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Stronger Diff

Ken Bradley

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I have a Spitfire 1500 that has a Fiat Mirafiori 2 litre engine and gearbox.


Great, everything in the garden is rosy except it appears to have destroyed a recently refurbished diff! 


The previous owner rebuilt the diff so I'm not aware of the detail of what was done, other than to say he appears to have been very capable in all other works he undertook. There are some terrible noises from the diff and an alarming backlash.


Before I replace the diff I would like to reassure myself that the standard fitment is capable of handling the increased power. This engine is pushing out about 120 BHP, through gear ratios that seem spot on when using the standard diff ratio and 14 inch wheels.


Is there a more robust diff out there that can be fitted without major rear end engineering?    

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large amounts of back lash is likely to be the planet gears and/or their shaft has  broken up,  have you also checked its not a driveshaft  uj failure.


is this a black car buy chance ??


I had to look at a clean bodied  black one with engine in boxes the owner wanted it all assembling.. interesting,  I walked away as you could spend hours and end up with a smoker 


to reduce the torque on a std diff you could go for a lower ratio like 3.89   but the 1500 diff has a fairly good survival record


Clives best on this one 



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The Mirafiore engine in production form never got above 74bhp.

I've run a Quaife LSD off a 2.5 with about 130bhp, and it has never failed, even in racing, even when the Woodruff key on one axle broke, and it drove the car through one wheel only.

The LSD components bolt to the original crown wheel, and replace the planet/sun wheels.

Maybe that;'s the way to go, but they are expensive.



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John, surely some mistake! the fiat twincam 2.0  produce way way more power (100-140) in production form, and many way more than that these days....

Anyway, stronger diffs... easiest is the quaife, though it still utilises the normal crownwheel and pinion, which doesn't seem to be an issue but still worries me a little.

I am fitting a Subaru R160 diff to my spitfire in the nearish future. This is a conversion from New Zealand, and requires a few parts that mean it is not cheap (broadly similar to a quaife) and also needs a rotoflex type suspension setup. (I now have most of the required bits, diffs are really cheap and many are viscous LSD. CV "rotalex already in place)

Others have fitted ford diffs, but that seems more of a challenge. No off the shelf conversion is available.

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No pete its not a black one, although plan "B" may involve a colour change.


The car performs so well I sure the output is easily 120 BHP, and thereby may be the problem.


I have considered going to a lower ratio diff and/or reverting to 13 inch wheels. Whilst reducing the torque on the diff I do wonder what might be the effect on performance.


Thanks for the input lads, it seems further research is required.

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Alternatively, another diff may last OK.


I broke a succession of 3.27 diffs in my 2.5 vitesse, and then fitted a recon 3.63. That lasted fine, and went into my last Zetec spit (155bhp) that was hooned about and did a couple of autosolos with no damage. That same diff is in my current zetec spit(again approx 155bhp), but I am getting old and worry too much......


A 3.89 may well suit the car better, get to use all those lovely revs! I am considering a 4.1 subaru diff, rather than the 3.7 I have lined up.

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