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Spitfire Spitfire and GT6 diffs...

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TheAssociate

TheAssociate

Jedi Hopeful
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Question about the diff between the diffs...(couldn't resist).

I'm doing a lot of highway driving lately, the low cruising speed and the high rpm is getting to me. I'm thinking of solutions. 5 speed - expensive. Overdrive - expensive for nice, still expensive for worn out.
I've got a shockingly nice running 4-speed in my Spitfire that is almost feels like new in every way. I can't stomach taking out and replacing with an overdrive tranny that might have all sorts of unforseen problems.
So, I am thinking about putting a GT6 diff in its place which I hear is 3.27:1 ratio versus the stock 3.89:1 ratio Spitfire diff.
Firstly, does anyone know if mechanically there is anything stopping me from doing this? Such as the leaf spring not fitting, or the drive shaft not bolting up right? Axle flanges?
Secondly, a question about the ratios. I think it would go quite nicely considering the way it is now you can almost pull away from a stop in second its so low geared!
Any input would be greatly appreciated.


Adam.
___________________________________________________________
1973 Triumph Spitfire.
 
Standard GT-6 diff was 3.27; GT-6 w/overdrive came with a 3.89 diff.
https://www.britishcarforum.com/tikiwiki/tiki-index.php?page=GT6#_Transmission_amp_rear_axle

They are all the same size & will generally swap, although I think sometimes there is a difference of flange size (which can also be swapped).

The word is that the best diff for GT-6 w/overdrive is the 3.63, which was available w/Spitfires (but unfortunately few US Spitfires).

I wouldn't be too fearful of an overdrive transmission. My D-type transmission is very well behaved. Almost everyone who installs an overdrive loves the thing. I can't recall reports of anyone who removed it & went back after the installation was sorted out. Just make sure you do the installation right - don't skimp on the inhibitor switches which keep you from engaging the OD while in reverse.
 
A long time ago I went about half way to building a Spit6 with my first spitfire and a GT6 donor car - I did a diff swap and had no problem. I do recall that the GT6 had an extra stud or two on the top that the spit didn't - to fit the spitfire spring to the diff I had to replace the stud(s) with some allen set screws otherwise there would have been an extra vent hole or two on top of the differential. It really was an easy swap, though I can't remember if I noticed the cruising speed difference that much with the swap.

Randy
65 TR4
 
The number of holes for studs on top of the GT6 differential will depend on whether the diff came originally with a car equipped with the rotoflex or swing spring rear suspension.

The ones which came with the rotoflex suspension have 2 more holes for studs.
 
The biggest problem with the 3.23 diff is the carrier is prone to failure. Also, a Spitfire will not really have enough torque to pull a 3.23. So I've been told. I know my Mk I with later OD will not pull a hill in 4 OD.
 
Adam, compatibility should not be a problem with your '73. Unless the GT6 diff came out of a '73 GT6 with the swing-spring setup like your car, it will have six studs at the top. Most people, when doing a swap like this, will cut off the center two studs flush with the top of the case and possibly also cut slots in those studs just to fill the holes. All the flanges will be as your current diff.

You may or may not be happy with this setup. I've heard other Spitfire owners say that they WERE happy doing this. I suspect most of them were Spitfire 1500 owners; 1147s in particular (from the early Spitfires) probably do not have enough torque. And you WILL lose something "off the line" with a 3.27...but 20 mph per 1000 rpm in top gear is nice for economy and lowered stress! (Oh, you will have to either recalibrate your speedometer or grab one from a GT6.)

The 3.63:1 was only available on UK-spec. 1500s with the dual carbs; it's a very rare beast in the US.
 
Thanks for the tips. Rotoflex: I was more referring to the questionable and unknown condition of a used transmission, with overdrive. My '78 had a nice overdrive trans, the overdrive worked when it wanted to and the trans was toast(popping out of gear, bad syncros etc). I like the overdrive very much and would love to have it again, but only if it were mechanically good.

I know the Spit will not pull as hard with the lower ratio, but I plan on installing dual SU's or a new Mikuni which should balance out well(I hope). I'm excited knowing that this will work, even with small sacrifices. Couldn't possibly be as bad as the engine being at 3100 rpm at 60mph.

Andrew: you think it would be 1000 rpm? That would actually make me very happy.

Adam.

____________________________________________________________
1973 Triumph Spitfire.
 
I have an 80 1500 Spitfire w/overdrive and LOVE IT! Never any problem going up hills in 4th and OD. (in spite of what you may have heard, Kansas does have some hills). I've thought of the same swap to lower the RPM even more, or to simply allow for a comfortable 80 MPH cruise.

Don
 
CaptDon01 said:
I have an 80 1500 Spitfire w/overdrive and LOVE IT! Never any problem going up hills in 4th and OD. (in spite of what you may have heard, Kansas does have some hills).
Don
Click to expand...

Yeah but your hills are not very long and the steeper they are, the shorter they are. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif /bcforum/images/%%GRAEMLIN_URL%%/jester.gif
 
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