What's the Diff?

[smithy]

My Spit IV has a GT6 engine and the diff whines horribly and has a runny nose. It's a 72, so the ratio's 3.89 I suspect.
I believe the "sensible" replacement's a 3.63, which is the same casing but contains the same cog ratio as the TR7 - and is "as fitted" to the GT6 Mk 3's. Am I right?

James excellent site http://www.mintylamb.co.uk/gearspeed/ tells me what the differences I can expect in terms of numbers with the diff swapped, but I'm having difficulty reconciling the seperate notions I have that the car will accelerate faster (?) with a 3.63 diff, AND have a "taller" top gear (ie: make more MPH for the revs - be a better cruiser).
I suspect one of those notions is wrong - I think I probably do know which, but any info/opinions raspberries gratefully received!

BTW I haven't yet pulled the diff, but I will later this week if anyone wants any photos or summat.  :)

[Clive]

3.63 was fitted to spitfire 1500, so should be plentiful. In fact I several in my shed, hoping they will last OK in my zetec car, but spares ready to bolt in.

Mk3 GT6 had the 3.89 (if overdrive) or 3.27 if no OD.

Car will be marginally slower off the line with the 3.63, and a little more relaxed on the motorway. But a very good ratio for a 2 litre (or indeed, 2.5)

[smithy]

Ah! Clive, that's as it says in:
http://club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1154292216/
But not what Bill said here:
http://club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1224872868/s-84/highlight-spitfire+differential/#num84

....and the ratios are just as you say, and in my Haynes manuals, and here:
http://www.teglerizer.com/triumphstuff/spit_and_gt6_specs.htm
...but then there's a contradiction saying that a GT6 diff is a better idea, and that a 3.63 exists on the "drivetrain" section, here.
http://www.triumphspitfire.com/substitute.html

Hence my confusion. Dag Nab It!

I think you're right on the money though, so I'm going with you.
(And you might want to lock up your shed nice and tight, in case I get desperate for one).
Anyone got this combo who happens to be reading this, I'd love to know watcha make of it....

[GT6 M]

Mk3 GT6 had the 3.89 (if overdrive) or 3.27 if no OD. says mr Clive,
erm not quite true, some came out with the 3.27 and OD. :P
mainly MK2,s going by some factory blurb i seen yonks ago.

Smithy, go for a 3.27. you  have a lighter car than a ..Propppper GT  ;)  .. so should prove to be ok.
Note, that you also have a lott more low down grunt than a Spitty, even a tuned one like Bob Spits. and he running a 3.27, told me its great.

as for being slower off the mark,  well yes and no,  
slower of the actuall mark yes, but slower further up, !!!!
you got to mek moer gear changes, , gear changes take time,  so its about even

also Royston Hairbear  was telling me, he took folk,s advice,and went for a 3.63, but now wishes he gone for a 3.27.
so, 3.27 its got to be, so nee need to raid Ciftys shed now. ;)

Marcus

[heraldcoupe]

Though logic suggests the GT6 diff would be expected to handle higher power, the 1500 was the 'last of the line', and benefitted from all the improvements along the way. Result is that the 3.63:1 diff from the 1500 Spitfire is the strongest of all the standard production types, and most would reckon it's the best all-round ratio for a 6-cylinder Spit or GT6,

Cheers,
Bill.

[GT6 M]

Bill, folk talk of the GT ones being weak, well its the actuall pinnion ive found, not the carrier,
broke three, all lost a tooth of the pinnion, never any thing gone wrong wid the crown wheel, carrier,or planets.

Marcus

[CRAJ]

My Dad has a 3.27:1 in his GT6 mk1 and a D-type o/d, it sports a T6 fabrications bodyshell but is road trimmed so a little lighter than standard, it was a 3.89:1 originally.
He now has a very relaxed o/d top that equates to around 25mph per 1000rpm. He gets a genuine 38mpg travelling up to us at a steady 60-65mph, but still has 5 other ratios to play with if he wants to give it some beans. Still pulls well in o/d top for overtaking and long drags.

Colin.

[smithy]

Ah, Marcus, the 3.27!
There's some stuff about the option here:
http://club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1231192341/
But again not definitive, perhaps, though Clives quite definite on the subject - and isn't the 3.27, indeed, rare? Ho hum!
I should add, I've got the GT6 3 o/drive box in.

[James]

A GT6 with a 3.27 and OD is not fun to drive.

I reckon 3.63 with OD— enough pep and a decent enough cruiser.

They're sports cars after all, not for getting high MPG or worrying about a few hundred RPM at motorway speeds.

[thescrapman]

A GT6 with a 3.27 will  be quicker to 60 than one with a 3.63 or 3.89 as it can do 65mph in second at the redline so you don't need that additional gearchange.

Very long legged in OD-top sums it up though.

Good for economy and relaxed cruising.

I did know one guy who fitted a 2.9:1 diff, now that was a bit sluggish.

Fit a 3.63 and get the best of all worlds.

Cheers

Colin

[Clive]

3.27. rare??
Again, my shed had a few. In fact I broke 3 in my vitesse, and had acquired over the years another 3 spares in the shed. They have all found new owners.
All 3 had mangled crown wheel and pinions. well, only took 2 apart, didn't bother with teh 3rd. Just fitted a 3.63 and didn't break any more.

Marcus, I too have read that some of the earlier GT6's had 3.27 and OD. I thought it was the mk1's but have never seen an original car in that spec. In fact the only mk1 to pass through my hands had no od and a 4.11. Hmm, it was a well passed its best in all respects :'(

[piman]

Hello Colin,

"A GT6 with a 3.27 will  be quicker to 60 than one with a 3.63 or 3.89 as it can do 65mph in second at the redline so you don't need that additional gearchange."

That's being over selective in your data, but a lower geared car will always be livelier than a higher geared car.

Alec

[GT6 M]

I did know one guy who fitted a 2.9:1 diff, now that was a bit sluggish., said Colin,

and here was me with a 2.79 BMW diff in last year,!!! only took it out, cos I did no want to change the drive shafts,


thing is, with a  a PI set up or some more poke, a 3.27 gt / spitty  is very nippy even in 4 th OD ,
[  infact 4 th  j type od, is  as  quick  as a  standard in 3 rd, thru the increments. !!! ]
so it does not make a 2.5 gt sluggish at all.

Clive , not only the GTs, but quite a few Spittys came rom tha factory with a 3.27 as well,
as did Dolly 1850, and some TR 7s

Marcus

[James]

A Spitfire with a 3.27 is nippy?

A light Spit with a 2.5 on PI and a 3.27 is quick— driven one of those lots :)

In fact it's quite nice as the rev limit feels further up because you are going quicker faster (makes sense in my head :) )

[smithy]

So, I tend to think 3.63, despite the many good arguments to the contrary, at the mo.
I won't be doing much touring, (I won't be on the Motorway at all if I can help it), so the 3.63 seems to be about the middle ground - and that's where I usually am in most things. That's current thinking.

How much to pay for the privilege of this swap do you think? A known good 3.63 should be costing me...?
(I've been to Reamers web site and enjoyed the prices - I don't fancy an unknown from Ebay particularly. I can source one from a splendid bloke I know not too far away from here, but I could use a few "real world" price pointers from you cognoscenti, please...).

[GT6 M]

Smiffy, ev a talk with Bob Spit, before you jump the wrong way,
maybe an ..Officer of the Law.. will sway you. ;) cos his is a 1300, and 3.27

[smithy]

Right-o Marcus, fear not, I won't jumped until pushed, and I always listen to my brothers in Blue!

[Clive]

I'll be honest, never heard of a spitfire with a 3.27.
I know the dolly 1850 autos had them, but again didn't know about the TR7's.

If (when??) I get through my stock of diffs I will be looking at the subaru conversion. How hard can it be?

[Nick Jones]

Subaru don't do anything taller than 3.54:1 and most are 4.11:1.

I quite like the 3.63 in the Vitesse, but that is only 2L with brick outhouse aerodynamics.  3K rpm at 70mph with a 0.79 OD 5th.  It would pull a 3.27 but it would take the edge off.  2.5 would definitely need at least a 3.27 IMO.

Nick

[Scottie55]

My vitesse estate 2.5PI has a 3.63 diff and has O/D. Feels a pretty good combination. I guess it could probably live with a 3.27 to lower the higher speed revs but I'd worry the lesser acceleration would be a disappointment. If my calcs are right, its a 10% reduction in acceleration gearing. But on the other hand, its still 90%  :)  So eg, 0-60 in 8 seconds would become 8.8 seconds. Probably quite noticeable.  :-/

[thescrapman]

piman wrote:

Hello Colin,

"A GT6 with a 3.27 will  be quicker to 60 than one with a 3.63 or 3.89 as it can do 65mph in second at the redline so you don't need that additional gearchange."

That's being over selective in your data, but a lower geared car will always be livelier than a higher geared car.

Alec

slightly over-selective.

:-)

[Richard B]

After breaking a 3.27 diff, I then went to 3.63 in the 2500 PI Spitfire. However I have 14" wheels with 195/65 x 14 tyres.

Now considering the Subaru diff conversion.

[smithy]

Richy, may I ask why are you moving on to Suby, especially if Nick's right about the ratios? (He usually is). Strength? Ratio? LSD?

[thescrapman]

796 wrote:

I did know one guy who fitted a 2.9:1 diff, now that was a bit sluggish., said Colin,

and here was me with a 2.79 BMW diff in last year,!!! only took it out, cos I did no want to change the drive shafts,

I should have qualified that a bit more, he had a standard Mk2 2000 saloon engine I think, so not exactly endowed with power, but he was making use of the torque (so he said :-) )

Cheers

Colin

[Richard B]

smithy wrote:

Strength?

Yes, I broke the 3.63 Autosolo'ing as well.   :o

smithy wrote:

Ratio?

3.54 is pretty close to 3.63 (I have 14" wheels)

smithy wrote:

LSD?

Yes   ;)