Changing the FDR: What do I need?
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HT4100 4.1, 4.5, 4.9 Discussion, Changing the FDR: What do I need? in Cadillac Engine Technical Discussion; Well, as some of you know, I've been looking to change the final drive ratio on my base Deville ('91) ...
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    Rodya234's Avatar
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    Changing the FDR: What do I need?

    Well, as some of you know, I've been looking to change the final drive ratio on my base Deville ('91) to that of the '91-'93 Deville Touring Sedan.

    Seeing that both are 4T60-E's and 4.9's, I assume that I shouldn't need to many parts. Rockauto has the final drive gear for the touring sedan, but I feel like I would need something more.

    Would the final drive gear be all I'd need? Or would the change require something more?

    Thanks in Advance.

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    4.9fiero is offline Cadillac Owners Member
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    Re: Changing the FDR: What do I need?

    I think you'll need the PROM and likely the whole ECM from the Touring Coupe. The PROM will treat the car like it's a TC, I don't know if the shifts will be firmer or not but it will be ready for the final drive ratio of the TC, beware of tire rolling radius too, try to use about the same size tires. (I think the TC had larger tires, but they might have the the same rolling radius with a different profile (60 vs 70 series)

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    Re: Changing the FDR: What do I need?

    Well, the base sedan tires are P205/70R, 15 inches, While the touring sedan tires are P215/60R, 16 inches.

    Will the change in size make a noticeable difference?

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    N0DIH's Avatar
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    Re: Changing the FDR: What do I need?

    205 70's vs 215 65's? I didn't think they got 215 60's.

    I can burn proms for the 90-94 4.5/4.9L cars, so I can reset to anything you want and do any tuning on the 4.9L....

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    Re: Changing the FDR: What do I need?

    Quote Originally Posted by N0DIH View Post
    205 70's vs 215 65's? I didn't think they got 215 60's.
    I'm just quoting the owners manual on that one, so I don't know.

    I can burn proms for the 90-94 4.5/4.9L cars, so I can reset to anything you want and do any tuning on the 4.9L....
    I'll most certainly be keeping that in mind, thanks !

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    4.9fiero is offline Cadillac Owners Member
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    Wink Re: Changing the FDR: What do I need?

    Plug some tire sizes into this handy little gizmo.
    http://www.fieroshop.com.au/tyrecalculator/

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    Re: Changing the FDR: What do I need?

    Well, my speedo is already 2.1% too slow because I'm using 215/70R/15's on the base sedan.

    With stock tires, using the Touring Sedan gear would cause it to be .5% too fast.
    Would the -2.1% be added on to the .5%, making my off percentage be -1.6%?

    Either way, even 2.1% makes the speedo margin of error at 60mph +-1mph, which doesn't bother me.

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    Re: Changing the FDR: What do I need?

    Alright, I've done some research and found that I only need to change the final drive sprokets to change the FDR.

    The Deville has a 3.06:1 Differential Ratio, and a 37/33 Sproket ratio.
    Another 4T60-E (I forgot which) has a 3.06 differential and a 35/35 (or 1:1)sproket.

    The Deville has a FDR of 2.73:1, while the other one has a FDR of 3.06:1.

    Does that mean If I change my sproket ratio to 1:1 (i.e. 37/37) that I will have a FDR of 3.06:1?

    Because changing just the driven sproket to 37 teeth would be a simple swap (the sprokets are just behind the side cover of the trans) and it would only cost me about 50 bucks. And I'd much rather do that then take apart the whole trans to change the differential gear.

  10. #9
    White on white caddy's Avatar
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    Re: Changing the FDR: What do I need?

    My Trany On My 91 Went Out And Its The Touring Deville If I Install a Trany From a regular Deville Will It Work Hers The Trans id Plate I Have On My Brokin Trans 1AMW The Trans Im Trying To Put In Is 1AHW Or 1 AVW

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    Re: Changing the FDR: What do I need?

    Physically you shouldn't have any problems, but the PCM is going to make speedo calculations for 3.33's, so your speedo will be way off. You can have the PCM retuned. I have the hardware to retune it, but need to purchase some software definitions for the 91 Cad.

    Will have to have your eprom/memcal to work with first.

    Give Intense racing in Ohio a call about the trans, see what they recommend. They build 4T65E's for racing for high strung supercharged 3800's.


    Quote Originally Posted by White on white caddy View Post
    My Trany On My 91 Went Out And Its The Touring Deville If I Install a Trany From a regular Deville Will It Work Hers The Trans id Plate I Have On My Brokin Trans 1AMW The Trans Im Trying To Put In Is 1AHW Or 1 AVW

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    White on white caddy's Avatar
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    Re: Changing the FDR: What do I need?

    If I Put In a Prom From the 91 Im Geting The Trany Will It Work

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