gears for highway driving and posi

Discussion in 'Got gears?' started by shadewolf, May 17, 2013.

  1. shadewolf

    shadewolf Well-Known Member

    Sorry for the delay guys. Had pictures of the frame to upload. My gears are 2.56 currently and look a bit worn down, my mechanic said that's normal but the rear end should get rebuilt soon if I plan to do anything serious with it and did not recommend long road trips. I told him afterwards about the 256 mile trip it went on over the weekend. He suggested I go to the 12 bolt again but to change the gears out to a lower number. I think I want to stick with a 10bolt but with four points and a 3.23 gear ratio. Now it time to look for one used or should I buy a kit and do it in the garage?
     
  2. 67skylark27

    67skylark27 Brett Jaloszynski

    If you are thinking about swapping out the tranny to an overdrive you could go a little higher. I am considering 3.36 or 3.55's. I am currently lookin for some gears so I'll keep you in mind as to what I come across.
     
  3. shadewolf

    shadewolf Well-Known Member

    Overdrive would be nice, I think that's a project for next spring though with the installation of cruise control. We have one confirmed trip to New Hampshire next year in July. My wives family live out there and they want the car, her uncle has a corvette with a 427 in it. Wonder if I could get him down to the quarter mile next year. I'll look into 3.55 gears and see if I can find them locally. Thanks for the advice with the od tranny to gear ratio.
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If you have the stock 72 rear, it's an 8.5 10 bolt. That's a good strong rear with great parts availability, and it has bolt in axles. No reason to go with a 12 bolt, but if you do, you'll have no trouble selling the 8.5 to someone on this board.
     
  5. shadewolf

    shadewolf Well-Known Member

    Sorry but I think there is some confusion I was looking at a 12 bolt but decided against it after the information provided in this thread. I have purchased another 8.5 " 10 bolt with posi and 3.23 gears. I'm not sure if I'll sell my stock rear end or not yet. To be honest I don't know what they are worth.
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    ''

    Very Good, carry on.
     
  7. hdpegscraper

    hdpegscraper Well-Known Member

    Arround here, demo derby guys like em cause they dont have "C" clips, and if they bust an axel, the wheel dosnt necessarily come off the car.
     
  8. shadewolf

    shadewolf Well-Known Member

    I plan on keeping my original rear end for a little bit but want to know what I should sell it for when the time comes. Never shopped for a stock before so no idea what sounds right.
     
  9. snake

    snake Well-Known Member

    Heres food for thought. A ST 400 VP w/ a converter from a 300 ( higher stall) 3.23 posi gears good for highway cruise and off the get go with a switch pitch toggle or timer box, 28'' tires and a big q-jet. Heres my take on a smart mans OD. the math goes like this 65 mph x gear ratio x 336 divided by tire dia. = 2546 RPMs this basically what the OD trans is going to get but with no lugging. Plus added a trans go premium shift kit. with the q-jet running on primaries but has the option to step on it and open up coupled with the switch pitch keeping it within the sweet spot driving no need for a OD trans, has the option to race if need be or economy for the long trips, hopefully looking at 20 mpg with this stock set up. 425 nailhead and ST400 VP was the best set up buick came up with, in my opinion way ahead of there times.


    The Hot Rod Homepage

    Keepin it Hot Rod

    [HR][/HR]
    The Switch Pitch TH-400

    Share

    Summary:
    The reasoning behind installing an overdrive transmission is running a low rear gearing to get off the line, but still retain manageable RPMs at freeway cruising speeds. This seems like the best of both worlds, but this article will examine some reasons why an overdrive transmission is not always be the best solution.
    Traditionally GM overdrive transmissions such as the 700-r4 and the 200-4r were viewed as anemic pieces for gas sipping transportation appliances. The last several years have revealed that these transmissions can be modified to handle significantly more power then in base configurations. A well-built and properly adjusted overdrive transmission should be able to handle around 450 ft/lbs of torque. Is that enough for the engine combinations that people run in their hot rods? In many cases the answer is no and I for one, have been through enough built 700-r4s for a lifetime.
    So what then is the solution?
    The solution is the tried and true TH-400, more specifically the variable-pitch version of the TH-400.
    The variable-pitch, or switch-pitch TH-400 is nothing new as it has been around since 1965. Originally found in Buick, Olds, and Cadillac the switch-pitch offers two stall speeds, a low stall and a high stall. Applying a positive 12-volt signal to the appropriate terminal on the transmission case makes the selection of low or high stall. The high stall gives you the advantages of torque multiplication to get off the line with a taller rearend gear and the low stall decreases slippage for cruising or on the big end of the track.
    The variable-pitch stator is not the only advantage that the TH-400 holds over its more modern overdrive counterparts. A TH-400 will generally cost less then an overdrive and it can be built to handle some serious horsepower numbers. The TH-700 and TH-200 also require a T.V. cable to be installed and adjusted correctly or serious damage and shifting problems will result, and getting it correct is often harder then it seems at first glance. Another advantage of the TH-400 is a small spread between gears. First lets put some gearing numbers out there for reference:
    Gear Ratio Comparisons:
    [TABLE="class: cms_table"]
    <TBODY>[TR]
    [TD][/TD]
    [TD]1st[/TD]
    [TD]2nd[/TD]
    [TD]3rd[/TD]
    [TD]4th[/TD]
    [/TR]
    [TR]
    [TD]TH 400[/TD]
    [TD]2.48[/TD]
    [TD]1.48[/TD]
    [TD]1.00[/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]TH 700[/TD]
    [TD]3.06[/TD]
    [TD]1.63[/TD]
    [TD]1.00[/TD]
    [TD]0.70[/TD]
    [/TR]
    [TR]
    [TD]TH 200[/TD]
    [TD]2.74[/TD]
    [TD]1.57[/TD]
    [TD]1.0[/TD]
    [TD]0.67[/TD]
    [/TR]
    </TBODY>[/TABLE]

    It is clear that the two overdrive transmissions in the comparison chart have lower first gear ratios as well as have the advantage of overdrive, but let us look at the numbers that are hidden between these ratios.
    Between Gear Gap Comparisons:
    [TABLE="class: cms_table"]
    <TBODY>[TR]
    [TD][/TD]
    [TD]1st-2nd[/TD]
    [TD]2nd-3rd[/TD]
    [TD]3rd-4th[/TD]
    [/TR]
    [TR]
    [TD]TH 400[/TD]
    [TD]1.00[/TD]
    [TD]0.48[/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]TH 700[/TD]
    [TD]1.43[/TD]
    [TD]0.63[/TD]
    [TD]0.30[/TD]
    [/TR]
    [TR]
    [TD]TH 200[/TD]
    [TD]1.17[/TD]
    [TD]0.57[/TD]
    [TD]0.33[/TD]
    [/TR]
    </TBODY>[/TABLE]

    Notice the large gaps between gear ratios. The TH-700 has a particularly nasty gap between 1st and 2nd. In fact if you combine the 1st-2nd and 2nd-3rd gaps on the TH-400 you get roughly the same gap as the 1st-2nd on the 700-r4. The result of this is that your first gear in the TH-700 may launch you like a rocket, but the huge gap is going to lug your engine and drop RPMs, hurting performance. The TH-200 has a more favorable gear separation then the TH-700, but the TH-400 still has a slight edge.
    Now to address the flaw in my argument that I am sure many of you are thinking about. The TH-400 does not have an overdrive gear and if you dont run an overdrive then how can you possibly expect to have a respectable freeway cruising RPM?
    The solution to this is easy, run a taller rearend gear. I argue that the many hotrods rarely, if ever, see the drag strip and spend a lot more of their time on the road. So why then do we put in a low rearend gear that makes freeway driving unpleasant and limits out terminal velocity to 115 miles per hour. An overdrive transmission will make up for our low rear gearing, but this introduces several problems.
    1. Overdrive was never intended to be an acceleration gear, which it is being forced to become if you are running a deep rearend.
    2. High drive shaft speeds
    Running a 4.11 gear with a TH-700 sounds like a good idea until you consider that with a 275/40R17 tire at 100 mph your drive shaft is spinning at somewhere in the neighborhood of 5383 RPM for comparison a TH-400 with the same tire and a 3.08 rearend gearing will turn 4034 RPM. The same comparison at 150 mph yields 8075 for the overdrive versus 6051 for the TH-400. Spinning the drive shaft that fast takes horsepower and can have other problems like magnifying the effects of driveline alignment problems, drive shaft imbalance, and driveline component wear.
    With the right setup, a switch-pitch TH-400 provides a viable alternative to the modern overdrive.
    Controlling the converter:
    There is a multitude of possible methods for controlling when and how the converter changes from high to low stall. Use a delay box to have the converter go to low stall after you leave the starting line. An RPM switch could have the converter lock up once your car is into its power band. Wiring a relay to the brake switch would allow you to put the converter in high stall for stoplights in a car with a radical camshaft. A simple switch can be used to allow full driver control of the system. For most those who want to have the most control of their driving experience this probably the best option. [article on wiring a switch pitch transmission]
    Where do I get my hands on a switch pitch TH-400?
    I purchased the unit in my 1966 Malibu from PAE Enterprises in Texas. They offer both Chevy and BOP cases for a resonable price. PAE also sells conversion kits so those of you that already have a TH-400 are in luck. PAEs contact information can be found in the sources section of this article.
    Sources:
    How to Work with and Modify the Turbo Hydra-Matic 400 Transmission
    by Ron Sessions
    ISBN: 0-87938-267-8
    P.A.E Enterprises, Inc.
    4401 Turf Rd. Bldg E
    El Paso, TX. 79938
    915.855.6009
    915.857.4727 -> Tech Line 8-10am and 4-5pm MST M-F
    www.paeenterprises.com

    Posted in Articles |

    ---------- Post added at 08:02 AM ---------- Previous post was at 07:17 AM ----------

    Hell I might even think of getting that 72 Airstream I had my eye on and towing it with my 65 Rivi to car shows. God knows this rig has the power and trannty to do this.:cool:
     
  10. 67skylark27

    67skylark27 Brett Jaloszynski

    A buddy gave me some mid 3's carrier and gears, non posi. I might have to figure out what it is actually. Said
    they were from and 8.5. Early 70's chevelle.......

    ---------- Post added at 11:44 AM ---------- Previous post was at 11:29 AM ----------



    Hell I might even think of getting that 72 Airstream I had my eye on and towing it with my 65 Rivi to car shows. God knows this rig has the power and trannty to do this.:cool:[/QUOTE]


    Very interesting read on the overdrive! I will read through it a couple more times and keep that all in mind for the next car. There is always a next one, right!
    One thing to consider for towing.... My current setup with the 200-4r and the b&m lockup converter allows me to dial in where I want lockup to occur.
    So if I ever put a hitch on it, and tow something at 55-60 mph, I'm thinking, put it in 3rd gear and dial the lockup to come in at 45mph. Then I am
    at about 2000 rpms and in lockup so the tranny stays cool - no slippage and still decent mileage. Yes????? I chose this setup for tranny availability
    and cost. I was able to find a cz code tranny (GN's and Monte SS) with the factory hardened parts and shift kit. 300 bucks and the b&m was 190.00.
    A little messing around with the linkage/tv cable and I was driving the same day. My St300 had burned up and I needed a replacement. It bolted
    right up, driveshaft fit and I just had to move the crossmember back to the factory drilled holes. Can't beat that
    if you don't need a fully built tranny for big horsepower. Now I am on to gears as the 2.78's have me too low at 60mph in rpms.......
     
  11. snake

    snake Well-Known Member

    Cant beat that, almost free! A couple of old tranny dudes said to keep a switch pitch 400, they would hook them up to motorhomes use a toggle or dimmer switch on the floor board and when needed on the red light or take off hit the switch pull that big SOB thru the gears then shut it off. Looks like a good trans cooler and breaks to add for this set up. The 300 converter is smaller thus higher stall than the 400. The tranny dude told me to use 3.23 posi instead of the 3.08 for better take off and just use a 28 inch tire or bigger. He said the set up I have is an unbeatable combo and the best all around for street and possibly strip if I do that once or twice a year.
     
  12. 67skylark27

    67skylark27 Brett Jaloszynski


    I knew I should have kept that 300 converter instead of selling it! Oh well, all the parts from that tranny went to people on the
    board here. I believe I got 300 for all the parts making it a wash on the new tranny. Your setup seems pretty awesome.
    Thanks for posting all that - it's why I am on the board - read, learn, read some more then build something.
     
  13. snake

    snake Well-Known Member

    Yes, the parts went to good homes, and also like you I learn a lot on this board . I will be getting this car running. By mid summer hopefully.
     
  14. shadewolf

    shadewolf Well-Known Member

    Well ok, my last question was not answered but another question was. To bad I sold my switch pitch tranny. I'm not going to worry about overdrive, I'll just run the th350 for now and hopefully the parts car might have an extra goodie or two with it.
     
  15. shadewolf

    shadewolf Well-Known Member

    reardif1.gif reardif2.gif reardif5.gif Ok, so this is the rear diff I'm looking at getting. Looks like it 3.08 gears but I'm to new at diff to know if its a true posi. Some one told me that people were putting plates in to turn cars into posi's don't know if that's true or if that's what they did here. Any advice would be appreciated, does it look good, what should I settle for on a price etc.
     
  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Looks like a posi to me. See George N's article, http://www.buickperformanceclub.com/posi.htm

    Are you going to transplant the Posi and gears into your rear?
     
  17. shadewolf

    shadewolf Well-Known Member

    Is transplanting the easier to the two routes to go? I would not mind doing it that way but thought that swapping rear axles would be easier. Either way I will end up with an extra axle and gears. Will most likely sell them now. If I go to the new axle its a four point, I think that would be better for me for stability and would allow me to change out the shocks and get some more ride in the back.
     
  18. TheSilverBuick

    TheSilverBuick In the Middle of No Where

    You can't just add plates to an open carrier to make it limited slip, infact you can't take the guts of a limited slip carrier and put them into an open one because they are not cast the same. People sometimes weld the spider gears to make it a solid spool, but that is different (and not really recommended). I have this same style differential, which is less desirable than the Eaton style (with 4-springs), but still works pretty good. I have not had any trouble with the limited slip function on my car. Depending on the kind of limited slip, extra plates can be inserted to increase the pressure and break away force needed to let the tires slide differentially.
     
  19. shadewolf

    shadewolf Well-Known Member

    So the pics are a limited slip or a posi? I'm confused, looks like a 4 point posi rear end. Please explain if I am wrong what to look for.
     
  20. ceas350

    ceas350 "THE BURNER"

    Good old gov bomb:)
     

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