2023 L8T 10 Speed

Epsilon Plus · December 20, 2021

I don't see the last few replies being hard to believe considering 3500-4000 is the classic SBC torque sweet spot. The engine was designed to sing here.

Further, I don't understand why this engine would specifically have cooling issues (heat rejection) over any other SBC considering it has a better internal cooling design than the LSx series and that the radiator and cooling surface area of the 20+ HD is the largest it has ever been. The engine also has a monster clutch fan. If anything is hurting the cooling performance of these trucks I would look right to GMs thermostat setting and clutch fan engagement strategy.

Member when (some of us) said 195+ thermostat in the trans was too hot for the K2s? Member? Member when GM just recently said "uh, yeah it turned out you end users who were deleting the thermo were right, retrofit this new on that opens at 158 in there instead"?

I member.

Don't put it past them to mess up their "calibrations" once in awhile.

Edited December 20, 2021 by Epsilon Plus

Ray Pickle · December 20, 2021

On 12/18/2021 at 10:07 PM, MTU Alum said:

You have your opinion and I have mine. Mine is based on running grades with powertrain calibrators. The heat rejection is high on this L8T and you have to control it eventually by keeping the rpms lower by commanding upshifts, denying

If the calibrations of ecm are attempting to deny downshifts under the circumstances of TFL test I will stick with my comments on poor programming parameters. After rewatching the footage and seeing the truck downshift just prior to the peak of ascent after allowing engine to lug to approximately 2k "not good". I am convinced that poor programming is the issue. As another has posted and I agree- the cooling capacity of this engine platform is robust and at least the equal to both Ram and Ford . There is no mechanical reason this powertrain combo cannot deal with the Ike in its powerband- 4 to 5500 rpm under most circumstances. Do you have powertrain calibration data indicating that heat soak or other parameters are at play actively causing this programming need? If so do the numbers indeed indicate the 6.6 is poorer at heat dissipation than Ford and Ram? I am genuinely curious and not above learning something new, just based on my own experience and actual use I find it hard to believe.

MTU Alum · December 20, 2021

I'm not going into details. You ultimately need to manage all the fluid temps and emissions to be pull long grades at altitude. If you compare the three brands, only ford does it signifantly better. None of three brands tested on the same day with same trailer and weights. Ford and Ram tested in winter where as Chevy tested in summer. The Ram tested with 2000 less lbs.

The L8T was designed as a fleet engine where reliabilty is the most important factor. It may not win many races but it will go to work everyday.

#iworkforGM

MTU Alum · December 20, 2021

On 12/19/2021 at 1:59 AM, C/K Man said:

I have heard that the heat rejection of the L8T is high, and I have been wondering if that's been keeping it out of the Silverado 4500/5500/6500. Then again I see that the L8T is now in some of the Isuzu based LCF's with GVW's approching 15,000#'s, so one would think the L8T ought to be O.K. at least for the Silverado 4500's.

BTW, I have a hunch that the heat rejection of the L8T is at least better than the Ford 7.3L Godzilla.........

I don't know the exact reason but I was heard it was an emissions concern. The emissions test is totally different between class 3 and 4. You can see how much the L5D is derated vs L5P to pass emissions. If you have to do the same to L8T, it may not be worth the trouble. Navistar may not want to screw with it as well. Navistar runs the program and we just supply cabs and engines.

Electric trucks are ultimately going to keep the L8T around. The electric trucks are class 2 vehicles and supply credits to keep the L8T around.

#iworkforGM

Another JR · December 21, 2021

1 hour ago, MTU Alum said:

...

The L8T was designed as a fleet engine where reliabilty is the most important factor. It may not win many races but it will go to work everyday.

#iworkforGM

That’s what I like to hear. While I only have one of these trucks, no failures and no limp modes when I’m on highway trips or remote camping was my top priority beyond basic capability. The gas engine and 6 speed were ideal for me.

Ray Pickle · December 21, 2021

1 hour ago, MTU Alum said:

I'm not going into details. You ultimately need to manage all the fluid temps and emissions to be pull long grades at altitude. If you compare the three brands, only ford does it signifantly better. None of three brands tested on the same day with same trailer and weights. Ford and Ram tested in winter where as Chevy tested in summer. The Ram tested with 2000 less lbs.

The L8T was designed as a fleet engine where reliabilty is the most important factor. It may not win many races but it will go to work everyday.

#iworkforGM

Iam not an engineer but have a little experience in the field of auto repair and basic understanding of thermodynamics, and would love to hear how Ford has managed this issue better. Further I have curiosity as to what has negatively impacted the new 6.6 vs the old 6.0 which apparently didnt suffer from this problem TFL tested a 2015 and it maintained 4 to 5k throughout duration of ascent other than the last mile where it lost rpm to 2900 before downshifting and then revs to the summit. Just rewatched that and they mentioned the programing issue. If it was trying to protect itself from being outside safe operating parameters the downshift is hard to explain- same as the new 6.6.

Ray Pickle · December 21, 2021

Minor point of clarification: it appears the old 6.l wasn't nearly as pronounced of an issue as the 6.6

C/K Man · December 21, 2021

22 hours ago, MTU Alum said:

I don't know the exact reason but I was heard it was an emissions concern. The emissions test is totally different between class 3 and 4. You can see how much the L5D is derated vs L5P to pass emissions. If you have to do the same to L8T, it may not be worth the trouble. Navistar may not want to screw with it as well. Navistar runs the program and we just supply cabs and engines.

Electric trucks are ultimately going to keep the L8T around. The electric trucks are class 2 vehicles and supply credits to keep the L8T around.

#iworkforGM

That makes sense, but somehow they got the L8T in the class 4 Isuzu LCF's for 2020. The transmission could be a factor as well, the LCF's get the 6L90 but I would think an Allison of some sort would be desireable for the Silverado medium duty trucks. Some Isuzu LCF's continue with the L96 6.0L teamed with the Allison 1000 6 speed. That combination is used by Freightliner for stripped van chassis.

newdude · December 22, 2021

On 12/18/2021 at 7:15 PM, vucelick said:

The allison 1000 was used behind the 8.1l big block gas motor and the diesel. The new 6.6 is similar in power to the older big block, i believe the 10/1000 is more than appropriate for the 6.6's power.

Would be pointless. 10L1000, the torque converter alone is 2-3 times the size of a 6L90 converter. There would be tons of power loss just spinning that transmission. 10L90 is all that is needed.

GM also used the 4L85 behind the 8.1 in 3/4 ton Suburban/Yukon because they never did an "HD" to then try to fit the Allison. 01-07 HD trucks had a "2 inch rise" compared to a 2500 non HD to fit the diesel and the Allison.

Edited December 22, 2021 by newdude

vucelick · December 22, 2021

2 hours ago, newdude said:

Would be pointless. 10L1000, the torque converter alone is 2-3 times the size of a 6L90 converter. There would be tons of power loss just spinning that transmission. 10L90 is all that is needed.

GM also used the 4L85 behind the 8.1 in 3/4 ton Suburban/Yukon because they never did an "HD" to then try to fit the Allison. 01-07 HD trucks had a "2 inch rise" compared to a 2500 non HD to fit the diesel and the Allison.

I am not sure what you mean by "2-3 times the size of a 6L90 converter" are you referring to the torque converter fluid capacity, or its dimensions?. The older allison 1000 and the new 10L1000 have the same "size" converter, both diameter and fluid capacity. I can easily see them offering it with the gas motor.

newdude · December 22, 2021

3 minutes ago, vucelick said:

I am not sure what you mean by "2-3 times the size of a 6L90 converter" are you referring to the torque converter fluid capacity, or its dimensions?. The older allison 1000 and the new 10L1000 have the same "size" converter, both diameter and fluid capacity. I can easily see them offering it with the gas motor.

Its physically massive and heavy. Converter alone in the 10L1000 weighs in twice as much as a 6L90 converter. GM even has a special tool to engine hoist it out of the trans while its on a bench.

The Allison 1000 wasn't offered with the 6.0 for similar reasons. Not enough engine power to spin the trans without stressing the $hit out of the engine.

vucelick · December 22, 2021

17 minutes ago, newdude said:

Its physically massive and heavy. Converter alone in the 10L1000 weighs in twice as much as a 6L90 converter. GM even has a special tool to engine hoist it out of the trans while its on a bench.

The Allison 1000 wasn't offered with the 6.0 for similar reasons. Not enough engine power to spin the trans without stressing the $hit out of the engine.

I believe the differences are much smaller than you believe, the bolt pattern on the back of the engines is the same, the bellhousing internal dimensions are the same. The allison converter probably holds more fluid than the 6L90 and perhaps the lockup clutches weigh more, but the weight difference would be trivial.

C/K Man · December 22, 2021

As I said Freightliner uses the L96 6.0L with the Allison 1000 series in their MT chassis line:

https://www.fccccommercial.com/chassis/mt/

Isuzu as well in there NQR, which is also availabe from Chevy dealers:

https://www.isuzucv.com/en/nseries/class_5_gas

I believe Powertrain Solutions International did the setup and markets the Allison-compatible L96 to other OEM's. The Freightliner MT with the L96/Allison drivetrain seems to be UPS's preferred chassis for package cars these days. No question about it's durability.

Epsilon Plus · December 24, 2021

Read between all the lines of this thread and you'll find if you're the average home owner/truck guy not needing to tow more than 10,000lbs with a travel trailer or dump trailer/equipment trailer then the L8T/6L90 is the most reliable and satisfactory combo you can buy in a modern truck today.

.....and as one of those people that warms my heart.

Mike7769ss · February 3, 2022

On 12/10/2021 at 9:07 AM, Epsilon Plus said:

I wouldn't be surprised to see it in LTZ or higher trims, or maybe even a stand alone option for LT, perhaps included with LT. Reason being is the 6L90E is the bulletproof fleet trans on the market. I had time to wait for this truck but pushed on this year specifically to get the 6L90. Probably sounds silly to those who want the latest and greatest, but I only see the 10-speed increasing city MPG and that's about it.

This L8T is great but it's not a 1,000lb/ft @ 2,000RPM monster. Being a traditional pushrod SBC it gets its torque meat and potatoes up at 3-4k with the HP starting to jam in good around 5k. I can't see shifting it out a bazillion times as its hooked to a heavy load really being beneficial. The 6L90E is perfectly matched to this combo to keep it where it needs to be (maybe some 4.10s for those that really want to sweeten the pot). It's ridiculously butter smooth and damn near indestructible (with a stock truck). Yeah, I don't really need/want the added complexity of the 10-speed just to keep up with the multi-gear Jones' and get another city MPG...

I hear alot of people saying that the 6L90 is bulletproof. I wish that was the case... my 2011 3500drw work truck is on its 4th trans in 160k miles. The torque converter failed at 102k on the first one. Then the rebuild by a local trans shop died within 6k miles got screwed on that phony ATRA warranty. Then the next rebuild lasted about 60k. Now I have a gm replacement. I can't really blame the 2 rebuilds on anyone but the local trans shops that did the work (two different shops). My truck weighs about 10k lbs with tool box body and stuff inside it. I occasionally tow a scissor lift (maybe 6 to 8 times a year). I'm pretty disappointed that it failed at 102k. For that reason alone I'm waiting until a new trans is offered behind the gas engine.