Ls Swap Checklist

[Slooptin]

LS Swap gurus, I'm getting pretty close to pulling the trigger on a 6.0/4l80e combo, but I want to make sure I've got my ducks in a row before I get in over my head

Assumptions:
1. using stock pcm/tcm & wiring harness
2. engine and trans are running when pulled
3. comes with all accessories
4. comes with pedal (if it's not a cable throttle body)
5. no modifications right now to engine or trans. get it running and driving and I can do a cam or whatever down the road
6. reusing NP205 t case

Things I'll need to buy that I know how to source/have figured out:
1. motor mounts
2. NP205 to 4L80e adapter (this also takes care of OSS sensor if the 4L80e I buy doesn't have one)
3. Headers
4. Gauges
5. VSS for NP205 output speedo cable
6. I can reuse my current radiator (doesn't leak & my current 454 doesn't overheat) I'll just need to reuse the LS electric fans or find new ones + shroud & hoses.
7. I'll possibly need to shorten/lengthen driveshafts
8. I can figure out ps hose routing

Things I'm still unsure of:
1. I can use my existing fuel tanks/sending units, I'll just need to source a frame mounted fuel pump/regulator and figure out how to plumb a return (will this require tbi sending units for the tanks?)
2. NP205 "mount". Currently there is a truss rod that connects the 205 to the body of my th400. I'll need to figure out a mount for this
3. will the accessories from a 6.0 truck motor be fine where they are or will they need to be relocated?
4. I'll need to figure out how to run ac lines to/from compressor to/from condenser & drier
5. wiring-wise, if I keep the stock PCM/TCM, the only additional wiring I'll need to do are for the new o2 sensors (if the locations are different and I need to extend wiring), outputs for gauges & obd2 port, alternator/starter, ignition switch, potentially fans, & potentially gas pedal

Anything else I'm missing?

 

[Originalthor]

Two things I can think of is for you factory guages to work there's a adapter for both the oil and temp sending units.

Depending on the year of motor there is a lot of extra wires that don't need to be in the harness and are not to hard to remove and can clean up the harness Aswell. This is the site I used for mine. And lots of good info here.

Wiring Harness Information

 

[Bextreme04]

You don't need an OSS for the 4L80. You also shouldn't need any new adapter. It should be a direct bolt-up to your existing round pattern 205 using the stock TH400 adapter. At most you will need to have the index ring enlarged(or buy one that someone else has already modified).

I'm using this sensor screwed into the speedometer gear on the output of my 205. https://www.summitracing.com/parts/...g0y7Cd8bjN5yVLxrQEURUVA92f0RxLnQaApVWEALw_wcB

Be careful using one with less resolution as the factory ECU will have problems reading it at lower speeds.

I was planning on either extending the brace bar or adding a 1" spacer at the bellhousing. The 4L80 is only 1" longer than the TH400.

 

[CheemsK1500]

You may have to to change the oil pan, depending on the the clearance you get. In regards to the fan, the 6.0 LS has a mechanical fan like your current 454, an electric fan would have to be added after-market.

 

[Slooptin]

Two things I can think of is for you factory guages to work there's a adapter for both the oil and temp sending units.

Depending on the year of motor there is a lot of extra wires that don't need to be in the harness and are not to hard to remove and can clean up the harness Aswell. This is the site I used for mine. And lots of good info here.

Wiring Harness Information

I'll probably end up going with some dakota digital gauges

You don't need an OSS for the 4L80. You also shouldn't need any new adapter. It should be a direct bolt-up to your existing round pattern 205 using the stock TH400 adapter. At most you will need to have the index ring enlarged(or buy one that someone else has already modified).

I'm using this sensor screwed into the speedometer gear on the output of my 205. https://www.summitracing.com/parts/...g0y7Cd8bjN5yVLxrQEURUVA92f0RxLnQaApVWEALw_wcB

Be careful using one with less resolution as the factory ECU will have problems reading it at lower speeds.

I was planning on either extending the brace bar or adding a 1" spacer at the bellhousing. The 4L80 is only 1" longer than the TH400.

From what I've read, the 4L80e tailshaft is too long and you need a spacer to make up that difference or cut the tailshaft down.

 

[Slooptin]

I guess another question I have is, I have two options right now that I've sourced from pick n pulls. One has the complete combo minus wiring harness and pcm/tcm. the other has complete pcm/tcm. both are 2wd transmissions (so I'm assuming I'll need a 4x4 tailhousing). If I went with the combo without the wiring harness/cpu, could I just use any computer for a 6.0/4l80e combo, or does it have to be the cpu that came with the truck?

for refrence the "bare" combo is about 50% of the price of the "complete" combo

 

[Bextreme04]

I guess another question I have is, I have two options right now that I've sourced from pick n pulls. One has the complete combo minus wiring harness and pcm/tcm. the other has complete pcm/tcm. both are 2wd transmissions (so I'm assuming I'll need a 4x4 tailhousing). If I went with the combo without the wiring harness/cpu, could I just use any computer for a 6.0/4l80e combo, or does it have to be the cpu that came with the truck?

for refrence the "bare" combo is about 50% of the price of the "complete" combo

The harness will be the big one. You can get pigtails and a P01 or P58 ECM for about $50. There is no 4wd tailhousing. You just remove the 2wd tailhousing and you need to replace a plug at the rear of the case from the 2wd one with a hole in it to a solid plug for the 4wd. There are about 6 different 4l80e output shafts. For your application a 4wd output shaft would be a direct fit with no modifications needed. My 205 is an old figure 8, so I needed a long 2wd output shaft that was then cut down to ~5". If the one you get has a long fully splined 2wd output shaft on it, then you can just get the advanced adapter 1" spacer that will convert the TH400 index ring size to 4l80e index ring and then cut the excess off the shaft... it wont hurt anything to cut it down.

This is the adapter ring I got. https://northwestfab.com/products/gm-4l80e-to-th400-np205-adapter-spacer-kit

I have the original 4wd output shaft I removed when I put in the longer 2wd if you need it.

 

[Bextreme04]

If you can get a complete 6.0l/4l80e from a 4wd that includes the harness and ecm... you will save yourself a lot of trouble and added expense. It will basically only require that adapter ring and the VSS for the output on the 205. Motor mounts, fuel system, wire in the battery and ignition power and off you go.

 

[TotalyHucked]

There's some differences for the 4x4s over mine, but if you have Facebook, join up on the "LS Swapped Square Body GM Trucks ('73-'86 C/K & '87-'91 R/V)" page. Tons of good info there.

For my 2wd, I used the Tejas Steelworks engine mounts and trans mount and they worked fantastic. I think you can use the engine mounts but probably not the trans. Also, Tejas has a fantastic shroud for the 454 radiator. I've got that with dual 13" Spal fans and it keeps my truck cool even in south Alabama traffic in August.

I used a PSI Conversion harness, it was all plug and play except for about 5 wires that you have to run for power or to integrate in the stock harness.

I've got Speed Engineering 2 3/4" headers on mine and the clear with tons of room. I believe they work for most 4x4 guys as well.

For gauges, Dakotas are the way to go. Super easy install and they look great. Check out the Squarebody Syndicate versions if you like an OE look, that's what I'm running.

For driveshafts, I would definitely suggest getting new ones built and tell the shop your exact setup/planned upgrades. If you join that page, there's also some good discussion about "critical speed" and how the shaft is spinning so much faster with the overdrive, so stock shafts aren't meant to handle the additional stress.

For the fuel system, just buy '87 TBI sending units and Walboro 255 pumps. (one tip, if you swap them side to side, it'll give you some extra line for dropping the tanks in the future. As in, put the LH in the RH tank and vise versa) I plumbed my whole system with nylon AN line, super easy, and used an '06 Corvette fuel filter/regulator for the return (the returnless style fuel system from '03-up works the best and is most common. If you get a '99-02 engine with a return, then you just need a regular inline filter and plumb the return back to the tank). You can change tanks to the '87 TBI style as well to get the little baffle/fuel basket but it's not a necessity. Just be aware, you may starve for fuel around corners below 1/4 tank.

I've got a build thread for my truck on here if you wanna check that out or just let me know if you have any questions. I'd be glad to help

 

[CheemsK1500]

If you get a complete driveline out of a 4x4 truck, make sure it's a floor lever transfer case and not a push button style. This is irrelevant if you just adapt the 205 you already have to it though.

 

[CheemsK1500]

Speedhut sells a digital conversion box to plug your factory speedometer cable into.

The Easiest Speedometer Conversion: SPEEDBOX™ | GPS, Transmission | SPEEDHUT®

Designed to operate a cable-driven mechanical speedometer using built-in GPS, modern transmission signals, or CAN-BUS - the SPEEDBOX™ is the easiest speedometer conversion around. Shop the box now.

speedhut.com

 

[Bextreme04]

There's some differences for the 4x4s over mine, but if you have Facebook, join up on the "LS Swapped Square Body GM Trucks ('73-'86 C/K & '87-'91 R/V)" page. Tons of good info there.

For driveshafts, I would definitely suggest getting new ones built and tell the shop your exact setup/planned upgrades. If you join that page, there's also some good discussion about "critical speed" and how the shaft is spinning so much faster with the overdrive, so stock shafts aren't meant to handle the additional stress.

Since he's keeping the stock 205 and swapping from a TH400 to the 4L80, he shouldn't need to do anything to the driveshafts. The trans is only an inch longer and the LS motor mounts can actually move the entire assembly forward enough to make the transfer case be in the exact same place or an inch further to the rear at most.

The overdrive makes absolutely no difference to the driveshaft speed. Driveshaft speed is mechanically linked to the rear wheels... period. The only difference is going to be how high the engine RPM is at a given vehicle speed. If he can drive at 70mph before the conversion without the driveshaft coming apart, he will have no problem doing it afterwards. The donor 97 K2500 suburban that I got my 454/4L80e out of used the exact same yoke, driveshaft tube, U-joints, and rear end as my 1980 K25 pickup that everything is going into.

 

[Camar068]

get the fan controller module from the vehicle that the engine comes out of. Makes it pretty much plug and play

 

[Slooptin]

Damn, lots of good info guys, thanks.

 

[Bruce Wingate]

I'm planning on a very similar swap, except I'll have a 4l60e instead of the 4l80.

What are you planning for fuel? I would like to keep my dual tank set up, so I think i need to change the switch and the fuel selector valve from an '87. My tanks are brand new so I might go with the holley in-tank pump with the bigger mat that doesn't need baffling (almost the same cost in the end). I might also just run with one tank for a while