mikeringland Posted August 4, 2023 Share Posted August 4, 2023 I have a '72 350/TH350 with a 12 bolt rear end with 2.73 gears single tire fire. I put in a 454 with 440HP/450 torque about 2 years ago. The rest of the drivetrain is still the same. I want to do an overdrive transmission, as well upgrade the rear end to posi. Talking to car buddies around town, they all told me to steer clear of the 700R4 & 4L60, and to go with a 4L80 and maybe 3.73's. This is a car I take out with my wife/boys to car shows, cruises, nothing crazy. I want a decent setup for highway driving when I do go on the highway. What do you guys think, what is the best setup for what I'm looking for? Also if I go the 4L80 route, is it a simple enough swap with the TH350, or are there all kinds of modifications I'll have to make to make that thing fit in my car? Thanks, Mike Quote Link to comment Share on other sites More sharing options...
EVC Posted August 4, 2023 Share Posted August 4, 2023 It's all a matter of opinion … Unless you're racing or trying to impress the cops at the traffic light, the 770r4 with some milder gearing is always a great choice …. My 700r4 was built to handle the horsepower you're running, but I'm a little over stock at about 325hp with 273 gearing and do everything I need below 3600 rpm …. That's great for me , longevity, dependability , some seriously long road trips, and the wallet. Driving the Interstates at high rpm is tiring and not for me. The only change I'd consider for my car would be 308 gearing… Based on your comments, I'd think you're not into racing ,but would enjoy a street car most. That's a lot of horsepower and either transmission you choose will be fine if properly built and you have the correct stall converter. I am a big fan of the 273 and 308 gearing regardless of the transmission. Anything else you need will be handled with the horsepower and torque of your engine. EVC. Quote Link to comment Share on other sites More sharing options...
Scott S. Posted August 4, 2023 Share Posted August 4, 2023 Going with the 4L series transmissions will require some electrical work for the shifting and torque converter lock up.... And re designing the cross member supports to the tail shaft as well as the output shaft/drive shaft. 700r4 was the "recommended" choice locally, but again, the output shaft/drive shaft and cross member will change.... But the wiring is much simpler. I went with a 200r4(BOP)hooked to a 350 hp 350 cid that I had rebuilt by a well seasoned mechanic. With all the upgraded internals, it's rated/guaranteed to 700hp. Not that I'll ever push that kind of hp. The 200r4 has the 2.67 to 1 first gear, and 0.67 to 1 in overdrive. The nice this about it is that, other than moving cross member back 6" to the other set of holes in the frame, everything else was a bolt in..... And with the lock up being internal, all that was required is a switchable power source to activate the torque converter. I'm using a 2400 rpm stall converter and 3.31 gears..... And on the highway I'm pulling about 20 mpg.... Originally had the 2.73 rear gear, but had a hard time finding the correct speedometer gear to get accurate mileage, and it was a touch sluggish in the city. That's why I went to the 3.31. Best recommendation is talk to a good transmission guy..... There are a lot of varied opinions and ways to go. Best of luck. Scott Quote Link to comment Share on other sites More sharing options...
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