Automatic Transmission Recommendations for an '81
Safety Car
Joined: Jan 2009
Posts: 4,000
Likes: 299
From: New York
Anyway sure toss a 2004R in it just run it in 3rd in town and I guess use O/D on the highway .. at 65 your at 2560 rpm ? If you cant get a 2004R the 7ooR4 will have added costs for cross member and drive shaft shortening .
With a 200 unit you will be at 1600 at 65 and the torque converter is gonna lock on and off on any grades ..unless you get that feature eliminated .. In town the O/D will dog you right down. Not sure if it is worth going over 65 to limit the car in so many areas with an extra gear but you decide.
Look at the rpm drop , and realize the energy needed to keep the car going is the same but at which RPM is the engine better suited to maintain correct in traffic driving behavior. Thats why when people usually go to the O/D on a carb motor your rear gear should be lower then you have because it gets the motor closer to an rpm it can behave most correctly with.
It also allows gear heads to put a very low gears in for killer acceleration and still have it able to drive on the highway and get ok gas mileage doing it.
With a 200 unit you will be at 1600 at 65 and the torque converter is gonna lock on and off on any grades ..unless you get that feature eliminated .. In town the O/D will dog you right down. Not sure if it is worth going over 65 to limit the car in so many areas with an extra gear but you decide.
Look at the rpm drop , and realize the energy needed to keep the car going is the same but at which RPM is the engine better suited to maintain correct in traffic driving behavior. Thats why when people usually go to the O/D on a carb motor your rear gear should be lower then you have because it gets the motor closer to an rpm it can behave most correctly with.
It also allows gear heads to put a very low gears in for killer acceleration and still have it able to drive on the highway and get ok gas mileage doing it.
Last edited by diehrd; Feb 13, 2014 at 07:49 PM.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
You really need to consider the use and what the owner wants beyond just making broad recommendations.
With a 27" tall tire & 3.08 rear gears;
I'd say 1541rpm is rather low for a carb'd engine but 1800rpm should be doable if it's a milder engine that is properly tuned. Well, maybe not if you expect to climb mountains but overall a carb'd engine should be able to pull the car at 70mph down the interstates in much of the US without issue.
finallygotit - I don't believe Tucson, AZ is too hilly but I've never been there so maybe it's hillier then I'd expect. I'd say if you want the OD to make those 75-80mph runs better then go for it. You ~300hp engine should be mild enough it'd pull smoothly at 1800-2000rpm range you'd be running.
Another thing to note is that the 700R4 has a 0.7:1 ratio compared to 0.67:1 for the 200R4. So, the 700R4 will bring the rpm's up just slightly compared to the 200R4. It also have a steeper 1st gear to make taking off from a stop more fun.
Overall, you're likely not using OD unless you're running on the highway with that rear gear ratio but the same basically applies to a 3.08, 3.31 or 3.55 gear. You'll be using 3rd unless you hit the highway.
For a bit I ran my Impala with a 700R4, 27.7" tire & 3.31 gears and it cruised down the highway just fine running ~1700rpm @ 60mph. I did use drive in town - because it would try 4th and lug under certain circumstances. We're talking ~250hp engine here using a 220* duration hydraulic flat tappet cam so fairly mild. I'd bet it has 500-750lbs over a C3.
Another option is to use an 4L60e with a controller. Then, you can program each shift curve so you never lug the engine without any driver intervention required. Just stick it in "OD" and drive.
With a 27" tall tire & 3.08 rear gears;
Code:
Speed 60 70 80 TH350C 2300 2680 3068 200R4 1541 1798 2055
finallygotit - I don't believe Tucson, AZ is too hilly but I've never been there so maybe it's hillier then I'd expect. I'd say if you want the OD to make those 75-80mph runs better then go for it. You ~300hp engine should be mild enough it'd pull smoothly at 1800-2000rpm range you'd be running.
Another thing to note is that the 700R4 has a 0.7:1 ratio compared to 0.67:1 for the 200R4. So, the 700R4 will bring the rpm's up just slightly compared to the 200R4. It also have a steeper 1st gear to make taking off from a stop more fun.
Overall, you're likely not using OD unless you're running on the highway with that rear gear ratio but the same basically applies to a 3.08, 3.31 or 3.55 gear. You'll be using 3rd unless you hit the highway.
For a bit I ran my Impala with a 700R4, 27.7" tire & 3.31 gears and it cruised down the highway just fine running ~1700rpm @ 60mph. I did use drive in town - because it would try 4th and lug under certain circumstances. We're talking ~250hp engine here using a 220* duration hydraulic flat tappet cam so fairly mild. I'd bet it has 500-750lbs over a C3.
Another option is to use an 4L60e with a controller. Then, you can program each shift curve so you never lug the engine without any driver intervention required. Just stick it in "OD" and drive.
Last edited by lionelhutz; Feb 14, 2014 at 01:09 PM.
Instructor
Joined: Dec 2007
Posts: 201
Likes: 1
From: Tucson AZ
yeah, I have decided to definitely go to OD with my new 350 and the stock 3.07 rear. If i encounter the "falling on my face" I will use 3rd more and look at changing the rear end.
Tucson has mountains all around, but the highways are flat.
Can't wait to get this thing reassembled!
Tucson has mountains all around, but the highways are flat.
Can't wait to get this thing reassembled!
1st Gear
Joined: Aug 2020
Posts: 1
Likes: 0
Thm350
I am a tranny guy , I dont make a suggestion because I think it is acceptable I post because I know from experience.
HIS BEST choice with those gears on a 1980 with a factory lock up system is the oem th350c .. He gains a lot of hassles going to the 700R4 or the 2004R . I can list each hassle if need be but he gains nothing in way of performance , gas mileage , or driving experience.
Unless he plans for motor and gear upgrades most importantly "gear" up grades a 3.08 diff is a joke for a o/d trans , matter of fact monster and or BTO suggest taller gears with a o/d conversion. I think they say 3.31 and up with higher ratios ideal.
I made a typo earlier ... I meant 45mph .. not 60.. I went with 45 because so much driving is between 45 and 60 that it is a very important speed to consider.. And where he will fall on his face with a o/d trans
HIS BEST choice with those gears on a 1980 with a factory lock up system is the oem th350c .. He gains a lot of hassles going to the 700R4 or the 2004R . I can list each hassle if need be but he gains nothing in way of performance , gas mileage , or driving experience.
Unless he plans for motor and gear upgrades most importantly "gear" up grades a 3.08 diff is a joke for a o/d trans , matter of fact monster and or BTO suggest taller gears with a o/d conversion. I think they say 3.31 and up with higher ratios ideal.
I made a typo earlier ... I meant 45mph .. not 60.. I went with 45 because so much driving is between 45 and 60 that it is a very important speed to consider.. And where he will fall on his face with a o/d trans
