Anything and everything GT6.

Rear axle, brakes, wheels and tyres.

I have a Mk111 GT6 with sports exhaust and HS6 carbs. I have overdrive and the 3.89:1 diff to go with it, but have a 3.27:1 ratio diff and am considering fitting it to give the car 'longer legs' as, compared to modern cars, it always feels like you need an extra gear on motorways (can't always avoid them!) even with OD.

A previous thread talks about the 3.63:1 diff I notice. I run 175/70 tyres too.

I'm not too bothered about loosing a little performance (it won't affect the sound!).

Is this likely to make a significant difference in giving cruising speed at lower revs? I am no expert, so any views (including any 'Don't Do Its' would be welcome to aid the decision making process.

Tim

tim I have a 2.5 pi spitfire with o/d and a 3.27 and I find the tall gearing to be exactly what I want

the 3.63 diff is a good compromise between the 327 and the 389 especially with an overdrive gearbox

If you decide to go for a 363 I would like to offer a new home for your 327 diff...

jonathan

It's a question of maths really...

using your current set up you would be cruising on a motorway in 4th-o/d at 70mph and pulling 3300 rpm (3.89 dif).
with a 3.63 you'll be doing 70mph @ 3000 rpm
and with a 3.27 you should be using about 2700 rpm

So effectively you'll be dropping around 300 rpm for each ratio.
You don't need to worry about the non-o/d gears as the difference will be minimal.

While MOST GT6's had 3.89:1 with overdrive and 3.27:1 without (giving the same overall gearing in top/OD top) there were some GT6 Mk2's which the factory sold with the 3.27:1 ratio AND overdrive. This gives about 25mph per 1000rpm in OD top, so very relaxed on motorway. Would not recommend it (with 2l engine) in a Vitesse, but should be fine in a GT6.
It's really a matter of what you want and how you drive.
The acceleration will be the same as now (I take it you have no OD at the moment), just a more relaxed highest gear with OD. With the 3.89 diff, you will get slightly faster acceleration through the gears, but just the same engine speed in OD as your current direct top).
As already suggested, the 3.63:1 is a compromise and arguably the best of both worlds.
However, if you have decided you want OD, fit that and run the car and see how you like it. You can always change the diff later (without undoing any of the work already done).

Thanks Jonathan, Nigel and Chris for your responses. I e-mailed them to my mate Jon who's been a Triumph nut for years and he came back with the following;

"My choice now having driven with all 3 different arrangments with and with out O/D over the years would be a 3.69 ratio.

The reason is that as you have an overdrive, combined with a 3.69 ratio the engine revs are in my opinion spot on for Motorway cruising. The revs being low enough to sound comfortable but high enough for the engine to maintain 70-80 mph with out having to move up and down the gears if you need to accelerate.

Also the 3.69 diff is significantly physically stronger and less prone to stripping the pinion teeth (Iknow Ive done it ). The 3.27 diff is in demand by non overdrive owners so if you went with the 3.69 option, you should easily get back the money you spent on the 3.27 diff. Also I have that 3.69 diff I bought sitting in the corner of the work shop already available!

I think at around 70-80 mph you are going to be pulling around 3000 rpm with a 3.69 and around 2600 with a 3.27.
You might find that 2600 is a touch low for maintaining speed and could actually cost you more in fuel as the engine will be quite a way 'off the cam' and not spinning and sucking at its most efficient speed.

Dont equate Lower rpm with better mpg [WHICH I DON'T - I JUST WANT LOWER REVS ON MOTORWAY]. Certainly if you are reducing from say 4000 which would be sort of beyond the peak point down to 3000 you would expect a fuel improvement, but if you go to far down the revs the engine starts to struggle because its gulping the fuel and air but cant burn it at an efficient rate.
Like trying to pull away in a lower gear sort of thing.

If it was mine I would fit 3.69 as it is the best compromise in my view.
If you fit 3.27 you could expect to run lower revs all the time with slightly less acceleration. Possibly a higher comfortable top speed cruise of say 80-85 and possibly a fuel improvement on long fast runs. That last point is debatable as I say".

I don't know if anyone wants to add comment to this? i.e. do you agree.

Chris - Didn't make it clear, I have already got overdrive fitted with a 3.89, but I still feel I'd like easier motorway cruising. Interested in your comment, "Would not recommend it (with 2l engine) in a Vitesse, but should be fine in a GT6". Would you care to elaborate? Is this to do with the weight of the Vitesse as opposed to the GT6? (just guessing).

Jonathan, if I do go with the 3.69, I bought the 3.27 from Mike Papworth. He has, I believe, fully reconditioned it. I paid approaching Â£300 for it to give you an idea. Also, don't know where you are, but I'm in Oxford.

Thanks again all.

Tim

Vitesse/GT6 difference in diff: last Sept during the 10CR going up the Alps my GT6 with a 3:27 diff couldn't shake off Craig Gingell's Vitesse with a 3:89; just couldn't pull away! So now I have a 3:63 fitted and accelleration is great and, once its working, the OD that I've got should give me a higher and quieter cruising speed. Other than that I think the 3:27 diff is fine in normal circumstances.

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