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Thread: TH400 replacement with overdrive transmission

  1. #1

    TH400 replacement with overdrive transmission

    Have just bought street/strip pro street pop, it will be purely street car so need to replace TH400 with overdrive transmission for motorway use. Engine is SBC ( allegedly 435bhp 450 ft/lb torque ) th400 transmission and ford 9 inch with detroit locker eiiher 3.77or 4.1 gears and 31 inch M/T tyres. Don't want to use 700R4 as the ratios are not good so left with two choices either uprated 200-4R or gearvendors overdrive unit. Anyone using either of these.

  2. #2
    NSRA member rem's Avatar
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    I haven't used either, but isn't the Gearvendors overdrive an add-on Lay**** J-type variant, so will add a foot or so to the overall length, and shorten the propshaft accordingly? I had a Pop with TH400 and would have struggled to fit one behind that. I think The Skid Factory fitted a Hughes Transmissions' 200 to their recent Kingswood build on Youtube?
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  3. #3
    Have the 700r4 rebuilt with the lower ratio (numerically) 1st gear and it solves your problem, I'm sure Zane will be able to advise better, but it's probably a cheaper option
    https://www.monstertransmission.com/...ios_ep_52.html

    http://www.zannetec.com/

  4. #4
    I'm using a 200R4 with my built 383 SBC using a formula I found the optimum rear axle ratio for my Jag irs diff (4.27:1) to give me the right engine speed (2000 rpm) at normal cruising speed (60 mph). Only went with it initially as it's the same dimensions as the TH350 I had before & space is limited in a T coupe, since found out they're a very strong & reliable trans & used a lot in drag racing. Had mine overhauled by Tony Gransden as one of the last trans he rebuilt before his sad departure & had the TC upgraded too.
    Last edited by MisteR Tee; 14-11-19 at 05:41 PM.
    MisteR Tee
    Ah, Nostalgia, it ain't what it used to be!

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  5. #5
    Had you thought about changing your rear ratio to something nearer 3 to give you the cruising rpm's you need - unless you are concerned about loosing some acceleration. Cheapest option of the lot.
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  6. #6
    Quote Originally Posted by MoreCubes View Post
    Had you thought about changing your rear ratio to something nearer 3 to give you the cruising rpm's you need - unless you are concerned about loosing some acceleration. Cheapest option of the lot.
    that would be my choice too, no engineering work at all. somewhere between a 3.25 and a 3.55 gearset would make quite a difference. you say it has a 3.77 or 4.1 now, you need to check that to be sure before ordering new gears if you chose that route. i wouldn't be too concerned about losing any acceleration, with that much hp/torque in a pop you'll be just fine.
    neil.

  7. #7
    In fact I've now found my calculations & at 65 mph I should be turning over at 2082 RPM using 30" tall tyres, in comparison to the TH350 with a 2.88 diff at the same speed turning over at 2096 RPM. the 4th gear ratio (0.67 :1) will drop the effective rear axle ratio down to 2.86 (or up!). I could go for a 3.77 ratio which would drop it down to 1838 RPM but at the time I couldn't find one, however this ratio combined with the o/d trans gives me a low first but a high top, the best of both worlds (race on Sunday, cruise on Monday!)
    MisteR Tee
    Ah, Nostalgia, it ain't what it used to be!

    No longer wanted: Steel 26/27 Ford Model T coupe rear fenders.

    My Intro My website my cars

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