39 replies to this topic

[solihullian]

hi all, i'm building a straight cut box for my mini, the gearset that i'm using is a tran-x 6 piece kit, my question is :- what would be the best diff to use? i have a 3.1 cwp and a 3.6 cwp, some people have said that 1st gear will be rediculously tall geared with the 3.1 diff but on the other hand i dont want the engine revving its nuts off in 4th gear with the 3.6 diff at 70mph either.

ant help/suggestions would be helpful

thanks.

Edited by solihullian, 07 March 2013 - 06:00 PM.

[Guess-Works.com]

You'll be fine with the 3.1, as long as you're not putting it under an 850.

But in all cases the final drive you choose should be governed by what you intend on using the car for, not necessarily what you have available.

[solihullian]

You'll be fine with the 3.1, as long as you're not putting it under an 850.

But in all cases the final drive you choose should be governed by what you intend on using the car for, not necessarily what you have available.

thanks john, the car will just be a road car mostly but i will be using it for shows as well and i didn't want to drive down to somewhere like L2B with the engine revving highly at 70mph ~(i never drive over the legal limit..... )

[solihullian]

just another question actually, i assume that the final drive in my original gearbox is a 3.1??? the engine is out of a 1992 mini 1275 sprite but its still in the car so i cant take the diff out to check just yet.

[Guess-Works.com]

Yep, that'll be a 3.1, unless someone has changed it.

[Vipernoir]

I've got a 3.105 on the back of an ST ratio gearset underneath a warmed up 1150 and it is an utter ******* to get off the line. Hillstarts are "entertaining" when fully loaded.
Although it's nice on the motorway, next time it's apart I will be dropping to a 3.44 or shorter.

You need to be honest with yourself about the main use for the car, and build a box to suit.

[solihullian]

I've got a 3.105 on the back of an ST ratio gearset underneath a warmed up 1150 and it is an utter ******* to get off the line. Hillstarts are "entertaining" when fully loaded.
Although it's nice on the motorway, next time it's apart I will be dropping to a 3.44 or shorter.

You need to be honest with yourself about the main use for the car, and build a box to suit.

mmmm, my engines pushing 90lbs/ft of torque so it should pull it i reckon

[Cooperman]

The final drive ratio is one part of the total specification for engine & transmission, all of which must match up to the use to which the car will be put.
It depends on the cam, head, carburation, capacity, etc.
For example, with a 286 cam, an 11:1 C.R., big twin crabs and a lightweight flywheel if you fit a SC close ratio gearbox a much lower final drive ratio (FDR) is necessary in order to avoid poor initial acceleration and lots of clutch slipping. With that sort of spec a 3.44 is about the highest you can use effectively and a 3.76 might be better.
Remember, you fit a close ratio gearbox to match a hotter cam with a narrower torque & power band. In turn that means you are seeking better acceleration and expect to be using a lot of revs all the time. That is the only reason for a SC CR gearbox.
I had a 544 cam in a 1330 'S' with a high-comp big-valve head and even a 3.44 was too high, so a 3.9:1 was fitted. Cruising speed was 60 mph at about 4200 rpm, but acceleration was fantastic and it was easy to keep it 'on the cam'.

[MRA]

Thats 3 gear changes to get to 60 mph ? I think only a die hard would be happy with that combination...

A standard MPi on 13" tyres ran a 2.76:1 final drive, with 90lbf ft of torque you will easily get off the line with a 3.1:1 final drive

[Cooperman]

Thats 3 gear changes to get to 60 mph ? I think only a die hard would be happy with that combination...

A standard MPi on 13" tyres ran a 2.76:1 final drive, with 90lbf ft of torque you will easily get off the line with a 3.1:1 final drive

I ran that spec (544, Longman head, light flywheel, etc) with the 3.44 FDR on an international rally and when trying to pull away up any sort of hill the clutch really got a hammering. My current 'S' has a 3.9:1 FDR and I can pull 60 in 2nd gear at about 6700 rpm (approx) which gives a 0 - 60 time of 7.3 seconds.
Of course a standard gearbox will pull a 2.76 FDR, but 1st in a SC CR box is very high especially when using a 'cammy' engine. If the engine is standard it is not correctly matched to a SC CR box really as 1st gear goes on 'forever', then 2nd to 3rd & to 4th are simply too close together.

[Carlos W]

0–60 in 7.3 secs?

i'd just drive around blowing away boy racers

[MRA]

I agree a cammy engine requires a lower FD than a similar none cammy engine, however I have run loads of road going minis with SCCR boxes and fairly mediocre tuning, and found that you actually get more benefits than most people would have you believe.

Please also don't forget that a close ratio box changes its characteristics as you change the final drive ratio, transfer gears ratio and the size of your wheels and tyres.

Peter I don't doubt that your set up is excellent in rallies, and to some would be great on the road, however I have run with nearly every final drive ratio and gearset available to mini man and still I rate the higher final drives for road use, if the cam is a racier duration then yes of course the FD needs to be lower.

1st gear may go on for ever, this is subject to the amount of power, but a reasonable 1275 can easily pull a 3.1 FD with a SCCR box on 13" tyres and still be very sharp off the line, but more importantly, you get off the line nice and clean with a lot less wheel spin, and gearchanges.

[solihullian]

Thats 3 gear changes to get to 60 mph ? I think only a die hard would be happy with that combination...

A standard MPi on 13" tyres ran a 2.76:1 final drive, with 90lbf ft of torque you will easily get off the line with a 3.1:1 final drive

I ran that spec (544, Longman head, light flywheel, etc) with the 3.44 FDR on an international rally and when trying to pull away up any sort of hill the clutch really got a hammering. My current 'S' has a 3.9:1 FDR and I can pull 60 in 2nd gear at about 6700 rpm (approx) which gives a 0 - 60 time of 7.3 seconds.
Of course a standard gearbox will pull a 2.76 FDR, but 1st in a SC CR box is very high especially when using a 'cammy' engine. If the engine is standard it is not correctly matched to a SC CR box really as 1st gear goes on 'forever', then 2nd to 3rd & to 4th are simply too close together.

after reading the comments i might just stick the 3.6 diff in it, the cam in my engine is a 286 with a CR of 10.4:1 with MG metro sized valves and is well ported, the car is currently running a single HIF44 but normally runs twin HS4's, the gearbox will be running helical transfer gears.

Edited by solihullian, 07 March 2013 - 10:04 PM.

[MRA]

What tyre size are you running, a 286 would in most cases be fine with either, excpept that your motorway journeys would be a painful experience.

[Cooperman]

A 3.6 FDR would be about ideal.
If you are anywhere near St. Ives, Cambs, you will be welcome to come and drive my 1310 cc car with 286, twin H4's, light flywheel, 35.6/30.5 mm valves, 11.2:1 CR, light flywheel, SC CR gearbox with BMC ST ratios, 3.9:1 FDR. I would be happy to go to a 3.76, but really not much higher.
The wheel size doesn't make very much difference as a 165/70 x 10, 165/60 x 12 or 175/50 x 13 all have virtually the same rolling radius within a few percent of each other.
With a 286 you do need a CR box to keep it in the optimum power band, but then a 286 is not really a normal road cam, rather it's a good rally cam when high revs are used most of the time. Mine has little below about 3200 rpm, but will pull right through to 7000 rpm with best power from about 5000 rpm to 6700 rpm.