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700R4 vs TH400

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  • 700R4 vs TH400

    I know the difference between the two and one member pointed out to me that I was going to have a problem with shifting with the really tall gear that is in my '63 Catalina. I think it is a 2.41 gear and they were right. I does not like to downshift on hills and you have to almost come to a complete stop for it to shift to first. On level ground cruising it is great but I think a TH400 is in order cause a new center section for these cars are not cheap. Is the 700R4 and the TH400 the same size and would it be a direct swap?
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  • #2
    Th400 is shorter so you will need a longer ds. Mount point on th700r4 is closer to engine so you will need to move cross member toward front of car. I'm guessing you are either using an adapter or a Chevy engine. 2.41 is a pretty steep rear gear. Maybe a rear gear swap.
    Rich

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    • #3
      Yes, it has a 350 chevy motor in it. The rear end is the stock Pontiac unit. I've priced some center sections and they are high. Every Catalina I've ever had has come with a 2.41 or a 2.56. I was told that it needs to be in the 3.08 range in order for the 700R4 to work properly. Thanks for the input. That has me rethinking.
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      • #4
        even rock auto carries the gears 147 to move up to a 3.55. Its really whatever you feel more comfortable doing, considering the mileage you will be losing with the gear swap.

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        • #5
          If it has already been updated to a chevy engine and has a 700r4 then throw a gear at it and have some fun
          Drag week 2009 Quickest street rod
          Drag week 2010 Quickest street rod

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          • #6
            a 3.55 gear x .7 overdrive ratio is equivalent to a 2.48 gear without overdrive.
            There are calculators online to help you with gear choice that factor in your tire size and cruise speed on highways. How much time do you spend cruising around town versus how long you spend on interstates cruising, how fast you go, etc, are factors you should consider in making your choice.

            I personally would go with a 3.73 gear if I had an old car with an overdrive. This way you get the "get up and go" of the 3.73 gear around town, and you get a final drive ratio of 2.61:1. Again, run your tire size and cruise speed through a calculator, but for example, with a 28" tire at 80 mph, you'd be turning about 2573 rpm. At 70, 2251. At 60, 1930.



            The lower rpm (based on a numerically lower gear) doesn't necessarily mean better gas mileage. Lugging the engine at too low of an rpm can cause too low of a vacuum signal, reducing your timing and possibly opening your power valves...

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            • #7
              That helps out a bunch guys. I just remember from my '65 Catalina it was next to impossible to find a gear for it. All of mine had the smaller carrier and it was finding the right carrier to get the right ring and pinion for. I guess this car is different? The car is used mostly around town so that 3.73 would probably be the ticket. I think the local transmission guy can do the rear gear swap cause I drove it for the first time after she has driven it a bunch and the trans is whining under load. It won't do it in park or neutral and only does it when you accelerate so from what I've read that is the torque converter.
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              • #8
                A downgrade to a mouse, yeah you will need gears. The 700R4 has a 3.06 first gear, that was designed to help weak on torque chevy engines move vehicles, that is more important for you driving in town than having the OD. The Th400 has gear ratios very close to Th350s, so you won't notice a difference if you have the Th350 now. If the detent cable isn't connected on a Th350, or is otherwise far too loose, it won't down shift just like you describe. If you have a two speed any of those is an upgrade to help your tiny engine move that car. You might as well gear the snot out of the thing so you can get away from stoplights.

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                • #9
                  We purchased the car with the 350/700 combo in it. My wife is not running stop light to stop light for pinks in the car. The current set up is not bad considering it is not a 389. The impala is basically the same car and they put straight sixes in those and they moved them just fine so the 350 will do for what she is going to use the car for. The more I think about it, the longest drive it will make is to Daytona for the rod run so a 3.73 should be fine because it will be a around town car for her when she wants to go to cruise ins and such. Going to make some phone calls and see about a ring and pinion as I know a complete pumpkin for this car will cost a fortune, but then again having that set up installed may not be cheap either.
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                  • #10
                    Gears and installation will be a lot cheaper than a transmission swap.... and will make the car more fun to drive.

                    Did you run the numbers with your rear tire size? I used 28" as an example because I run a 275/60/15 on my Chevelle. If you don't know how tall your tires are, you can go to tirerack.com and look up your tire size and get an approximate height. (or you can measure them with a tape measure). If you have 26" tires, 3.73 may be too much gear...
                    Last edited by yellomalibu; April 27, 2015, 05:56 PM.

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                    • #11
                      26.70 and I've located a 3.23 complete pumpkin for it. He is sending me pics today and says it's good. Only downfall is its in California. So I'll probably pay as much for shipping as I do the part but still cheaper than a trans swap like you said.
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                      • #12
                        Going from a 2.41 to a 3.23 will make a huge difference in the feel of the car. You're going to love it. And the highway cruise will be great, too.

                        I'm not familiar with the 63 Catalina, but if it's a complete pumpkin like a 9" ford, then the bonus is you don't have to pay a mechanic to set up the gears - just install it!
                        If, for some reason, the rear u-joint doesn't fit in the yoke on the pinion gear, most good auto parts stores can look up the right conversion joint for you so you don't have to mess with swapping out yokes and altering the preload on the pinion bearings.

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                        • #13
                          Yeah, even I can swap that out. Looks like four bolts for the axle retainer and the axles slides out, then remove the driveshaft and take out the twelve bolts that hold what they call "the hogs head" in and pull it out. After putting on a new seal it bolts back in, slide the axles back in and bolt up the retainers, fill with fluid and enjoy. Funny thing is I looked at the center section and it indeed looks like a wild hogs head LOL!!! Both napa and o'reilly have been good about parts for this car. They have some good people that know what they are doing.
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                          www.driftundertheradar.com

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