Section : Gearbox

Fitting Overdrive 

Posted By : Mark Elliott, 19-Sep-2006, 04:59pm

• Fitting Overdrive 

  Posted By : Jason Chinn, 19-Sep-2006, 05:24pm

  You can buy a kit if you want an easy life. Basically you need an overdrive gearbox, an overdrive, a prop, gearbox mountings, overdrive wiring, switch and that's about it - there are some incidentals - eg a drive for the speedo, relay if you go D type etc.
  Anyway - here's a picture of the components http://www.canleyclassics.com/featuredproducts.asp...

  • Fitting Overdrive 

    Posted By : Don Cook, 19-Sep-2006, 06:34pm

    You will need to change your diff. You have a 3:27 and need to change to a 3:89 but a good choice is the 3:63 (best of both worlds). if you don't you may find that the car can't comfortably pull overdrive in 4th.
    
    You will also need to cut back the propshaft tunnel to get at the prop UJ, I think the Spit 1500 has an extension piece that then fills the gap.
    
    Its a big job but provided you can get all the bits you can do it yourself - and it really is worth it, got 40 mpg going to Le Mans in July!

    • Fitting Overdrive 

      Posted By : Clive Senior, 19-Sep-2006, 09:19pm

      Disagree!! well sort of. If your car has the 3.27 diff in it (it should!) the car will be exactly the same as at present in the 4 gears, and give you a very relaxed overdive of aout 25mph/1000 rpm. If you fit the 3.89 diff you will be doing the same revs 20mph/1000 from memory, in overdrive top. Ie the same as you have at the moment. You do gain a bit of acceleration though.
      If you want more relaxed, keep the 3.27.
      Cheers
      Clive

      • Fitting Overdrive 

        Posted By : Chris Taylor, 19-Sep-2006, 09:49pm

        Some Mk2's were factory fitted with 3.27 diff and overdrive, so the engine CAN pull the very tall top gear ratio, at least on motorways.
        Agree though that the 3.63 (Spitfire 1500) diff is the best compromise, but you'll also need a Spit 1500 speedo to read correctly with that diff. (You MIGHT be able to get away with different speedo drive gears in the overdrive, but that needs specialist knowledge and dismantling the overdrive I think. NOT recommended for a novice!)

        • Fitting Overdrive 

          Posted By : Mark Elliott, 19-Sep-2006, 10:29pm

          is it worth it i ask myself?
          

          • Fitting Overdrive 

            Posted By : Chris Taylor, 19-Sep-2006, 10:49pm

            If you have a 3.27 diff (which should give you 20mph speed per 1000rpm engine revs in top gear) AND you don't feel the engine is overreving at motorway speeds, then the simple answer to your question is probably no!
            On a Spitfire, Herald, Vitesse, or 2000 saloon, the answer is almost certainly yes! With lower back axle ratios, those cars are undergeared on motorways. Overdrive is the simplest (and most authentic) way of getting a reasonable fifth gear ratio for modern traffic.
            It was only the GT6 that had a different diff ratio depending on whether overdrive was fitted, and it was only really the GT6 that could pull a suitably high back axle ratio to make a 5th speed unnecessary, albeit at a small loss in acceleration.
            Interesting to see how few cars make do with four gears these days!

            • Fitting Overdrive 

              Posted By : Mark Elliott, 19-Sep-2006, 11:42pm

              Hi Chris,
              
              thanks for info!
              
              Mark

              • Fitting Overdrive 

                Posted By : Jason Chinn, 20-Sep-2006, 08:31am

                Is it worth it? Well in my experiebce with Heralds, Vitesses and GT6s the answer is yes, it's the most reliable way to get a better crusing speed and some added flexibility - overdrive third is the same ratio as non-overdrive fourth but is a clutchless change up and down, ideal for 30 mph roundabouts
                To answer your question, get yourself along to a club night or tow and have a drive in someone's overdrive car and experince the difference - you can then make a self educated decision.