2000 3500HD (GMT455?) 7.4 to 8.1 swap

[deezlugnutz]

I have a 2000 3500HD cab & chassis with a dump bed that has a trashed 7.4L. Nothing really salvageable but the block so gave it to a kid I work with who wants to build it for his project truck. Believe it has the 4L80E transmission.

For a parts truck I have a complete 2006 C6500 with the 8.1L and an Eaton 6-speed. Trans is junk but the engine runs good. Was toying with the idea of swapping this 8.1 into the 3500HD. Would probably want to stick with the auto as nobody at our company can drive a manual without destroying it.

Wondering how feasible of a project this would be. I'm imagining needing to take most of the ECM and harnesses off the C6500 in order to get this thing to run in the 3500HD. Guessing I might be splicing together a lot of harnesses. Not sure if the 8.1 would have a different flywheel housing since it had a med-duty Eaton trans (FS-5406A iirc). Thoughts?

 

[someotherguy]

The ultimate donor truck would be a 2001-2002 3500HD with the 8.1 as those were factory setups. You'd have everything - harnesses, PCM, I believe the TAC module (pretty sure those were DBW trucks) ...

As far as the transmission, you're re-using the 4L80E in your existing truck, right? It'll bolt right up to the 8.1. You'll have some PCM stuff to figure out for sure, I'm not even close to an amateur here but I'm betting you will need a different PCM than your current 3500HD one or the one from the C6500.

Anyway sounds like a great project and having the HD with good transmission and an 8.1 you've got the big money parts covered already and it's likely down mostly to harnesses, PCM, and labor.

Richard

 

[Supercharged111]

It really depends on how savvy you are. Were it me, I'd amend the existing harness to add the coils and whatnot to the 7.4 harness, 411 swap it, and start with an 8.1/4L80 Suburban tune.

 

[Moofus02]

The 8.1 will bolt right to the 7.4 mounts and 4l80. 0411 or p59 computer with a suburban tune. They had the 4l80. The 06 probably had the p59 and if so I'd use that. Being a 2 ton donor truck does it have the high mount ac Brackett? Vans and heavy trucks were where they were found. You could segment swap a 4l80 tune into the donor computer. I know square body guys use the drive by cable 454 throttle body but I would keep the drive by wire. I would probably use the 8.1 harness and adapt to the truck. The 8.1 is a fantastic engine especially for a work truck. L31maxexpress has a van and Tahoe 8.1 swapped i believe. He is a wealth of knowledge. He also p59 swaps his stuff. The 06 is running the latter can sensor and crank sensor so the p59 would be the better computer. The 0411 is done a lot and he swaps from 0411 to p59 and could give you info on that. Good luck

 

[someotherguy]

If you look up some 2001-2002 3500HD's you should be able to find some with the 8.1 to give you an idea of the engine bay layout to determine what other misc. parts you may need like the air cleaner setup, how the PCM mounts, etc. I've got a couple pics but they're not high res. By those last 2 years of the HD the only 2 engines available were the 8.1 or the 6.5td.

And yes, GMT455 is the correct designation for the HD models. There's a FB group about them which may net you some info or parts as there will occasionally be someone parting out one they've given up on due to rust etc. https://www.facebook.com/groups/220730835136705

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Richard