Afternoon everyone,

I hope you're all well?

I'm just wondering, what difference will changing the diff in the seven make?

I've got a 2.0l zetec with throttle bodies and megasquirt, with an MT75 box. After some tuning problems it seems to be running reasonably well, but acceleration really isn't up to much. (Apparently the guy that built it had touring in mind).

I think the running gear is from a Granada, so my questions are:

1. How do I tell the difference between a 7in and 7.5in diff? I'm guessing there's a measurement to be take from somewhere.

2. How can I tell what the ratio of the diff is at the moment?

3. Which diff ratio would give me an average performance, more towards acceleration than top end?

4. Will it make that much difference changing it or is it more down to tuning?

Thanks in advance,

Chris

These are the diffs side by side


diffs

The higher the diff number the better the acceleration so a 3:1 diff will be slower on acceleration than a 4:1 ratio.

You can tell what you have either by reading the tag on the back of the diff if it is still present, or by taking the back off the diff and reading the ratio off the crown wheel, or most easily by jacking the car off the ground and counting how many turns of the prop it takes to make one turn of the wheels.

It does make a difference.

ETA: In my opinion a 3.9:1 or thereabouts is good for a zetec engine.

[Edited on 5/8/11 by mookaloid]

So if I've got a 7.5in diff fitted at the moment, could I change it for a 7in one or will I have to change other components to accomodate it?

Thanks,

Chris

The chassis will need modifying to accommodate a 7.5 inch diff. They are quite a bit heaver as well.

quote:

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Originally posted by MikeRJ
The chassis will need modifying to accommodate a 7.5 inch diff. They are quite a bit heaver as well.

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You can use spacers to take up the mounting clearances if you fit a 7" diff. Don't know if half shafts need changing.

Wheel/tyre size also has a big effect. 13" wheels or 17". Looks v acceleration.

If you think the ratio is bad there's not neccessarily any need to change to a 7" diff as I'm sure there is an equally wide range of standard ratios available in 7.5", isn't there? You just need to decide what ratio you want and find one in 7.5". And as Blake says make sure your engine's working properly too!

Thanks for your replies so far.

I've just got back from the garage, I jacked up one rear wheel and marked a reference point, I also marked a reference point on the prop shaft. I turned the prop shaft and it took just under 2 whole turns of the prop to turn the wheel one revolution.

Surely that doesn't mean I've got a 1.something:1 ratio diff? I'm confused?

I'm still working on the engine, it's standard bar the gsxr throttle bodies and megasquirt, I'm just wondering where to go next.

Thanks again,

Chris

no, because if you only turn one wheel, half of the revolution is lost to the diff. so if its just under 2 turns, you double it, so its a 3. something.
a more accurate way is to push the whole car backwards and forwards, with all wheels on the ground, and count the turns of the prop

if you take the back cover off the diff, the actual number is usually written on the crown wheel.

[Edited on 5/8/11 by blakep82]

quote:

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Originally posted by blakep82
no, because if you only turn one wheel, half of the revolution is lost to the diff. so if its just under 2 turns, you double it, so its a 3. something.
a more accurate way is to push the whole car backwards and forwards, with all wheels on the ground, and count the turns of the prop

if you take the back cover off the diff, the actual number is usually written on the crown wheel.

[Edited on 5/8/11 by blakep82]

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Sounds like something in the region of 3.6 or 3.9 then which are probably the most common ratios, but if you already have a 3.9 you can't get lower with a standard ratio I dont think (or are those later 4.somethings 7.5" diffs??). Your acceleration ought to be OK with a 3.6 or 3.9 I would have thought - what are your speeds in each gear? Maybe your engine really isn't running right?

[Edited on 5/8/11 by Liam]

quote:

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Originally posted by Liam
Sounds like something in the region of 3.6 or 3.9 then which are probably the most common ratios, but if you already have a 3.9 you can't get lower with a standard ratio I dont think (or are those later 4.somethings 7.5" diffs??). Your acceleration ought to be OK with a 3.6 or 3.9 I would have thought - what are your speeds in each gear? Maybe your engine really isn't running right?

[Edited on 5/8/11 by Liam]

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You can get a 4.27 7.5 diff from a scorpio, if changing from a 7 inch three of the mount holes will line up ,one top mount will need moving (passenger side viewed from back of diff).If you are going from 7.5 to 7 you just need a spacer on the one side of the top mount . The bolt on driveshaft flanges are the same distance apart on both diffs. The 7.5 is also around 10mm longer than the 7 on the propshaft end.