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8.1L & 4L80E/4-L85E Transmission


dmjr77

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Hello! I am new to the forum! I own a 2000 Chevrolet K2500 (not Silverado) ext cab 4x4 with a 454 with 60k miles on it that I picked up damaged and I have since re-built it. I also own a 1999 GMC K2500 4x4 Suburban with a 454. I must say that I enjoy the 454 and 4 speed combination. I dont do any heavy towning at all with these 2 trucks, these are daily drivers. I am posting on this forum is to ask the question why GM does not offer the 4 speed transmission with the new 8.1L big block in the 2500 HD pickups? I have tested out a new 2500 4x4 Suburban with a 8.1L and the 4L80E/4-L85E transmission combination. It had more power and torque then my 99 Suburban and 00 pickup. I then test out a 2500HD pickup with a 8.1L and Allison transmission, Extended cab (same configuration as my 2000 pickup). After driving the 04 pickup with the Allison, the 04 Suburban with the 4L80E/4-L85E transmission combination and my 99 Suburban and 00 Pickup have more lower end torque then the 8.1L and Allison transmission. I was very disappointed with the 04 2500HD pickup. It would be nice to see GM offer the same combination of the 8.1L and the 4L80E/4-L85E transmission combination that is in the new 2500 Suburbans with 8.1L as a option in the new 2500HD and 3500 pickups.

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I think we should be able to get the 4 speed also.It would be a more affordable option than the Allison.I'm sure the 4-speed would be a more peppy combo and get better mileage too.The Allison is a very stout tranny and carries a stiff price tag while soaking up a bunch of power.I bet most of the people that have a Allison don't even need it except in the most extreme cases.I think a 4l85 would do the job for the masses with the 8.1 while leaving the Allison as an option.The Duramax trucks should only be available with the Allison though due to the higher torque output.

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I've got an Avalanche 2500 4x4 with the 8.1L and the 4L85E tranny. Its the only tranny option available with the 3/4 ton avalanche. No complaints here. So it is an available combo, just need to "adjust" your truck preference. :confused:

Although, with the exception of the fact that the Allison is known for sucking up quite a bit of power, I'm not sure a 4 speed auto would do better on gas mileage than a 5 speed. I guess a simple comparison would be that at around 70 mph, I'm turning about 2700 rpm with a 4.10 rear with the 4L85. What are you 8.1ers with the Allison running at that same speed and gear ratio turning?

 

Gunslinger

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Gunslinger,

The Avalanche and the Suburban share the same 4-L85E transmission and that is the only way the the Suburban and Avalanche with a 8.1L come. I feel that the you should have the option of the 4-L85E in the 2500HD pickup with a 8.1L I think it would be a better combination

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dmjr77-

 

By seat of the pants feel, it may seem the vortec 8100 paired with the 4L85e tranny had better low end grunt than that same engine with the Allison, but I'll bet it doesn't actually put as much torque to the rear wheels on take-off or much of your acceleration time. It's true the Allison 1000, due to it's hefty design, is a power hog. I've seen some dyno tests on Duramax/Allison combos that show stock trucks losing about 70 lb-ft of torque from the flywheel rating. I'd guess it robs a similar amount from the big block. However, if you look at the gear ratios of the Allison 1000 versus the 4L80e/4L85e, it's pretty obvious there is a distinct gearing advantage for low end power to the rear wheels with the Allison tranny.

 

Allison 1000 gear ratios...

 

First: 3.10

Second: 1.81

Third: 1.41

Fourth: 1.00

Fifth: 0.71

 

4L80e/4L85e gear ratios...

 

First: 2.48

Second: 1.48

Third: 1.00

Fourth: 0.75

 

The extremely low 1st gear of the Allison helps put a lot of torque to the rear on take-off. Also, there is one more gear between 1st and the 1:1 direct drive gear, with the 3rd gear of the Allison being nearly as low as the 2nd gear of the 4L85e.

 

Doing the math...

 

2500 Burb/Avalanche with 8.1/4L85e - 3.73 gears (let's guess-timate the tranny uses up half the torque an Allison takes away, so about 35).

455-35=420 peak TQ to rear end

420 TQ

X 3.73 gear ratio

X 2.48 1st gear

= 3885 TQ to rear wheels at peak

 

2500HD Pickup with 8.1/Allison - 3.73 gears

455-70=385 peak TQ to rear end

385 TQ

X 3.73 gear ratio

X 3.10 1st gear

= 4452 TQ to rear wheels at peak

 

567 more lb-ft of peak torque to rear wheels in 1st gear with the Allison

 

Doing the same calculations with 2nd gear, you'll find the rig with the Allison puts about 281 more lb-ft of peak torque to the rear wheels.

 

At the upper end, the 4L85e will put a few more lb-ft of torque to the rear wheels, but maintaining speed is a lot easier than accelerating up to it. The Allison is designed to help give that power at the low end, and has the advantage of one additional gear before it reaches the 1:1 direct drive gear. It also has the enormous benefit of grade-braking, which is an excellent feature for those of us towing loads.

 

It's a good debate, and from an empty truck and/or hot-rodding standpoint, the 4L85e would probably work out better. For modest to heavy towing however, the Allison is, frankly..the cat's @$$!

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Bluenote,

I did not like the feel of the transmission shifting back and forth gears while you were doing about 45-50 mph. I dont do any heavy towing and where I use the truck as a daily driver, I just feel that the Allision is an expesive option

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dmjr77-

 

I understand what you're saying, and agree with your point about daily/empty driving. The features and function of the Allison are designed primarily for towing applications, whereas the 4L80e/4L85e are really more of an "all-around" use tranny. One thing however, mine doesn't shift back and forth at any relatively constant speed, even when towing my 5 ton fifth wheel. I'm wondering if the adaptive learning features of the Allison hadn't settled in on the truck you drove. Then again, maybe my diesel engine gives enough extra torque to convince the tranny to hold gears without hunting as much as the big block you drove. The Allison is a little quirky compared to any other auto tranny I've had. Now that I'm used to it though, I don't mind it a bit as my daily driver (45-50 miles round-trip to work, which includes both highway and inner-city miles). It seems like a nice option for those who want to brute force of the big block in a pickup body would be to offer the 4L85e just like GM does in the 'Burb and 'Lanche.

 

Gunslinger-

 

Thanks for the complement, it's amazing how lucid we sometimes can be at 1:30 in the morning. :confused: I did the math again with having 4.10 gears in both set-ups, and it would actually be even more of an advantage to the Allison-equipped truck at the low end. In 1st gear, there would be about 623 more lb-ft of TQ at peak, and in 2nd gear, there would be about 309 more lb-ft of TQ at peak to the rear wheels with the Allison. Like you say however, at the top end, there is actually a bit more power to the ground with the 4L85e and either gear ratio.

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