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I killed my TH350 recently. Not sure what happened and won't know till I tear it down but it was definitely metal to metal and locked up the tires at times. This happened shortly after a boost spike (just shy of 30psi). The fluid that I drained out looked like dark red mud. The fluid has only had about 1,000 miles on it(put it in last year). Gotta find a shop that will cut the PTC torque converter apart and see what's going on in there.
At any rate, I'd purchased a used (152,xxx miles?) 4L80E for a winter install anyway so I've been gathering stuff for it. One thing that stuck out in my mind was the harness and connector that everyone was saying they'd had to account for by either beating the underside of their Fbody's to gain clearance of sawzall, etc to allow the connector to fit. I thought to myself, "Why doesn't anyone sell a 90 degree harness connector for this application?". Enter BP Automotive. I have BP's plug and play harness for my 5.3 in my 96 Z28. Flawless installation and perfect connectors. Not cheap, but no headaches or worries about "where does this go?". I found the 4L60E to 4L80E harness quite by accident when looking and quickly hit the 'buy' button when I saw it was BP's. It comes with the additional speed sensor connector (the 4L80E has input and output speed sensors) and instructions (with color pictures) on how to interface the connector to your PCM.
I'll take a picture tonight when I go to the shop but yeah, it's right up against that support 'rib' as she sits right now.
What did you LT1 Fbody style guys use for a 4L80E transmission cross member? I'd hoped I could use one of my existing BMR (T56 and TH350 styles) but in mocking it up last night, the TH350 was an absolute no-go while the T56 version would only come close if I moved the entire bracket back and used the rearmost cars bolt holes with the front BMR bolt holes only. I imagine I can cut some steel strap and use that to extend the bracket back sufficiently, but it leaves it with poor vertical support since the trans mount is so far behind the cars trans mounting bolts.
I'll take pictures of that as well. I suppose even that's not really viable in terms of the torque arm mount, being that I'm putting the entire backet back 3 or more inches. Looks like I'll be fabricating one by cannibalizing the BMR T56 bracket to get some needed surfaces.
Originally Posted by 2kgtp
I still had to beat my tunnel in to accommodate the plug.
Here's the picture of the connector in place. I marked the 'rib' and took the tranny down so I can 'convince' it to have a different contour with my BFH.
And here's one side of the BMR LT1 T56 torque arm relocation cross member, one bolt hole back.
As you can see, it *almost* lines up with the 4L80E trans mount. I checked my Dennys nitrous ready drive shaft and YAY, it looks like I can use it as is. Also, the torque arm might still fit if I can come up with a way to make the BMR cross member work.
I went over to the shop tonight and played around with a few things.
The BMR torque arm relocation /trans cross member will likely work with a notching here and there. Hits the trans oil pan in a couple places but other than that, I think I can make it work if I cut off the old trans mount flange and weld on a new one.
Mounted my Denny's Nitrous Driveshaft with the billet 1350 400/4L80E Yoke and mocked it up and it looks like it fits perfect. Can't be certain till everything is in place but right now, looks like it'll bolt up.
Almost finished with fabbing up the 4L80E cross member. I'll have pic's by tomorrow. I used my old BMR LT1 T56 torque arm relocation cross member, cutting off the pad and welding a new one on there. Truth be told, I could have left it intact and just put some 1/4" mild steel spacer plates on the Left and right cross member chassis mounting flanges and probably been fine. The 'real' fab work was creatine a strong flange that holds the trans mount itself.
Just an update - The homemade flange works great. Driving around in town, on the tollway, etc is really enjoyable now that the engine can utilize the overdrive. Completely a great upgrade. I hope to buy a 2nd Junkyard 4L80E to disassemble and put a bunch of good parts into it. I already purchased a new PTC torque converter and its sitting here in the shop.
Underboost, the contrast between the race built TH350 and the bone stock junk yard 4L80E is vividly evident in the HPtuners scans and just the impression in the drivers seat. The TH350, stalled with a 3200 PTC torque converter, if you were to see the HPTuner scan, you'd see it shoot down in a linear but vertical manner getting into the upper KPA ranges quickly. With the stock converter/stock 4L80E, when it goes into boost, the cells in the scan shoot out near horizontally in RPM before starting to climb the KPA. But while the TH350 screamed and flung you forward, you didn't feel the shift so much from 2nd to 3rd. Then doing the same thing in the 4L80E, when it shifts from 2nd to 3rd, in the drivers seat you seem to feel an extra 'G; of compression, It seems like you're being flung forward ever harder. Very enjoyable!
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Converting from 4L60E to 4L80E Harness 
