Differential Ratio

[Dicky]

Is it vital to replace the differential like for like that comes with my Spitfire? From what I can see a MKIV has a ration 3.89:1 whereas MKI-III have 4.11:1 and 1500 3.63:1. Would I, as the driver, notice the difference?

[Clive]

yes, you will notice.
If you currently have a 3.89, 3.63 will feel quite relaxed. Bit like having half an extra gear. Downside is the car will feel a little more sluggish.

Changing to an earlier 4.11 diff does the opposite, but comes with a whole host of fitting issues as all the flanges are different to the later cars.

[James]

If you have a the MkIV engine then 3.89 is ideal.

The 3.63 makes it feel a bit sluggish as Clive says— I had that on my Mk3 when it had a IV engine.

[Dicky]

Thanks guys, it sounds like I'll need to match it up. Is there anyway of telling what ratio I have without taking the car apart? Should it be noted somewhere?

[jonnyk5614]

You can check the diff number but that does not mean that what is inside the diff is the same as when it left the factory!
Just jack the car up and count turns of prop in relation to turns of wheel - canleys have a good article http://www.canleyclassics.com/?xhtml=xhtml/infodatabase/diff.html&xsl=infodatabase.xsl

[jonnyk5614]

Hmmmm, either my rev counter is wrong or someone sold me a diff that wasn't 3.89!

[Clive]

If you get under the car, wearing goggles and carrying a wire brush, plus a good light.
The Diff number is a bit behind the prop flange, on the bottom of the diff. Front section, not on the rear casing. Can be hard to see.
FR is a 3.63,
HC, KD, FH (and FD I think) all 3.89
KC is 3.27, but if that is fitted you will be struggling to pull away at all!

All those diffs have the correct sizes flanges/ Some will have 4 spring fixing bolts, some 6. No problem as the ones with 6 can have 2 blanked off by using sawn off bolts and sealant (very easy/quick)
Your car should have 4 spring fixing bolts.

[rotoflex]

If you're thinking of changing the differential, you might should think of which one you'd want with an overdrive, & start working that way.

[Dicky]

Spitfire Graveyard have a Diff 3.89:1 for £85 which I'm very tempted to buy, do you think it's a bit risky getting a second hand one without knowing the full condition of it?

Also @rotoflex mentions the need for an overdrive, sorry for my mechanical ignorance but should the Diff come with Overdrive? I have overdrive at the moment but didin't realise it was part of the Diff!

[Richard B]

856 wrote:

Also @rotoflex mentions the need for an overdrive, sorry for my mechanical ignorance but should the Diff come with Overdrive? I have overdrive at the moment but didin't realise it was part of the Diff!

No Bill refers to the fact that you want to select your final drive based on your revs with an O/D fitted.

For example Triumph sold the GT6 with a 3.27 (Non-O/D) or a 3.89 (O/D). All the other Triumphs only had one diff ratio regardless of gearbox.

But for example a 3.63 ratio would be about the same final drive as a 3.89 with O/D.


A normal gearbox gives 1:1 engine to propshaft, an O/D gearbox gives about 1:1.18 engine to propshaft.
O/D increases the number of revolutions that the propshaft does in 4th (and 3rd).

[Dicky]

Thanks for all the help  :)

Would I not want a 3.89 regardless and then the overdrive ratio is a bonus?

[Paudman]

That's what I used to have, acceleration was just ok but motorway or long-distance cruising was effortless.

[Richard B]

As Clive said;

cliftyhanger wrote:

yes, you will notice.
If you currently have a 3.89, 3.63 will feel quite relaxed. Bit like having half an extra gear. Downside is the car will feel a little more sluggish.

Changing to an earlier 4.11 diff does the opposite, but comes with a whole host of fitting issues as all the flanges are different to the later cars.

If you have a standard 1300 MkIV engine stick with 3.89 (quite common, also fitted to 2 Litre Vitesses and O/D GT6's). Then upgrade to an O/D box.

[GT6 M]

For example Triumph sold the GT6 with a 3.27 (Non-O/D) or a 3.89 (O/D). All the other Triumphs only had one diff ratio regardless of gearbox. ,   Said Mr Brake.  nearly true, ;) ;)

But a few MK 2s came oot with 3.27 and OD.

[Dicky]

This diff that's on offer of 3.89 plus my O/D gearbox should be good then. Do you think it's a bit of a risk getting this second hand one or worth a punt? I'm thinking the later  ;)

[Dicky]

Well I've gone and bought it, spitfire graveyard said it will be with me tomorrow  :)

[PHOENIXGT6]

8)I always thought that the 3.89.1 Ratio O/D final drive was equal to the 3.27 non O/D. In fact the non O/D is a few revs per mile FEWER!. ;)

[Peter McEvedy]

I have a 3.27 with O/D in my GT6 convertible and it makes for an extremely relaxing drive on long trips. Seems to be perfectly quick off the mark as well - the car has HS6s and if feeling lazy, you can drive it in third at almost any speed!