LS transmission questions and other Ls stuff.

[Awann99]

So I’ve finally decided to begin the process of slowly piecing together everything I need to LS Swap 1969 Lwb gmc 1500. I drive 66 miles round trip 5 days a week. Most of it is highway driving and the 350/350 combo getting 11 mpg is cutting it. Fuel economy is my main goal in this swap so I was thinking a 5.3 mated to a 6L80e. Double overdrive = better mpg in theory. I already know I will have to install a big jump in the truck for the transmission to fit. I would keep everything stock and get the stock harness rewired. I’m not look for preformance. My truck has a stock 308 rear ended and I just got a parts truck with a 354 rear end. What rear end would help keep the rpms lower? I’m swapping discs onto the truck as well. I am most likely going to find a junkyard combo because it’s cheaper. My question is what are ways to make this things extremely efficient. I’m not worried about power. I won’t be pulling anything. I just want a good daily driver. Will a older 5.3 mate up to a 6l80e? Please give me your advice.
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[Overdriven]

Let’s start with the 6L80e, unlike the 4L80 and 4L60 which are controlled by the ecu, the 6L80 has its own control unit built in that communicates via CAN bus to the ECU, this requires a compatible ecu regardless of what engine you get. While possible to adapt a newer ecu to an older engine, there’s not likely to be any cost savings but likely more headaches. You’d also be better off getting a complete engine and trans package from the same vehicle to avoid tuning headaches. For example a 2014 engine from a pickup and a 2014 trans from a van (or if the engine and trans are different years) this may require a “segment swap” in the ecu which doesn’t always go smoothly and can complicate the tuning process. Lastly let’s talk about 6l80 mileage vs a 4l60. The 6l80 should get you better mileage around town since it has more gears, can keep the rpm where the engine is most efficient. But cruising on the highway, both trans are going to be in the highest gear, torque converter locked up meaning very little difference if any in mileage. I haven’t looked up ratios for either trans but maybe one trans has a slightly better final ratio for a 1mpg advantage.

Rear end gears. 3.08 gears will lower engine rpm compared to 3.54 gears. But there comes a point where lower isn’t always better. With 3.08 and overdrive, rpm will be so low the trans isn’t likely to shift into its final gear and lockup the converter until you’re going way too fast to be fuel efficient. There are guys with 5.3 4l60 3.73 getting 20-22 mpg highway, 3.54 might increase mileage, it might hurt mileage as you might need too much throttle to maintain speed. It’s a balancing act and it helps if you run some numbers through a gearing calculator, there’s a bunch of them online. Check out the LS swap thread in the “Engine and Trans” section of the forum to see what others run. I always like to look at what the factory does for rear gears as a reference, they pour lots of $ into getting the best fuel efficiency. If mpg is the end all for you though, I hear the 2016 and up v6 Camaro gets 27-28mpg (real world owner reports) on the highway.

[57taskforce]

Originally Posted by Overdriven

Let’s start with the 6L80e, unlike the 4L80 and 4L60 which are controlled by the ecu, the 6L80 has its own control unit built in that communicates via CAN bus to the ECU, this requires a compatible ecu regardless of what engine you get. While possible to adapt a newer ecu to an older engine, there’s not likely to be any cost savings but likely more headaches. You’d also be better off getting a complete engine and trans package from the same vehicle to avoid tuning headaches. For example a 2014 engine from a pickup and a 2014 trans from a van (or if the engine and trans are different years) this may require a “segment swap” in the ecu which doesn’t always go smoothly and can complicate the tuning process. Lastly let’s talk about 6l80 mileage vs a 4l60. The 6l80 should get you better mileage around town since it has more gears, can keep the rpm where the engine is most efficient. But cruising on the highway, both trans are going to be in the highest gear, torque converter locked up meaning very little difference if any in mileage. I haven’t looked up ratios for either trans but maybe one trans has a slightly better final ratio for a 1mpg advantage.

Rear end gears. 3.08 gears will lower engine rpm compared to 3.54 gears. But there comes a point where lower isn’t always better. With 3.08 and overdrive, rpm will be so low the trans isn’t likely to shift into its final gear and lockup the converter until you’re going way too fast to be fuel efficient. There are guys with 5.3 4l60 3.73 getting 20-22 mpg highway, 3.54 might increase mileage, it might hurt mileage as you might need too much throttle to maintain speed. It’s a balancing act and it helps if you run some numbers through a gearing calculator, there’s a bunch of them online. Check out the LS swap thread in the “Engine and Trans” section of the forum to see what others run. I always like to look at what the factory does for rear gears as a reference, they pour lots of $ into getting the best fuel efficiency. If mpg is the end all for you though, I hear the 2016 and up v6 Camaro gets 27-28mpg (real world owner reports) on the highway.

All good points. If you go 5.3 I would steer clear from the afm engines as they are prone to lifter problems. That’s just my personal preference. Those engines get the best mileage but it’s no fun when you have to pull the heads to replace the lifters... if you lucky that’s all you have to replace. If you do end up with an afm motor, id look into deleting the afm and keeping the vvt that they come with. Vvt will help you get better mileage and has proven to be reliable. The Camaro v6’s in the 5th and 6th gen both get good mileage with exceptional power. I will say the l99 6.2 ls3 variant in my Camaro has gotten 27mpg a couple times on long trips keeping my foot out of it (hard to do with 400hp)
As said the ls swap section has TONS of info.

[webfoot]

A 5.3 will also bolt right up to your th350, FYI. You will just need a different flexplate, and advance adapters sells these.

I'd go over your swap build carefully and price out how much the build will cost for a gain of 7 mpg max.

[Awann99]

Originally Posted by webfoot

A 5.3 will also bolt right up to your th350, FYI. You will just need a different flexplate, and advance adapters sells these.

I'd go over your swap build carefully and price out how much the build will cost for a gain of 7 mpg max.

I feel like it would be pointless to go through the work of putting a LS in the truck without a modern transmission. I could swap in a 700r4 and put a stand-alone fuel injection kit on the 350 to gain a few MPG but the cost for that would be almost as much a full ls swap. I will be commuting for the next two years minimum so I want the gas milage most.
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[mongocanfly]

i would just look for a 5.3/4l60 combo pullout..look for the earlier yr models..then youll have everything you need.. if you go with dbc itll be a little cheaper...if you get dbw be sure you get all the accelerator parts with the engine....the LS has already proven itself to be dependable in stock form .....no real need for a 6l80...the final od gear in a 60 and 80 (4th&6th )is practically the same...but I would not go any higher than a 3.73 gear and that depends a lot on your tire size as well..if you have a 3.54 rear it may be ok ...there again,depends on your tire size..im running 4.10s in one and 4.56s in the other with a od trans.....as stated above you can get the rpm to low with a high gear

4l60
1-3.059 2-1.625 3-1.000 4- 0.696 r-2.29


6l80
1-4.027 2-2.364 3-1.532 4-1.152 5-0.852 6-0.667 r-3.064

[95 S_Trucker]

If you get 20mpg(might be a stretch) and you go 18,000 miles a year, and gas is 4.00 a gallon, you would have a fuel savings of 2945 per year.

[First c10]

My sons truck 70 lwb has a 2011 lc9 5.3 6l80 with 3.08 rear gear. It gets in the low 20’s I disabled the DOD/AFM in the tune. It has 30k on it since swap. It had 77k on it when I got it. With the 6l80 3.08 gears are great but you will need a posi.
Try and get it as a unit don’t peicemeal as it will be tough to get it to work together that way.
I am working on a mild cammed 4.8-4l65e. It was a 305hp before doing anything. I hope to get 20 mpg. They are cheap mine complete package was a 2008 delivered from Ohio with 83k for 2K$ lots of options out there just depends what you need.

Oh and if you go 4l60-80 you will want 3.42-3.73 rear gear as 3.08 maybe to high.

[Overdriven]

Originally Posted by Awann99

I feel like it would be pointless to go through the work of putting a LS in the truck without a modern transmission. I could swap in a 700r4 and put a stand-alone fuel injection kit on the 350 to gain a few MPG but the cost for that would be almost as much a full ls swap. I will be commuting for the next two years minimum so I want the gas milage most.
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There are scenarios where LS with th350 makes sense. Like if you don’t have the budget for a modern trans, you don’t need to modify your driveshaft saving some more $.

Also depending on the rear gears used you could end up with similar highway mileage. 2000rpm at 65mph is going to net a certain mpg, doesn’t matter what trans and rear gear combo you use to get there.

[Wgesnerjr]

Not sure but you may want to consider a 4.8/4L60E swap. The cost of the unit from one vehicle might be cheaper and MPG might be better. The swap would be the same amount of work but the cash saved might be worth it.