6.6 Gas L8T Engine Swap in 2015 Suburban

[Tremek]

the 4l60 ratio spread reminds me why I never want to regularly drive anything with 3 gears + OD (or worse, just 3 gears and no OD like the THM400 in my ‘77 GMC 3/4 ton camper special pickup with a 454) again!

Years ago I bought my wife a 2008 Saab 9-7x Aero (occasionally known as the ‘Trollblazer’) with a LS2 + 4l65e and again the transmission was easily the worst part of the entire experience. That truck sounded great but drove in the same underwhelming way of all GMT360s, and would have benefitted from a 6l80 or 90 but I’m sure old GM wasn’t doing any driveline development work on badge-engineered TBSS stuff at that point (although the interior was substantially better than a TBSS although again that’s not saying much.)

 

[Marky Dissod]

10L80: ... 4.69 ... 2.99 ... 2.15 ... 1.77 ... 1.52 ... 1.28 ... 1.000 ... 0.85 ... 0.69 ... 0.64 (MF6 in suvs)
8L90E: ... 4.56 ... 2.97 ... 2.08 ... 1.69 . . . . . . . . 1.27 ... 1.000 ... 0.85 . . . . . . . 0.65 (M5U)
6L90E: . . . . 4.03 . . . . 2.36 . . . . . . . . . 1.53 . . . . . 1.15 . . . . . . 0.85 . . . . . . 0.67 (MYD)
4L70E: . . . . . . . 3.06 . . . . . . . . . 1.63 . . . . . . . . . . . . . 1.000 . . . . . . . . 0.69 (M70)
4L85E: . . . . . . . . . . . 2.48 . . . . . . . . . . . . 1.48 . . . . . . 1.000 . . . . . 0.75 (MN8)

(Once GM determined that the GMT360 / GMT370 were taking big bites out of Tahoe sales,
that's when the GMT360 / GMT370's demise became guaranteed.)

 

[TollKeeper]

10L80: ... 4.69 ... 2.99 ... 2.15 ... 1.77 ... 1.52 ... 1.28 ... 1.000 ... 0.85 ... 0.69 ... 0.64
8L90E: ... 4.56 ... 2.97 ... 2.08 ... 1.69 . . . . . . . . 1.27 ... 1.000 ... 0.85 . . . . . . . 0.65
6L90E: . . . . 4.03 . . . . 2.36 . . . . . . . . . 1.53 . . . . . 1.15 . . . . . . 0.85 . . . . . . 0.67
4L60E: . . . . . . . 3.06 . . . . . . . . . 1.63 . . . . . . . . . . . . . 1.000 . . . . . . . . 0.69
4L80E: . . . . . . . . . . . 2.48 . . . . . . . . . . . . 1.48 . . . . . . 1.000 . . . . . 0.75

(Once GM determined that the GMT360 / GMT370 were taking big bites out of Tahoe sales,
that's when the GMT360 / GMT370's demise became guaranteed.)

That and the bankruptcy...

 

[L8T BURB]

I am considering the thought of selling. Anybody interested in this thing? 193k miles as it sits. Daily driven. Clean, clear Kentucky title. Own one of the VERY few 6.6 swapped Suburbans! Only around 10k miles on the 6.6 L8T w/6L90 combo. This thing hauls! My mind isn't completely made up, but just gauging interest at the moment.

I have a HUGE list of upgrades. Far too many to list here. This is the closest to a fully bulletproof setup that you can get in this platform with the engine and transmission setup.

Shoot me a message if truly interested and i can give more details and pictures.

 

[Tonyrodz]

I am considering the thought of selling. Anybody interested in this thing? 193k miles as it sits. Daily driven. Clean, clear Kentucky title. Own one of the VERY few 6.6 swapped Suburbans! Only around 10k miles on the 6.6 L8T w/6L90 combo. This thing hauls! My mind isn't completely made up, but just gauging interest at the moment.

I have a HUGE list of upgrades. Far too many to list here. This is the closest to a fully bulletproof setup that you can get in this platform with the engine and transmission setup.

Shoot me a message if truly interested and i can give more details and pictures.

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I wish I had the $$. Awesome ride.

 

[Marky Dissod]

I am considering the thought of selling. Anybody interested in this thing? 193k miles as it sits. Daily driven. Clean, clear Kentucky title.
Own one of the VERY few 6.6 swapped Suburbans! Only around 10k miles on the 6.6L L8T w/6L90. This thing hauls!
My mind isn't completely made up, but just gauging interest at the moment.

I have a HUGE list of upgrades. Far too many to list here.
This is the closest to a fully bulletproof setup that you can get in this platform with the engine and transmission setup.

Shoot me a message if truly interested and i can give more details and pictures.

If you rush to sell, you will lose LOTS of money.
Patience will find you a buyer who truly appreciates what you've done.

 

[L8T BURB]

If you rush to sell, you will lose LOTS of money.
Patience will find you a buyer who truly appreciates what you've done.

Yes sir. I planned to make this a long process to hopefully bring in the right buyer. Having no lien on the vehicle allows me to wait this thing out without getting frustrated that I'm still making payments on it lol!

 

[Marky Dissod]

What axle gear? (Forgot.)

 

[the 18th letter]

A niche buyer will pay for it, a regular car shopper is most likely scared off by the numerous modifications. Need to be very mechanically inclined, the average shop would probably mangle a repair on that rig.

 

[L8T BURB]

What axle gear? (Forgot.)

It is the standard gearing, I believe 3.08.

 

[Marky Dissod]

Normally I bytcz & whine about 3.73 or 4.10 with 6L80,
but with an L8T, I would not be surprised if it actually gets impressive MpGs with 3.08 ...

 

[L8T BURB]

Yes I have regularly seen 26-28 MPG on 50+ mile drives through rural 55 MPH state roads. Usually brings home 22 MPG at 70 MPH interstate travel.

 

[Marky Dissod]

Yes I have regularly seen 26-28MpG on 50+ mile drives through rural 55MpH state roads.
Usually brings home 22 MpG at 70 MpH interstate travel.

For the sake of clarity and posterity:
22MpG (or better), @≈55MpH, with a 6.6L V8, a 6L80, 3.08 axle, in a 3ton vehicle with the aerodynamics of a box?

Yet the gov'ts of the world have spent BILLIONS trying to convince us that engines need to be smaller?

 

[L8T BURB]

For the sake of clarity and posterity:
22MpG (or better), @≈55MpH, with a 6.6L V8, a 6L80, 3.08 axle, in a 3ton vehicle with the aerodynamics of a box?

Yet the gov'ts of the world have spent BILLIONS trying to convince us that engines need to be smaller?

At full disclosure, my wife has a 2021 Toyota Sienna Hybrid and I have owned 2 other Toyota hybrid vehicles in the past. I love hypermiling in them. A lot of times I will use the same hypermiling techniques in the Suburban, and do not use cruise control. It is a bit of a job, but the MPG returns puts a smile on my face lol.

 

[Marky Dissod]

For the sake of clarity and posterity:
22MpG (or better), @≈55MpH, with a 6.6L V8, a 6L80, 3.08 axle, in a 3ton vehicle with the aerodynamics of a box?

Yet the gov'ts of the world have spent BILLIONS trying to convince us that engines need to be smaller?

At full disclosure, my wife has a 2021 Toyota Sienna Hybrid and I have owned 2 other Toyota hybrid vehicles in the past.
I love hypermiling in them.
A lot of times I will use the same hypermiling techniques in the Suburban, and do not use cruise control.
It is a bit of a job, but the MpG return puts a smile on my face.

In other words, the driver's driving technique is roughly AS important as the choice of vehicle being driven?

(Problem with hypermiling in metro-urban NYC, is NYC - traffic lights make hypermiling almost impossible,
everyone else finishes making it impossible.)
(Problem with hypermiling on the highway, is everyone else INSIDE your vehicle.
Not easy to get them on board with 'pulse & glide', if I remember correctly.)

By the way, when you had your Suburban L8T's tcm tuned, did you bother to tell the tuner that you hypermile?

 

[Bigburb3500]

This is a great build thread! I just purchased an 18 Suburban 3500 with the 6L90 already so I wonder how easy the 6.6L would drop in?

Thanks for posting and good luck with selling. I also agree the Toyota hybrids are pretty great! Too bad our Suburbans are not offered with true meaningful hybrid systems. Oh well.

 

[Marky Dissod]

... wonder how easy the 6.6L would drop in?

No meaningful external dimensional differences between any 4.8L / 5.3L / 5.7L / 6.0L / 6.2L / 6.6L / 7.0L GM Corporate Small Block V8 ...
... physically, as easy as any other GM Corporate Small Block V8. Estimate the physical labour at 2 business days, give or take.

It's the programming that takes extra time.

 

[Bigburb3500]

It's the programming that takes extra time.

Marky, if extra time was my only issue in life I am not sure I would be on this forum… Having enough $$$ to buy the extra time is the issue hahaha
Not sure I can rationalize spending more money after just buying the truck. I will let you know when my $2 scratch off pays off or if the 6.0 gives up the ghost (doubtful).

 

[tooleyondeck]

If you are seriously considering selling this BEAST, you probably have a good shot at getting it featured in a magazine with as many pics/updates you provided on here. I could see this featured in Motortrend, C&D, or Street Truck...

 

[L8T BURB]

UPDATE

I have officially decided to sell the Suburban. I was wanting to get some meaningful opinions on a reasonable list price? My initial thought is $18,000. What do you guys think? I plan to clean it up this weekend and list it on marketplace.

A few highlights below...

2015 Suburban LT (all options that an LT can get w/rear seat screens, leather captains, 2nd row heated seats, moonroof, etc.)
197k miles
2WD
Full RST conversion with 22 inch RST wheels from a 2022 Suburban, RST grille and RST badging throughout

Within the last 10k miles the following has been replaced...

Engine AND transmission replaced with a 5 miles 6.6 liter L8T engine and upgraded 6L90 transmission (swap is well documented in this forum)
New shocks/struts in front and rear
New upper/lower control arms in front
New front wheel bearings
New upgraded custom driveshaft
Rear diff fluid flush
Brake fluid flush
ALL exterior lighting converted to LED

I am likely missing some of the details but I believe this sums it up. Please let me know a reasonable list price. At 18k, I am losing BIG but I am not sure what this thing is really worth, or if its even worth 18k... and its time for me to move on.