Deep Dive: 2020 HD L8T 6.6L Gas and L5P Duramax 6.6L Diesel Engines

[ThomCannell]

Thom Cannell

Contributor, GM-Trucks.com

March 7th, 2019

 

Heavy Duty trucks mirror the contest for market domination in light duty trucks.  This year both GM and Ford announced significant upgrades to the engines powering their all new 2500 and 3500 HD trucks as each company upgraded their diesel engine, and delivered new gas engines.

 

At GM, the launch event centered on Chevrolet, who brought in truck writers from every segment—popular to fleet management—to Flint, Michigan’s Flint Truck Plant. Flint is the original home of General Motors trucks and the spiritual and historical home of the UAW. So, Flint Truck Plant is receiving an all-new facility constructed and designed for just HD trucks, with the former truck assembly areas destined for warehousing and future projects.

 

New L8T 6.6-Liter Gas Engine

We first spoke to Mike Kociba, a GM engineer and part of the Small Block team to learn about the new 6.6-liter V-8 engine we'd been anticipating.

Mike told us the new motor “is a marriage of the six-liter it replaces and an upgrade in technology levels to Gen 5 architecture.” A careful look will disclose similarities in key areas where GM has maximized their experience with the six-liter’s durability and improvements in performance levels derived from Gen 5 architecture.  “Specifically, new here is the gray cast-iron block which is unique for this application, hyper-eutectic purpose-built pistons for this application and heavy-duty requirements, forged powdered-metal connecting rods, and a forged steel crankshaft,” Mike continued.

The most significant change is the addition of Direct Injection. It’s all new, an industry first for the heavy-duty market and new to GM trucks. “When we added DI, we took the roughly 400 KPa fuel pressure from the low-pressure pump and dialed it up to roughly about 15 mPa for engine operation under key conditions,” Mike continued. “That allows us to increase compression ratio, now 10.8:1 using regular fuel. Without DI you're not going to hit those numbers without  losing a lot of spark efficiency. With those additions, and the six millimeter longer stroke, that gets us up to 6.6-liters.”

“That suite of changes allows us to hit class-leading gasoline engine torque, at 464 foot-pounds at a lower engine speed than the outgoing six-liter was optimized for. I'm proud of its 401 horsepower, which is SAE Certified, no games, legitimate.  This (engine) is purpose-built to crank out those numbers day, after day, after day with no compromise in durability. Customers can have confidence they're going to pull, tow whatever trailer you need.”

 

There are other new features like an all-new water pump and a massive cooling fan to meet the demands of the HD customer base. New is how the water pump drives the fan through a one-inch shaft using purpose-built bearings to handle loads.  

 

Another first for HD is a variable-output oil pump. “No mater what the severe operating condition is for the customer, the pump is capable of dialing in more, or less oil pressure regardless the requirement. 

 

The engine features an aluminum oil pan, nylon 6-6 air intake, and stainless steel exhaust manifolds unique for the Heavy Duty market. That’s because HD market has specific requirements for (fuel) enrichment and these stainless manifolds will meet those requirements. “We have variable dual-equal valve actuation, like on light duty, where intake and exhaust are phased together and controlled through the actuator on the front cover. It's chain driven for accessories.” GM designed this engine specifically for upcoming standards for particulates and NOx emissions standards. “With this architecture we're not just making power and torque, but improved emissions and improved efficiency.” Mike continued.

We noted the massive valves, which Mike said are common with Gen 5 architecture for valve layout and their pushrod technology. “That's how we get this compact shape. When you compare the size of the two engines, they're similar, which is due to the common 4.4-inch bore spacing.”

 

A unique feature of the new engine is inter-bore cooling. Coolant flows between the Siamesed bores, notably in the upper bores where there’s a tendency to generate higher temperatures. 

 

“For two-valve technology of course you've got the spark plug, and the fuel injector, splayed outside. To avoid heat, we have the coil mounted directly on the rocker cover and the boot mounted next to the manifold with industry-standard individual coils for each cylinder.”

This is great stuff, we though, but engine development isn’t cheap. So, why a new 6.6-liter when the 6-liter was doing well? 

 

“We needed to improve to Gen 5 level of technology to be sure (the engine) is capable of delivering on durability requirements. Customers love the convenience of gas, but if you look at the market—for instance trailers with more gadgets and slide-outs—everything is getting heavier. Customers want to be sure they can tow with confidence, no compromises, whether it's fuel economy, power, torque, emissions, efficiency, they don't want to pull up to their neighbor and have to make excuses.

That's what we targeted. No compromises. With the significant technology we put into this engine, it makes segment-leading torque without compromising efficiency or emissions.  Peak torque is at 4,000 rpm, 400 rpm lower than the 6.0-liter. Three things enable the new 6.6-liter's better power output. Direct injection (DI) allows us a higher compression ratio; longer stroke is good for increased torque (but not as good for horsepower as piston speeds are high) and for heavy-duty application where you need torque everywhere it’s why we focused on a longer stroke to get to 6.6-liter displacement. Those changes enabled us to broaden the torque curve, which is up 20% everywhere, for greater work potential.”

 We thanked Mike and asked if we’d missed anything. “Small engines with turbochargers allow them peak torque off idle, but for heavy duty we don't want that complexity. For the Heavy Duty segment we (General Motors) have durability requirements—Global Engine Durability—that are unique and very long and stringent requirements. We know customers need 401 horsepower and 464 lb.-ft. of torque today, tomorrow, and every day for years to come with no compromise in durability. 

 

We know our customers and, if they can't use their truck today, they might not get paid. That's why we focus on durability.”

 

Brand New HD 6-Speed Transmission

With that in mind, we next spoke to the systems chief engineer for six-speed FWD and RWD transmissions Rich Mardeusz. More power and more torque tend to break an older transmission. So, we wanted to know what changes had been made to the new transmission to carry the additional torque.

“We started with the 6L90 that's in the current HD vehicles and full-sized vans (and ZL-1 Camaro and CTS-V), received the horsepower and torque curves from the engine engineering teams and then performed an analysis of all mechanical components from front to back,” Rich said. General Motors uses specific simulation tools for different parts. “For instance, we have a "gear damage analysis tool" for analyzing the gear set and how much damage it may receive over the life of the vehicle,” Rich told us. The result was a need to improve the torque converter and the clutch pack, which needed to be more robust to accommodate the greater power output of the upgraded 6.6-liter V-8 engine.  

 

From a clutch pack standpoint, changes were simple, according to the engineer, as there was enough room in the case to add a clutch and one backing plate to each of the clutch packs to handle additional power. When it came to the torque converter, things changed. “We looked at the components from a heavy-duty diesel torque converter and a high-output gas torque converter and then took the torque-carrying components from the diesel and married them to the spring and damping components from the gasoline torque converter. That’s what was needed to accommodate the approximate 22 percent across-the-board torque increase.”

 

So, the new torque converter can A) handle the added torque of the new engine and B) damp out the firing frequencies from the gasoline engine, which are significantly different from a diesel engine. All of the shafting and gears were able to handle the torque. Interestingly, there is no dipstick. GM has the confidence to eliminate it, and only change fluid at suggest intervals of approximately 100,000 miles, more often for those who mostly tow, or drive over mountains with full loads. Another surprise, the transmission uses GM-spec Dexron VI fluid, GMs standard since 2005, as they found no reason to change.

 

2019 L5P Duramax 6.6-Liter

Once we’d completed our gas powertrain interviews, we turned to the diesel side of Heavy Duty. We spoke to Max Sala, whose Italian accent tipped us to an affiliation with GM’s diesel engine center of excellence in Turin, Italy. 

 

Max said that their objective for the new Silverado HD was to increase towing capacity and ensure functionality with the new Allison/GM transmission. Remember, the Duramax 6.6L Turbo-Diesel V-8 engine makes 445 hp. and 910 lb-ft of torque. “We added a bigger fan now 28-inches, a bigger oil cooler that is upgraded from 14 plates to 19 plates, and we fine-tuned the cylinder head gasket” Next up were improvements to the engine-brake capacity, taking into consideration towing capacity. “It’s better by 14-percent and we introduced smart activation of the engine brake,” Max continued, “There's still a button for manual activation, but for safety there's automatic activation at certain RPMs.” Under the new control system, the powertrain will recognize any need for the engine brake and activate automatically. For instance in driving down hill and forgetting to shift, the higher RPM means automatic activation. “With that, we have better after-run strategy. Every time you tow uphill, temps rise and you have a message to cool the engine when stopping. If, by chance you forget and close the door, the system cooling system activates automatically for up to 15 minutes to cool the engine for reliability.” 

 

That isn’t the end of changes, as the engine has been completely recalibrated to match the new 10-speed Allison transmission. “Emissions have been improved and fine-tuned to maintain the best efficiency the transmission can offer to our customers.” With these changes, most importantly, Chevrolet says they are now capable of delivering full torque at any time, in any gear, and that they have done everything to the engine, transmission, driveline, drive shaft and frame to improve strength and durability. “What's important is how safe (the new HD trucks) will be and how comfortable it will be for our customers to drive these huge trailers up, and down hills.” Max concluded.

 

Allison transmissions have gained a peerless reputation for strength and durability. Adding a 10-speed transmission branded with the Allison name is a great choice. David Ames, now GM assistant chief engineer on the Allison transmission and liaison with Allison, is a former Allison engineer. A natural fit. 

 

The 10-speed is a collaborative effort with joint development of the analysis, engineering, as well as testing. So, testing was performed at Allison and at GM, each with their own set of rules and test regimes. “We go back and forth”, David told us. “Today we have a ratio-span of five and this transmission has a span of 7.2, so the new 10-speed provides both more overdrive and a lower first gear.”

 

We asked about the projects’ starting point. “We (at GM) come out with a "here's what we're looking for" and we begin an internal development contract. It was a pretty clean sheet of paper. So, the controls on the bottom are from a smaller 10-speed, some pieces and parts, but not the entire controls package. For the most part, it's all new to handle the increased power and much larger torque. 

We collaborated with Allison on this transmission (GM does have a 10-speed transmission of its own) which made it necessary to meet their (Allison) design requirements, their analysis requirements, their engineering requirements, as well as our own. 

 

It's a very compact transmission. If you had a 6-speed for comparison, this more dense, more compact and solid to get ten speeds into a package that would still fit nicely into the vehicle and not take up too much space,” David continued.

This transmission’s torque converter has a lock-up clutch and is unique in that it will lock up in first gear, even under max loads. So, if you're pulling 33,500 pounds, you can do a first gear launch and lock up right away, which helps get rid of heat. We asked David why this is important. “Normally in first gear you're under high torque and generating a lot of heat, which puts a lot of demand on the cooling system. Locking up gets rid of that heat and the 7.2 ratio gives you a lower first gear. For instance, the six-speed uses a 3.1 first gear and the new transmission has a much lower 4.5 first gear. It's got four planetary gear sets, six clutches and the main place you'll notice the ten speeds, not only in launches and driving with heavier loads—it's very smooth—is going down a grade. Often you're trying to downshift to save brakes and having ten gears you can usually hold the right speed and not feel like you're running over the car in front of you, tapping the brakes or going too slow.”

“Also, we built in the first OEM PTO option. Note that the chain drive to the PTO is engine-speed driven rather than turbine-speed driven, which is important to many commercial customers, and it’s quieter drive than gear driven systems.”

We asked David for an overview of the combined Duramax-Allison package. “For those who need it, it’s a nice package, one we're very proud of because of the outstanding durability. I think we're going to do a better job of putting power to the road than anybody out there. Whatever torque the engine is putting out, it's getting to the road in an accurate way. I think this transmission will be far more durable than people need it to be. Four-five years from now people will understand how durable it is.”
 

[newdude]

Great write up.

 

I'm still stuck on three things with the gas 6.6.

 

- They keep referencing its tie in with the 6.0.  Is this engine a true Gen 5 engine?  Or a Gen 4 with DI and other upgrades?  Looking at your close up shots, it seems its a gen 5 engine.  I feel like they are missing a word, such as "we started with the dimensions of the old 6.0." 

 

- I still don't understand why the 6.6 gas doesn't get a 10 speed as well, especially with the final drive ratio change to 3.73.  That would have really maximized this new lower torque band potential with the shorter gear spacing of the 10 speed, and deliver even lower cruising RPM's than the 6L90.  I get the 6L90 is their least repaired/most reliable gas transmission, so why fix what isn't broke.  Part of me thinks that the GM/Ford joint venture had something to do with it.  GM got it in the ZL1 first, Ford stuffed it in the F150 in return, then GM in the K2 suvs, Ford with the Mustang GT now with the Super Duty.  3.73 gears makes me think the 10L90 is coming still yet. 

 

- 3.73 gearing.  This would be the right choice with the 8L90 and the 10L90, but seems strange with the gas/6L90 combo.  GM did 3.73 gears on gas HD from 2007-2014 with the 6.0.  They got better highway mpg, but towed 4000lbs less.  Somehow, this new combo 6.6/A6/3.73 gets the same if not slightly higher tow ratings than the K2 HD with 4.10 gearing.  I don't believe that 41hp and 80tq allows for similar performance to the 6.0/A6/4.10 combo. 

Edited March 7, 2019 by newdude

[L86 All Terrain]

Over 140hp and 400lb-ft of torque BELOW 2,000rpm from a naturally aspirated engine is a remarkable achievement and will undoubtable be more than a sufficient choice for the HD's. Considering the outgoing 6.0L makes around 120hp and 330lb-ft at 2,000rpm. Now I just wish I could have had this engine in my 2018 2500. But as newdude suggests, going from a 4.10 gear to a 3.73 will likely take away any of that gained torque IF all the 6L90e gear ratios remain unchanged.  

[newdude]

7 minutes ago, L86 All Terrain said:

Over 140hp and 400lb-ft of torque BELOW 2,000rpm from a naturally aspirated engine is a remarkable achievement and will undoubtable be more than a sufficient choice for the HD's. Considering the outgoing 6.0L makes around 120hp and 330lb-ft at 2,000rpm. Now I just wish I could have had this engine in my 2018 2500. But as newdude suggests, going from a 4.10 gear to a 3.73 will likely take away any of that gained torque IF all the 6L90e gear ratios remain unchanged.  

 

Per above and as I've read elsewhere, its the same 6L90.  Ratios should be unchanged.  It does appear that the ZL1 variant might have been the basis a bit more (higher capacity converter, etc.) than the current HD version. 

Edited March 7, 2019 by newdude

[SierraHD17]

3 hours ago, newdude said:

Great write up.

 

I'm still stuck on three things with the gas 6.6.

 

- They keep referencing its tie in with the 6.0.  Is this engine a true Gen 5 engine?  Or a Gen 4 with DI and other upgrades?  Looking at your close up shots, it seems its a gen 5 engine.  I feel like they are missing a word, such as "we started with the dimensions of the old 6.0." 

 

- I still don't understand why the 6.6 gas doesn't get a 10 speed as well, especially with the final drive ratio change to 3.73.  That would have really maximized this new lower torque band potential with the shorter gear spacing of the 10 speed, and deliver even lower cruising RPM's than the 6L90.  I get the 6L90 is their least repaired/most reliable gas transmission, so why fix what isn't broke.  Part of me thinks that the GM/Ford joint venture had something to do with it.  GM got it in the ZL1 first, Ford stuffed it in the F150 in return, then GM in the K2 suvs, Ford with the Mustang GT now with the Super Duty.  3.73 gears makes me think the 10L90 is coming still yet. 

 

- 3.73 gearing.  This would be the right choice with the 8L90 and the 10L90, but seems strange with the gas/6L90 combo.  GM did 3.73 gears on gas HD from 2007-2014 with the 6.0.  They got better highway mpg, but towed 4000lbs less.  Somehow, this new combo 6.6/A6/3.73 gets the same if not slightly higher tow ratings than the K2 HD with 4.10 gearing.  I don't believe that 41hp and 80tq allows for similar performance to the 6.0/A6/4.10 combo. 

 

Gen V architecture is the same an LS... so likely because they chose to stay cast iron with the block they like to relate it to the 6 liter as opposed to the 6.2 this block will be likely heavily based on.  

 

Now when I say Gen V architecture is the same I mean you can literally pull the rods and crankshaft out of a brand new Gen V whatever and drop them into a 1999 LS block..... or a 1997 Corvette LS1 block.  The journal sizes, thrust bearing, rod length, piston pin size( if it was a full floating pin LS piston anyway) etc is all identical.  Since most Gen V cranks are twist forgings guys are dropping them into LS's for a cheap upgrade.  The LT4 connecting rods specifically look like a trick aftermarket piece and it's the same thing..... you can literally hang gen 4  LS 5.3 pistons on them lol.  So really it's a stroked 6 liter with direct injection if you want to look at it like that.....  Or it's an iron 6.2 casting lol.  It has the same 4.065" bore size as any LS or LT 6.2 did and then a 3.85" stroke crank.  I would not doubt they used the shorter stroke and still use the same cylinder liner length as the current 6.2 does.  

 

Honestly unless they make massive changes.... which being GM they will not.. this is a good stroker setup for LS guys now haha.  And I think this engine pulled from the HD and dropped in a half ton will rock too.  Cam and tune should be 500+ Hp and 550+ lb ft of torque on the milder side.  It would not surprise me one bit that this uses the same cylinders heads as the current Gen V 6.2 which flow extremely well.  

 

The 6 speed is fine in my opinion as more gears is whatever nowdays.  A good shifting 8 or 10 would be cool.. but that fail of an 8L90e is not an upgrade so maybe the 10 speed down the line.  

 

The gearing is dumb.... the truck will probably weigh as much as the current one and with GM's stellar programming for 87 octane that thing will probably feel not a lot stronger than a 4.10 geared 6 liter does.  On paper it should blow the L96 away but time will tell that.  

 

4.56 gears like I did in my 2017 HD 6 liter truck and some reasonable premium fuel tuning might make one of these pretty fun.. if the ECM gets tuning support lol.

Edited March 7, 2019 by SierraHD17

[magnum74]

The L8T will be a noticeable improvement. It'll be making more than 75lb/ft off idle than my L96. I agree that a 4.10 should be an option but I can't see how the 2020 won't smoke an L96 even with a standard 3.73 especially if they make the same driveability improvements as they have with the 1500's. 

 

As far as the trans is concerned it is bulletproof and the gears are perfectly spaced. Of course GM is behind as usual by not using the 10-speed but if I'm happy with with how my L96 works with the same trans why wouldn't I be just a happy with a more powerful engine?

 

I will be taking a good look a the 7.3 Ford gas, though.

[L86 All Terrain]

On ‎2019‎-‎03‎-‎09 at 3:41 PM, magnum74 said:

I will be taking a good look a the 7.3 Ford gas, though.

That thing will make the current 6.0L look like a Prius at the gas pumps. I know how bad my 6.2L DCSB (5400lbs 1/2ton) can be when using the power, never mind talking DI big blocks in heavy trucks. I would expect you will see a F350 7.3L get about 9 highway and 6 city. I am sure Ford will put a 50-60 gallon tank in there though to convince owners they get good mileage, same trick with the F150. 

[C/K Man]

It looks to me that the L8T is an iron block Gen. V without AFM/Dynamic Skip Fire.  I see a lot to like in this engine, particularly the increased displacement, forged crank, and siamesed cylinder block with bored cooling passages.  I would like to see E85 capability in the L8T eventually, but I think the 6L90 and 3.73:1 gear ratio will be fine.  I hope the exhaust manifold bolts don't break under hard use, something I see in 6.0L's often.

 

As for the Ford 7.3L, I have questions!  I looks like a good truck engine, but so for I don't see much that makes me believe it will have much of a future as a high performance/crate engine.  While it certainly has displacement and the bottom end looks strong, the valve train doesn't look like it will tolerate very high r.p.m.'s.  The valve stems and springs look very tall, and the pushrods are also quite long.  Lots of inertia and I wonder about stability at high r.p.m.'s with an aggressive camshaft.  The bore spacing is 4.65" and they are running a 4.22" bore, so it looks like the 7.3L also has a siamesed cylinder block.  However, it appears Ford has machined slots between the cylinders to permit coolant flow between them.  These slots are open at the top (block deck), which might be a problem if someone decides to hang a blower or turbo on one of these things.  Heads look to have real decent porting, but I don't see a lot of cooling passages in them.  These are just superficial observations, and I do think the engine will be good in it's intended applications.  But it ain't no LS!  Just don't see that kind of versatility in the 7.3L.          

[mookdoc6]

I really,really like this L8T!~  It pretty much nails it in my opinion...GM took their tank 6.0 and upped the Game!  GM has no data on 10speed in HD setup?  They will wait couple years for data on 10spd's in 1500's

Say what you will about that decision but that is what sets GM apart and their powertrain which is a cut above the competition.  

[Colossus]

AWESOME article.  the engines and tranny has me excited.  But those things won't sell the truck if it is still ugly to look at. 

[SierraHD17]

Fleets will still buy them because looks really don't matter to them.  So will regular people because the second they see on 12 wide rubber bands its awesome lol.

[redwngr]

On 3/14/2019 at 2:25 AM, mookdoc6 said:

I really,really like this L8T!~  It pretty much nails it in my opinion...GM took their tank 6.0 and upped the Game!  GM has no data on 10speed in HD setup?  They will wait couple years for data on 10spd's in 1500's

Say what you will about that decision but that is what sets GM apart and their powertrain which is a cut above the competition.  

What kinda 'data' on the 10L1000 are you looking for?

 

 

 

[magnum74]

On 3/12/2019 at 12:15 PM, L86 All Terrain said:

That thing will make the current 6.0L look like a Prius at the gas pumps. I know how bad my 6.2L DCSB (5400lbs 1/2ton) can be when using the power, never mind talking DI big blocks in heavy trucks. I would expect you will see a F350 7.3L get about 9 highway and 6 city. I am sure Ford will put a 50-60 gallon tank in there though to convince owners they get good mileage, same trick with the F150. 

A lot of online people will be dissapointed when they find out it's nowhere near 500HP and 600TQ. It's the optional motor and will likely be a niche. The current 6.2 is a strong performer and standard fare. I still have to try a 7.3, though.

 

Yeah, the 'I can drive this far with my truck' mileage calculator isn't as accurate as the volume vs. distance calculator. Unbelievable how few understand mileage today but whatever helps them sleep well

[mookdoc6]

8 hours ago, redwngr said:

What kinda 'data' on the 10L1000 are you looking for?

 

 

 

Myself?  None I don't need any data....GM has Zero installed on HD's GM will not put that into their HD's until it has many,many units and data on the 1500's 10speed durability.  That is the reason for the 6speed in HD's and this pertains to the Gassers not the Diesels.

[redwngr]

11 minutes ago, mookdoc6 said:

Myself?  None I don't need any data....GM has Zero installed on HD's GM will not put that into their HD's until it has many,many units and data on the 1500's 10speed durability.  That is the reason for the 6speed in HD's and this pertains to the Gassers not the Diesels.

Hmmmm....

 

Thread title suggests both powertrains..