I'm contemplating changing the 3.75 diff ratio in my cortina atlas axle on my f27, to give faster acceleration,
can somebody advise me as to the best ratio to fit please?
axle connected to type 9 5 speed , 13" wheels fitted. engine is 2l pinto for now ( will be fitting vx 2l redtop in future )

the ratios I've seen so far are - 3.44, 3.75 , and 3.09.

what sort of speed figures would I be looking at, and what top end speed would I be losing out on?
many thanks
Greg

3.44 and 3.09 will give higher top end but less acceleration.

Somthing in the 4,s would do it.

I think


try this

[Edited on 31/7/06 by big_wasa]

Try this
http://www.westfield-world.com/gtc2.html

I'm sure there's a 4.1:1 standard diff in one of the Cortina models possibly an estate as they are usually lower geared.

Just been on the gear ratio calculator,
great reading

so far on 185 70 r13's , type 9 gearbox -

4.63 diff -
at 5000 rpm 1st 20.4 mph 5th 90.9mph

3.75 diff -
at 5000 rpm 1st 25.2 mph 5th 112.2mph

3.44 diff -
at 5000rpm 1st 27.5 mph 5th 122.3mph

3.09 diff -
at 5000rpm 1st 30.6mph 5th 136mph

top speeds at 8000rpm were - 145.4mph, 179.5mph,195.7mph ,and 217.9mph!!
if only!!

so would I be right in thinking that the 3.09 diff ratio would be quicker on the acceleration front?

No, the other way round. I have just swapped a 3.75 for a 4.11 diff in my Tiger. Gives me better use of the 8000rpm limit on my 4age engine.

Cheers for the replies guys,
just found one on e-bay for a 5.1 ratio!!
on abuy it now - too much dosh though!!

Is it an easy job to change them over ? or is it an axle off job?
will try to get a 4:1 or a 4:63 , I like the acceleration speed better than the top end!!
if anyone knows where I can get one from , give me a shout! ta

Had a 4.63 in my MK1 Escort Twincam many, many years ago.

Acceleration was brisk, but IIRC full revs equated to about 80MPH with a 4 speed box!

Also bear in mind that with a ratio like this, from a standing start you'll suffer from a LOT of wheel spin.

On the Atlas axle the diff housing seperates from the front of the axle. So chassis clearance allowing, you'll be able to change it without removing the axle. I seem to recall that there's some sort of pre-load to be set up when installing the new pinion.

Drew

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Originally posted by lotustwincam
On the Atlas axle the diff housing seperates from the front of the axle.
Drew

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yep, 4.1 and 4.6 are the real choices. We used a 5.1 on a mk2 escy rally car, good in forests but thats it, with a millington engine that revved to 9500 we could make 115mph!!

I used a 4.6 atlas in my RWD Pug 205 rally car and that have about 115 mph at 7000 revs iirc which is probably about what you want if you don;t do many long journeys.

If you plan any longer trips go for the 4.1, with the extra torque of the vauxhall engine it will still be bloody quick!!

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Not quiet that easy, that's the English axle. The Atlas diff is built into the axle casing.

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