Just purchased a 98 GMC 3500 dually 2WD to use as a tow vehicle. It has a blown up vortec 454. Previous owner tore into it but ended up not finishing what he started, so he sold it to me. My question is, a friend of mine has a complete drop in 8.1 from a newer model truck. Would it be possible to switch everything over to the 8.1 setup? Im pretty sure it will bolt up to the transmission thats currently in the truck (4L80E I believe), but correct me if it won't. I just need to know about the electronics, and heck, basically anything else you guys can tell me. Any information or recommendations will be appreciated. I tried to search forums on this but couldnt find anyone who had actually completed the whole swap. Thanks!!
CK5
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Help Needed! Vortec 7.4 to 8.1 Swap??
- Thread starter cdk5
- Start date
it will literally bolt straight in using the 454's engine mounts. You'll probably have to get the PCM flashed to eliminate the VATS system, and also likely have to change your exhaust, but otherwise you should be able to figure it all out fairly easily 
odin544
Registered Member
yup its a very simple swap. like already said just have the ECM flashed. 4l80e's came behind the 8.1L. cant get any easier.
Ok. Thanks for the info. So, the gist is that it will definately bolt straight in the truck and up to the tranny. On the electronics side, I will need to flash the ECM and take care of the VATS system. I guess I could bypass the VATS if necessary. Will my local chevy dealer be able to flash the ECM? Also, how about the wiring harness? Will the harnesses from the 8.1 plug into whats already in the truck? Anything else I'll need to know just let me hear it. Thanks.
Streetgliderx
1/2 ton status
i been debating this same swap i got a 98 3500 7.4 and my paw just bought a new one with an 8.1 its a strong motor. but is it not a distrubutor-less motor? i thought it had coils for every cylinder. will there not need to be some changes made to the wiring harness?
yeah, you'll want to use the full 8.1L wiring harness. You guys can both sell the fuel injected 454s for a very good price too, if you keep the harness, ECM etc with it.
odin544
Registered Member
I think you might confuse the stealership if you ask them to reflash your ecm.
jimsperformance.com can do any wiring harness mods and flashing of the ecm.
Streetgliderx
1/2 ton status
i think the vortec 7.4 will find its way to the blazer when i swap!!!!
Yeah, the 8.1 doesn't have a distributor, it uses a coil by plug setup. Im sure the motor will bolt up and work fine, just still kind of in the dark about the wiring harnessess and the computer. I would love to have the 8.1 in the truck. I have rode in a newer 3500 with the 8.1, and its strong. I need some more positive insight before I go and buy this 8.1. I want to be 100% positive it will work before I spend the money....
You'll need a wiring harness and computer for a suburban 8.1l. The pickups used an Alison trans with a seperate computer.
Possible issues include:
Throttle by wire- specificly getting and mounting a throttle pedal and sensor
A/C compressor to frame clearance, fabbing A/C lines
Longevity of 4l80, the 8.1 burbs used a 4l85. Not a big deal more of a note.
Mounting sensors for stock instrument cluster, 8.1 trucks use a smart cluster that receives information from PCM. They don't use conventional 1 wire sensors like the 454 uses. I haven't looked to see if there are additional plugs for additional sensors.
None of these issues are difficult to deal with but do require some thought and planning.
My personal opinion is build the 7.4, add some parts like a better cam, a good set of full length headers and an exhaust system and blow the doors off of any 8.1 for a lot less hassle. There are almost no performance upgrades for a 8.1 except a Whipple for a meesly 6 grand. What you install is what you get. In contrast to the almost limitless aftermarket for a gen 6 big block. Make sure you think this through before you get into a big hairy project that has limited benefis.
Possible issues include:
Throttle by wire- specificly getting and mounting a throttle pedal and sensor
A/C compressor to frame clearance, fabbing A/C lines
Longevity of 4l80, the 8.1 burbs used a 4l85. Not a big deal more of a note.
Mounting sensors for stock instrument cluster, 8.1 trucks use a smart cluster that receives information from PCM. They don't use conventional 1 wire sensors like the 454 uses. I haven't looked to see if there are additional plugs for additional sensors.
None of these issues are difficult to deal with but do require some thought and planning.
My personal opinion is build the 7.4, add some parts like a better cam, a good set of full length headers and an exhaust system and blow the doors off of any 8.1 for a lot less hassle. There are almost no performance upgrades for a 8.1 except a Whipple for a meesly 6 grand. What you install is what you get. In contrast to the almost limitless aftermarket for a gen 6 big block. Make sure you think this through before you get into a big hairy project that has limited benefis.
odin544
Registered Member
Not all the trucks used the allison. I just bought another 8.1 wednesday out of an '06 1 ton pickup with a 4L80E. You will need the full harness and ECU for the motor. Send the harness and ecu off to have it flashed. It's really that simple. Call jimsperformance and talk to the guy. He's very helpful. He will eliminate the wires you dont need and pull them completely from the harness. Not just cut the ends like some other places do. You can have him eliminate the VATS and other emissions you choose not to run. As far as the throttle goes, you can stick with the fly by wire or run a cable throttle body. There are many TB's that will fit. I am going to use the fly by wire. If you choose to run that you want to make sure you get either all new stuff, right at $300 from the dealership, or get all the parts from the same rig. There is the pedal assembly, and TAC module, and a short peice of wiring that connects the two. Your engine wiring harness also plugs into the TAC module.
Im not sure if the AC will clear your frame since they are located low on the passenger side. On my '80 it wont fit. Thats ok though I dont plan on running it anyways.
4l80E's will survive fine behind the stock 8.1. Afterall the came stock. I got my other 8.1L out of a motorhome which also had a 4l80e behind the 8.1
The aftermarket for the 8.1 just isnt there yet. I know a couple companies have carbed intakes in the works. They didnt give any indication on how long until they would be available. You could also run a sheetmetal intake from caompanies like Hogan for $3500. There is a set of heads and aftermarket intake. But they are BIG money! You can get a whipple used for about 2K, New for 6500. Comp cams makes cams. Honestly the Marine industry has more aftermarket than the auto. some things will swap over and some wont.
Im not sure if the AC will clear your frame since they are located low on the passenger side. On my '80 it wont fit. Thats ok though I dont plan on running it anyways.
4l80E's will survive fine behind the stock 8.1. Afterall the came stock. I got my other 8.1L out of a motorhome which also had a 4l80e behind the 8.1
The aftermarket for the 8.1 just isnt there yet. I know a couple companies have carbed intakes in the works. They didnt give any indication on how long until they would be available. You could also run a sheetmetal intake from caompanies like Hogan for $3500. There is a set of heads and aftermarket intake. But they are BIG money! You can get a whipple used for about 2K, New for 6500. Comp cams makes cams. Honestly the Marine industry has more aftermarket than the auto. some things will swap over and some wont.
