain Mode on the ZR2 and AT4X models. This is a one-pedal mode for slow off-road driving. This new mode is similar to the one available on GM’s light duty and midsize pickup trucks. The updated Off-Road is enhanced by the new EPS and eBoost Braking systems.
The ZR2 and AT4X models also get larger 47mm diameter DSSV dampers by Multimatic. The piston diameter went from 40 mm to 47 mm. GM says they found areas of improvement on the current off-road models. The new higher capacity shocks will do the trick. We cannot wait to drive these.
Updated 6.6L Gas V8
But wait there is more. The 6.6L gasoline V8 is updated to now deliver 412 horsepower and a class-leading 472 lb-ft of torque for an HD gas V8. This engine is also mated to an Allison-branded 10-speed automatic transmission.
| Power | Torque | |
| 6.6L V8 (L8S) | 412 hp @ 5,200 | 472 lb-ft @ 4,100 |
| 7.3L V8 (Godzilla) | 420 hp @ 5,350 | 445 lb-ft @ 4,250 |
| 6.4L V8 (HEMI) | 405 hp @ 5,600 | 429 lb-ft @ 4,000 |
These new 2027 Chevy and GMC trucks will go on sale before the end of 2026. Pricing is not available at this time. We will update you as soon as we know more.
Please join my video on our TFLtruck Youtube channel for detailed interviews and a hands-on look at the monstrous new turbo-diesel engine.
