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4bt to 6r80 trans

7839 Views 35 Replies 11 Participants Last post by  Redark1
I've decided to go another route on my 07 expedition. I've been rolling with a m5r2 trans for the last 5 years and 35k miles and something in the clutch system is not good. I also haven't been able to get 4x4, or cruise control working. So its time to change it up. I'm going with a 6r80 trans which is quite proven in the gasser world. So I have a few questions, I doubt the trans has been put behind a 4bt yet. Ill be machining the adapter plate myself. My current power is probably 225hp...assuming 450tq. The trans is easily rated for this from the 5.4.

Should I use a stock converter to get a baseline for how it will run? Will a stock HD flexplate be good for this application? Will a 5.4 starter start a 4bt?

Trans will be tuned with hp tuner.

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That is a seriously strange way of doing what you want.

That flexplate center thing you machined looks iffy. What is up with the big oops groove cut into the face? Are you a machinist or a hobby guy with a lathe and a mill?

When flexplates fail it's usually in the high stress area right where you put your weld. Welds done by hand are a bit iffy for stuff like this. Flexplates are rather intolerant of thickness changes like that. By design, it flexes, and flexes differently depending on load and line pressure (converter is basically a metal balloon).

Hope it works, but you have gone down a misguided rabbit hole IMO.
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