My 4L60E held up great in stock trim all the way down to 11.3's. My computer logging started to show the 1-2 shift slipping so I had 1 $1,100 race build-up done to it. I left everything computer controlled except for installing a vacuum modulator.
If you have a choice, I'd go with the new 4L65E from GM performance parts. It comes stock with about a 1/3 of the stuff I did in my race build-up.
Mark
1974 White Super Duty Trans Am
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1999 Red TA FireHawk #397
2002 Best: 11.68@116 NA (10.7@127 on the Juice!)
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71 Firebird Custom - Fuel Injected & Over-drived!
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