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Old 02-02-2014, 02:26 PM   #1
narnone
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Default Correct Diff gearing and stall speed for torque converter

I have a 1973 Cadillac 500 with #5 cam and higher compression pistons. The engine is 30 over.
I am mating it with a 700R4 which has a low first gear of 3.08 and a higher final gear of 0.7.
The car will weigh about 5000 lbs.
My understanding is my torque drops off at 2400 RPM.
I am not sure when the torque comes in and how fast it drops off.
Based on 235/75/15 tires my RPM at the following rear gears and cruising speeds are:
Speed - 3.73 ----- 3.53 ----- 3.23
60 ------1823 ---- 1725 ---- 1578
65 ------1975 ---- 1869 ---- 1710
70 ------2127 ---- 2013 ---- 1841
75 ------2278 ---- 2156 ---- 1973
80 ------2430 ---- 2300 ---- 2105
If my torque comes in early enough, I think 3.53 would be the best gears.
I believe my main cruising speed will be 70 mph.
Does anyone have any Ideas?
Does the 500 make enough torque to push the car comfortably at 1725 RPMs? Or will I be pushing the accelerator harder, taking out of over drive, to raise the RPMs to climb a small or medium hill?
Also, I thinking my torque converter should have a stall speed of about 1350. That way, at cruising speeds, my torque converter would be locked up and not slipping.
Is this a good stall speed?

Last edited by narnone; 02-02-2014 at 02:41 PM.
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Old 02-03-2014, 07:44 AM   #2
Nashalac
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3.53 and a stock convertor
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Old 02-03-2014, 08:35 AM   #3
narnone
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Thanks. I am still trying to find the gears for the Cadillac rear. It's a 1976. Any ideas?
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Old 02-04-2014, 08:07 AM   #4
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there are none
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Old 02-04-2014, 09:52 AM   #5
narnone
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I found that out. We are now looking at putting in a 9 Rear.
To pull this off, I need to change the rear. Even if I did not change the gearing, If I ever had to service the rear, I would be better off changing it.
I researched the original torque converter. It seems to have had a stall speed of 1200 rpm.
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Old 02-04-2014, 11:00 PM   #6
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I have no idea how you're going to get a TH700 to live behind an engine capable of that much torque, in a vehicle with that much mass.

I wouldn't put a lot of money into a torque converter. I think you're going to upgrade to a TH400, or a 4L80E fairly soon.
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Old 02-05-2014, 11:13 AM   #7
narnone
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My tranny guy said he could do it. If I have any doubt, I am will put a 4L80E in. My original idea was to put in a 4L80E, but he has assured me he could build a 700R4 that can handle it.
I am a little way from making this decision as I have to see if the rest of the project comes together.
If the project gets that far, I will strongly look at the 4L80E. The cost is not a lot inrelation to the rest of the project.
I have two big issues to deal with, will the frame work and changing the rear to a ford 90 rear. No gears are available for the Cadillac rear.
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Old 02-11-2014, 01:10 PM   #8
CadillacKid
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The mot practical method is going to a 9 inch. I have a 73 with a Moser 9", 3.73 gears a TH 400, and at 5200 pounds will run a 13.2 quarter. Gas mileage is around 7 (10:1, CadCo300SS cam, 2700 RPM stall converter, abnd a 1000 CFM Holly (clone). That said, figure out whether you are after performance, mileage, or cruising. If the last is your goal, a stock 400, stock converter and stock '73 rear end gears is just what the car was designed for.
Greg
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Old 02-22-2014, 09:55 PM   #9
Miklo II
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Quote:
Originally Posted by Schurkey View Post
I have no idea how you're going to get a TH700 to live behind an engine capable of that much torque, in a vehicle with that much mass.

I wouldn't put a lot of money into a torque converter. I think you're going to upgrade to a TH400, or a 4L80E fairly soon.
You are aware that a "built" 700r4 has beefed up gears and parts that are as tough and sturdy as a TH400 and that no one wants 2 or 3 speeds anymore, right? Yea 4L80E has an inferior overdrive gear too (0.75) compared to the 700 with a 0.70. Doesnt seem much like a numerical difference but it will be an RPM difference.
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Old 02-23-2014, 05:53 AM   #10
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Quote:
Originally Posted by Miklo II View Post
You are aware that a "built" 700r4 has beefed up gears and parts that are as tough and sturdy as a TH400
Yes, I am COMPLETELY un-aware that the 700 can be as tough and sturdy as a TH400. In fact, I'm in outright denial.
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