Hello!
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Hello! Newbie here from NC. Planning on rebuilding the drivetrain in my 2003 1500HD for better highway trailering of a 30' gooseneck with a tractor and some implements.
my plan is to stroke my engine, building it for maximum low-end torque in the 1000-3000 rpm range, replace the 4L80 with a 6L90 and replace the 3.73 with a 3.21 for unloaded highway cruising.
Any advice or parts offers.
my plan is to stroke my engine, building it for maximum low-end torque in the 1000-3000 rpm range, replace the 4L80 with a 6L90 and replace the 3.73 with a 3.21 for unloaded highway cruising.
Any advice or parts offers.
I see a couple problems you'll have to overcome, The 6l90 will need a newer style computer to run it so you may need a engine and trans harness swap and probably need a 58x Reluctor wheel on crankshaft to match the computer. Going from 3.73 to 3.21 gears is a step backwards for pulling heavy loads, It may improve unloaded mileage but will kill your pulling ability with heavy loads. I have a truck with 3.23 and 3.73 gears I can't run the truck in OD with the 3.23 gear unless it's light load, When I do try it's constantly downshifting and heating up the trans.
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I see a couple problems you'll have to overcome, The 6l90 will need a newer style computer to run it so you may need a engine and trans harness swap and probably need a 58x Reluctor wheel on crankshaft to match the computer. Going from 3.73 to 3.21 gears is a step backwards for pulling heavy loads, It may improve unloaded mileage but will kill your pulling ability with heavy loads. I have a truck with 3.23 and 3.73 gears I can't run the truck in OD with the 3.23 gear unless it's light load, When I do try it's constantly downshifting and heating up the trans.
Harness and computer, yes...I think so. The TCM is in the tranny. I haven't investigated the details (where the Devil is!), but communication for both is via serial data bus. Don't know if GM changed the protocol, but certainly the information between the TCM and ECM changed. I think the TCM is integral to the ECM/PCM for the 2003 generation.
If I have to change the ECM then, yes, I will need to install a 58 tooth reluctor to agree with the math in the ECM.
On the diff gear, the 6L90 is 0.67 6th and 0.75 5th. The 4L90 is 0.75 4th so, when towing, I can pull back to 5th in the 6L and depend on the added torque and programming to keep me on rpm when loaded. Between that, a bigger engine radiator, and a bigger trans cooler, I think I'll stay cool.
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