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Square Body LS Conversion

1981K20

1981K20

Up North
Joined
May 19, 2020
Member Number
369
Messages
219
Loc
North of MT but south of AK
Built a few rigs but never dabbled in the LS shit.

Looking to swap the engine from an 03 Silverado 4x4 into an 81 Silverado 4x4. I have both full trucks, both run and drive.

Easier to use entire driveline or just buy the parts required to mate the LS to the old transmission?
 
Entire thing.

Buy SBC to LS engine mounts, a conversion harness and e-fans, with an in tank fuel pump and be done.
You can use a TBI tank, with a Walbro 255 fuel pump on the TBI sending unit.
 
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I put an LS 6 in my 1985 chevy long before it was cool, back in 1990.
 
I would almost go with Grendel's solution except for the fact that you say "both run and drive"; conversion mounts for ~$70.

I would just use the existing harness and whittle it down; it is literally unbolt the main engine branch, track down a few wires, and burn out. Send out the PCM for $100 VATS and rear o2 sensor tune out(pay more if you want more tinkering).

I'd run the mechanical fan; they just work and if your radiator is up to it, a mechanical fan without a shroud WILL cool your engine fine. This is my setup in an old FJ55. It sticks 210 all day every day without a shroud using a Griffin alum rad in Sacramento 100 degree summers.

I'd probably look at making the transfer case of the 81 work so you can keep passenger drop; potentially cheaper than driver drop axle conversion. BUT, you don't say what the drivetrain is in both(ie auto vs manuals) but I'd guess the 03 is an auto.
 
Not hard to adapt the t-case from the truck.

If a 2003 auto, 27 or 32 spline output, with 6 bolt round pattern.

It would need a VSS.
 
Those $200 swap harnesses on eBay are not bad. I put one in a c10 the same time a buddy put a psi harness in his k20. Besides one had the cheap plastic loom and the other with the braided split loom they are almost identical. The quality of wire might be different but $500 different?

Besides that do what Grendel or LilJohn say, you can spend as little or lot as you want to do a swap.
 
Here is how "simple" the stock wiring harness is:
PCMelect1.JPG

In the dead center you can barely see what is left of the stock plug that goes into the fuse/relay box. Most of the wires are either always hot(orange) or hot in start/run(pink) with maybe 4-5 wires for "other" like starter, fuel pump relay, tach, OBD2. The above was pulled from the truck with the harness unmolested and maybe a day of leisurely fawking around to get the wiring cleared up.
 
X whatever the harness is very easy to modify

Being 03 it is dbw and very easy to retain cruise control. You can modify the stock 03 pedal to fit the firewall or you can get a 90’s diesel pedal, re pin a wire or two and bolt it up to your firewall.

In my ‘85 I used an 05+ radiator/Efan setup with factory relays to keep the swap as oe as possible.
 
I typically go with a new harness, as the OEM ones are old as fuck and probably half shorted.

A said above, they can be had cheap.
 
I'd want that over drive for sure

Mileage is a plus, but the sweetest part is humming along in the fast lane and seeing another old truck screaming in the slow lane:laughing:
 
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