As above, currently running a 7 inch 3.9 with ATB and looking to raise the ratio.

3.62 was a common and popular diff.
Not sure how your ATB parts fit to weather or not it can be refitted.

Probably fine for road use but not race.

as far as I know the only stock diff with that ratio was the 7.5" one found in the 2.0 8v scorpio's (95-98) - and the 7" ATB won't fit in a 7.5"
(also the 2.0 16v scorpio's had the same 7.5" diff but with a 4.27 ratio)


some places used to do aftermarket CW&P sets at 4.1 ratios - just not a cheap option though even when you could get them new....

https://www.raceline.co.uk/products/part_section.asp?categoryID=6%20&SectionID=86
https://www.burtonpower.com/crown-wheel-pinion-cwp-4-1-1-ford-sierra-7-axle-cws41.html
https://www.quaife.co.uk/quaife-products/crown-wheel-pinions/

you could ask Quaife for a one off set - but even if they will do one I doubt you'd like the price

Thanks for that - I'm just putting it out there in case anyone currently has a car with a 4.1 but finding it unsuitable for road use, or maybe knows where there are some suitable parts. You never know! I'm not even convinced it's necessarily the way to go but you never really know till you try it, hence not really want to spend a huge amount finding out. By the time you take in to account additional gear changes etc it's never a straightforward equation. Drag squaring with speed doesn't help either.

^^ I know were your coming from, but these 4.1 ratio 7" CW&P sets are rarer than rocking horse poo

even when they were available new they were £600+ just for the gears without even fitting them - so basically nobody bought them for anything other than racing (and I bet the guys who got those have them hiding away in the back of the shed for the next time they need it)


you could try swapping to the 7.5" scorpio diff if you wanted to try the ratio, but thats a lot of faf with driveshafts etc...


all for a 4.5% drop in the gearing - you could get a bigger difference just by changing the tyre sizes (I know there are other things affected by this, not least the sizes available)
but for example a 205/55 R15 to a 205/45 R15 is about a 7% drop in gearing on its own

I may have one, just removed it from my car (which was raced before i got it) but not entirely sure of its ratio, I think its a 4:1 but it has no markings and i havent taken it apart yet to count the teeth. It does have a whine but may just need the backlash adjusting.

At 70mph it was doing around 4200rpm hence the need to swap for road use.

Can let you know in a couple of days what ratio is.

Graeme

Thanks Graeme, appreciate the heads up and let me know what you think when you've counted the teeth. In the meantime I'll try and do some research and let you know how many teeth you may or may not be counting:-)

Hi

I cant crack the bolts on rear to count teeth (need a better nut gun) but ive done the old revolution trick and i think it may be a 4.4:1 diff as i get nearly 4 1/2 turns of the propshaft to every wheel rotation.

will get a better gun and crack open the diff and let you know though

Graeme

Well many thanks for the update, that certainly sounds interesting. Be nice if your counting was out a little bit and it was a 4.1 (!!)

By my reckoning 4.4 is right on the cusp of being workable. On UK tracks I'm running out of rpm in 4th at places like Thruxton and Snetterton but not early enough to make 5th a practical proposition. That is running a 3.91 FD with 1 to 1 fourth and .87 fifth gear. Limiter is about 7100 so that equates to a speed of 122. Going to a 4.4 FD would give me a max speed of 126, and I think that might be cutting it too fine. Or maybe not, I need to delve in to this a little more deeply. I'll drop you a pm.

I'll have a 4.1 7 inch diff available in a couple of weeks if that's any help?

Its currently fitted to my project chassis, but its going off to a specialist to have the ratio changed to a 3.14 as I'm using a bike engine. The chap is going to swap the CWP onto a donor open diff so I'll have a complete unit for sale.

Let me know if its of interest. If you're only after the 4.1 CWP I can speak to him about leaving it unfitted.

If it's easier than chasing rocking horse droppings, many mk1,2 and 2.5 MX5s were fitted with 4.1 torsen diffs.

It'll need some thought on how to mount it and some custom halfshafts but it's an option.... The mk3 diff (also available in 4.1:1) would probably package easier but it's a tochigi fuji super diff and they're a bit fragile

quote:

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Originally posted by geordie40
Hi

I cant crack the bolts on rear to count teeth (need a better nut gun) but ive done the old revolution trick and i think it may be a 4.4:1 diff as i get nearly 4 1/2 turns of the propshaft to every wheel rotation.

will get a better gun and crack open the diff and let you know though

Graeme

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