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Whats my diff?


Adrian
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Hello all

Just looking through some rebuild photos on a damp afternoon with the rugby on and noticed my diff is FR9376. Looking on various websites this seems to be a spitfire diff. I haven't had any problems with it and I think has been on the car since at least 1990.  The FR's are 3.63:1 whereas I should have a 3.89:1 on an overdrive. The ratio seems to sit in the middle between the 3.27 non-overdrive gt6 and the overdrive3.89

Any idea what the impacts are likely to be? top speed, acceleration? eventually crumbling?

Who knows what else i'll find is off a spitfire - I can be certain of the roof though!

Cheers

Adrian 

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Fitting a 3.63 diff in place of the 3.89 is a common mod with both the GT6 and Vitesse 2 Litre. The main reason is even with overdrive the 3.63 diff give lower engine revs when driving on a Motorway. Hence reduces the engine wear and the noise in the car. It does increase the 0 to 60 time by a small amount. It will reduce the speedo reading, therefore I would check it against a GPS. However, most GT6/Vitesse speedo's over read so it may well be reading spot on!

Dave

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I'm trying to work out if I'm remembering things incorrectly here, as I did the swap on my non-O/D GT6 - which I converted to overdrive - and found that while it gave better acceleration as Dave says, it also increased the revs and also knocked a surprising amount off the top speed; I remember the engine revving quite highly when at 60 or 70 mph. I got rid of it again (original diff rebuilt) and when I used the original along with the O/D conversion it gave very relaxed driving at motorway speeds, although again at the expense of acceleration; which I preferred anyway.

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I've run my Mk3 GT6 (O/D) with both the 3.89 and the 3.63 and have to say I like the latter. Apart from the more relaxed cruising it makes first gear that bit "longer". I prefer this as I always felt that first with the original diff' was rather too low - no sooner moving than you're scrabbling for second. Whilst it must have reduced the car's acceleration rate I can't say it has been noticeable. The change has made a (nowadays) legally significant difference to my speedo accuracy though and I'm presently reliant on GPS to keep me on the right side of the law. I hope to re-calibrate the speedo this year.

Wayne

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1 hour ago, Pete Lewis said:

Many used mintylamb but its being recoded,

Is this for gear ratio's as it suggests the SU needles graphs will be appearing sometime.

I downloaded the SU app (Which for the graphs is fine with the free version - its a tenner if you want suggestions of needles) http://www.winsu.co.uk/ You do need an old version of Dot.net though. (I'm running Win10 64bit and it works ok)

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Unbolt diff, bolt a 3.63 in its place....

If you mean change crown wheel and pinion, possible, but not really viable.

later 3.63 diffs are the strongest of all the diffs (down to the carrier) but I have never broken a 3.63 (even with 155bhp and used for autosolos etc) or indeed a 3.89. Lots of 3.27's have met their demise in my cars.....

 

(and Colin above would have had a 3.27 diff, so yes a3.63 would rev more for a given speed without overdrive engaged)

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When I got my GT6 it had a 3.27 diff and overdrive - gave a very long first gear and relaxed cruising car. I had it rebuilt as a 3.63 and have never regretted it - quicker acceleration, much easier to pull away on hills and still happy at 70mph on the motorway (or more on a private road)! I've not tried the intended 3.89, but suspect it might be a bit frenetic on modern roads!

Gully

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5 minutes ago, clive said:

True, butout of the 50?? diffs that I have had (some bought/sold, some from cars I dismantled, some in my cars) I have never had one where the ratio did not match the case number. So it is a very reliable indicator, but not infallable.

My 3.63 is built in the original 3.27 6 bolt case...

Gully

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As I said, not infallible! But the number of diffs where CWP ratios are changed must be very small. 

Out of interest, did you re-use the 3.27 diff carrier? I suspect a diff builder would have used the later spitfire type as it is so much better.

Re "odd" diffs, seen plenty where the rear cases have been swapped (ie changed from 4 to 6 stud, and the other way round) and similarly with 2 holes blanked off. I have also seen later type diffs with offset and redrilled front flanges,so they can be used with herald/early spits. Not to mention my mk3 spit where I fitted a 3.63 diff with late mk3 quarter shafts (so I could use the std driveshafts without swapping flanges)

 

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15 hours ago, dougbgt6 said:

What's my diff?

This would make a good game for the next Twiddle Day. For the unwary Haynes says GT6 without OD 3.89. With OD 3.27. Yes it does! Well, mine does.

Doug

Whereas my Haynes (1974 edition) is correct! 

Gully

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20 hours ago, clive said:

(and Colin above would have had a 3.27 diff, so yes a3.63 would rev more for a given speed without overdrive engaged)

Thank you Clive, I scratched my hear for an hour that morning trying to remember what I had originally... brainum unworkum is the Latin phrase for the phenomenon.

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What is always so surprising is how much difference a "small" change in diff ratio makes.

So the main choices are 4.1, 3.9, 3.6 and 3.3 (rounded to 1 dp) but changing from one to another feels like a big change. 

Other things to consider are power bands. If one has a cammed up engine, you really want to make sure you will still be in the powerband when you change gear, otherwise you will have wasted your money! (I suffered this with my old Zetec, it really came alive over 5000rpm, but when changing gear it dropped out the powerband, which was incredibly frustrating. I changed the diff ratio to sort that issue, then changed engine spec to give variable valve timing so a huge torque curve, and changed diff ratio back again. All fun and games! But I still use a smaller diameter tyre set on trackdays to change gearing about 5%)

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