L86 to L8T in '19 Yukon XL Denali

[Marky Dissod]

4L60's 3rd: 1.000 . . . . . . . . . . . . . . . . . . . 4L60's 4th: 0.696 . . . (5 speed manuals, woefully underappreciated)
6L80's 4th: 1.152 . . . 6L80's 5th: 0.852 . . . 6L80's 6th: 0.667 . . . (too bad 6L80 lacks a direct drive gear)
8L90's 6th: 1.000 . . . 8L90's 7th: 0.845 . . . 8L90's 8th: 0.652 . . .
10L80 7th: 1.000 . . . 10L80 8th: 0.854 . . . 10L80 9th: 0.689 . . . 10L80 10th: 0.636

In other words, 4L60's last gear ≈ 6L80's last gear ≈ 8L90's last gear ≈ 10L80's 9th gear.
In other words, only 10L80's 10th gear meaningfully improves MpGs -
in conjunction with direct injection at several thousand psi, 5W20 motor oil, newer ATF,
and Engine Confusion, which many of you are forced to disable by avoiding 10th entirely,
if you can't disable it with a plug-in.

 

[Bigburb3500]

The next-to-final and final gears of the 6L80 & 8L90 are pretty damn close to the same,
and 8th and 9th of the 10L80 are pretty damn close to the same too.
If you can take advantage of 10th (can keep the TCC locked) and downshift to 9th as rarely as possible,
yes, it can be worth a few MpG.

However, if you cruise at a high enough road speed, the 10L80 MIGHT vacillate between 9th & 10th?
Which I guess would still be better than just 9th?

Oh, by the way:
more to come ...

They might be close but the truck cruises so close to the sweet spot. I’m high jacking this thread so I’m going to stop, but we can continue this back on the 3500 thread.

 

[CMoore711]

The more I am seeing these swaps, the more I want to intentionally find a K2X with a bad motor specifically to do this...

I know. That’s a diamond in the rough, but it would be ideal to find a K2XX with lifter failure that someone wants to dump instead of fix.

13 MPG isn’t great, but it’s not terrible. That being said 13 MPG of regular unleaded 87 octane is leaps and bounds better than 13 MPG on 91/93 Premium fuel.

I’m sure your mpg will get better as you get the tune dialed in.

 

[tooleyondeck]

13 MPG isn’t great, but it’s not terrible.

Well I don't think anyone bold enough to do this swap cares about MPG, that's why they make the Traverse lol

It's only a matter of time until someone throws one of these in a single cab Silvy/Sierra, if they haven't already, make a mean little truck.

 

[chevman88]

@chevman88 What grade of fuel are you running in the L8T? It is designed to run 87, and that is what I ran in mine, but I am curious what your stance is on the matter.

Still running 91, I figure after the dyno I'll decide whether or not to drop back to regular, which for me is 85 .
Had to bump back the date to next Tuesday due to scheduling conflicts, but I'm pretty stoked. It drives pretty good as is, but getting it fine tuned should make it feel much better.

As far as mpg goes, the extra torque makes it hard to keep my foot out of throttle, so I'm ok sacrificing a bit and the wife tends to agree . The reliability alone was worth the swap to me.

 

[L8T BURB]

Be mindful of oil consumption issues. Before and after my L8T swap, I followed a few GM HD forums where owners would discuss oil consumption. I changed the oil in my L8T every 3000 miles. Perhaps a little excessive, but seemed like a small price for peace of mind. I didn't have any notable oil consumption issues with my L8T, but then again, I sold the vehicle after about 14,000 miles on the swap.

 

[Marky Dissod]

I didn't have any notable oil consumption issues with my L8T, but then again, I changed the oil in my L8T every 3000 miles.

Fixed it for you.

1. If you change your oil every x,x00 miles, and you get 0.5Qt of oil consumption (or more, duhr),
change your oil 500 miles sooner next time.
2. Repeat step 1 until you get UNDER 0.5qt of oil consumption, then stick with that interval.

A. If y'feelin froggy, and you get NO oil consumption at all, change your oil 500 miles later next time.
B. Repeat step A until you notice oil ANY consumption. If you get 0.50Qt or more of oil consumption,
see step 1.

Step C3. Install a catch can.

Direct injection engines spray fuel into the combustion chamber at ≈150Atmospheres,
compared to 3 or 4 atmospheres for Gen3 or Gen4 V8s.
Until the piston ring gaps get smaller, SOME of that fuel is going to end up in the crankcase,
which is the major cause of oil consumption in an otherwise properly working engine.