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Spitfire Diff with GT6 engine and gearbox


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Hi Kevin and all you good people here on this site. It was exactly a year ago, when I was last on this site begging for all your help with my overheating issues and acting on all your suggestions to improve the situation on my Spitfire, fitted with a GT6 engine and gearbox.


Well I shelved the project until a few weeks ago, where I picked up where I left off.


I bought a VW aluminium radiator and fitted it with an expansion tank from ford fiesta and with a little help from and electronic fan, and change of carburetors to original Strombergs , I managed to get the engine to tick over at a steady pace and the temperature down to semi normal. I have now got on an MOT for the car and with classic insurance in place with no road tax to pay (tax exempt), I hit the streets of west and central London today driving around and enjoying my open top car, while attracting all kind of attention, it made the last 5 years of blood sweat tears, all worth while.


Now that I have the engine all singing and dancing OK, I need some advice about the final drive.


The gearbox is an overdrive one, but I have not wired it in to operate with a switch, since I do not have the switched gear knob or the steering column lever operated switch. The diff is also the original Spitfire 1500 too.


The thing I noticed today after a long drive around the town, was that the engine can rev up to almost 5500 revs in 4th gear but the road speed will only reach 60 MPH.


I know that the O/D when activated will make some difference, but my question is, should I be looking to change the Diff too to get the maximum out of final drive? knowing that there is a marked difference between the spitfire diff and GT6 diff, or just activating the O/D in 3rd and 4th will make up the shortage in road speed?


as usual, I welcome all replies.



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At a guess you may have the 4 cylinder electronic rev counter fitted? That will over read by 50% I think.

Use a satnav to check your speedo accuracy first. That is important in terms of keeping your licence! Or indeed, there are free smartphone apps for a GPS Speedo. Chances are the Speedo is under reading by up to 20%


Then search for a 6 cylinder electronic rev counter from a 2.5 saloon or similar.

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I didn't wnat the faff of fitting a different steering column binnacle with the extra switch lever.

I have owned Triumphs with the gear lever switch, and it's a pain - the lever is a tube with a very small inner diameter, so small that Triumph wired the switch in the knob with wires insulated with varnish!   Which wears away, shorts out and the O/d fuel and temp gauges on the same fuse all fail.     


I have a steering wheel mounted Overdrive switch.    A 'latching' press button - press for On, press again for Off - and an indicator light are mounted on a small plate betwen the spokes of my wheel.

The light shows that the switch is 'On', not that the O/d is 'On', so acts as a warning if I change down below third with it on that it will be in O/d if I change back up.


Of course, it needs to be connected to the electricals, but a length of "curly-whirly" cable, three core, provides a flexible connection that tolerates turning the wheel and doesn't get in the way.

See pic - not well focussed, but the red dot is the switch, the white the light.      The gadget on the other side is a stop watch.




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It's reasonably common for GT6 owners to change to a Spitfire 3.63 ratio diff from the 3.89 ratio to lower cruising revs - something I did. From recollection, without the overdrive engaged 3000 rpm is about 55-60mph, so something seems adrift in your readings!



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Pete, Clive, John and Gully, thank you all for your invaluable input. It seems like I have overlooked the simple 4 cylinder Rev counter working with a 6 cylinder engine, as an issue. I thought the rev counters are all self adjusting and act on how many pulses are sent and read accordingly.  Well, electrics where never my strongest point, and I thank you all for pointing this out.


Clutch is new and not slipping, the road speed is sort of gauged with other road users at the time, only at what seems like 55 to 60 MPH, the rev counter reads around 5000 and engine has more to give, but I can't help feeling I am over revving the engine.


Thank you John for the photo and wiring option. 

Here is an idea for you, which I am considering to do myself, but need help with possible flaws in my idea.


I have fitted a classic sounding horn that operates with a long reach spring back switch mounted on the dash by the steering. This has left the steering center horn push button redundant.


I wonder if I can, with the help from a relay to use this circuit as a switching device for my O/D?


Any thoughts would be most appreciated.



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Would it not be better to use the horn push for the horn and and the dash stalk for the OD ????

That is why you are the advance member and I am a novice. don't know why I didn't think of that.

Of course it would be, thanks for that.


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