How much difference do the actualy make?

4x4 saph cossie, bout 450/460bhp

 

much better acceleration but limited top end

also need to have one of them plates on the front

possible 2nd gear launches if you are brave

 

what speed do they top out at dojj?

 

is it worth the swap though?

would be done with standard ratio front diff and drop gear in transfer box.

 

My mate has them on his cosworth he is running approx 380bhp and you have better acceleration. I think it's a good upgrade for the car.

 

is it not going to be alittle short gearing for the power though?

 

Rainbird runs 3.9's from memory. Best mod my mate ever did to his 3 door.

TiB off here still managed 178mph at Brunters running a 3.9.

I wouldn't say they are too bad on the road, just the cruising speed needs to be more like 70 not the 80+ that your maybe used too.

 

what sort of increase in rpm is there at 70mph over the standard?

 

Ask Mike, I'm quite sure my mates was doing something around 4k @ 70mph but dont take my word on that

 

the standard one is 3.62:1 so the "upgrade" to the 3.92:1 is only 1 less tooth on the pinion drive with 14 instead of 15 or something along those lines

it's "easier" to put the power down with less gears to turn and, in essence, makes your car "feel" more powerful without actually adding any more power to it

 

Was one of the best mods I did to my old 4x4 door.

 

No


You can still do over 180mph at brunters. My Sierra did it...

The speed limit on road is 70mph. The fastest you will go on any racetrack is what 150mph?

so what more you need?

 

Summed up perfectly.

 

I didnt mean so much top speed, i ment going through the gears. If that makes sense

 

Steve Scott summed it up better. He said to me '3.9 gearing makes em sparkle'

 

Love this subject but as always things are not what you think.


The Sierra left Ford with 205/50/15 tyres giving = 23.7mph/1000rpm
Every Tom, Dick & his Harry then fit :
225/40/17 = 24.75mph/1000rpm or 225/40/18 = 25.77mph/1000rpm if you change the Diff to 3.9 on the 225/40/18 = 23.92mph/1000rpm so you need the 3.9 to get it back to standard gearing.
The correct gearing is the shortest gearing that will give you grip no point fitting a 3.9 on mine it would spin its wheels at alltimes everywhere .
The shortest i can go is 3.54 diff with 235/40/18 = 25.72mph/1000rpm.

You can indeed reach 180mph at Brunters with a 3.9 provided the Tyres selected allow you to reach 180mph at roundabout max power.

 

Didnt think of that Rod.

Good point!

 

How much though for front and rear diffs plus the alteration to the box ?

 

And actually finding the 3.9 stuff

 

ive just done it to mine.there is guys in northern ireland that have new and 2nd hand 3.9 cwp on the shelf.

not dricen it yet as im waiting on my fuel tank back.

 

I've got a 3.92 front and 7" 3.92 LSD rear diffs for sale

 

i think i've got a few 3.92 front diffs in the garage

as for 3.92 rears, find any dohc rwd sierra 90/92 spec and you've got one (as they were fitted with the 7.5" 3.92:1 diff as standard

 

I'd like to go this way with mine too. So you need both diffs and the gearbox modded? How much would it cost?

 

if you can find a dohc 4x4 sierra then the diffs are a direct swap, buy the car, swap the diffs, sell the car

you only need to do the gearbox mod if you change the front OR rear diff as it's the step down gearing to keep the centre diff spinning at the correct speed

 

Oh so if I buy front and rear diffs then no need to touch the gearbox?
Chances of finding an old Sierra now are thin on the ground, is they no one that just sells them?

 

ebay throws up a few for around the Ł500 mark occasionally

http://www.ebay.co.uk/itm/ford-sierr...item3cbe2448ff

this one though is a bit much

 

How much Mark?

 

I've fitted a 3.9 to mine (2wd) It deffo makes pottering about and pulling away with an evil clutch nicer than before, the whole car just seems more driver friendly.

Can't comment on performance as haven't had my car on boost much since running it


Hi Rod,
Makes a lot of sense
How would my actual car speed compare to my indicated speed then? I'm runnning a 3.9 diff (2wd) with 235/40/17.
I guess not *too* far out at legal speeds?

 

Does anybody have a 3.9 front and rear crown wheel and pinion for sale?

Is it just the crown wheel that have a different number of teeth? Or the pinion shaft as well?

Thanks
Marc

 

Both are different afaik

Robrs2 above says he has some for sale.

 

I have pm'd him

Anyone need a newly shot peened and super finished 3.6 rear 7.5" crown wheel and pinion then give me a shout

 

is it worth fitting the 2.0 4x4 box as well as the diffs?

 

No as the cosworth 4x4 mt75 has a thicker main shaft i am told so will be slightly stronger,

Also if you change the front and rear diff to 3.9 ratio then nothing has to be done to the standard cosworth gearbox, you only need the drop gears if you leave the front diff as a 3.6 and change the rear to a 3.9 to make up the difference in rotational speeds

Thanks
Marc

 

had this on my old car and will be getting it again good mod imo...

 

Long time no see Ben

 

yeah got my self another money pit hows you, nice pug in your sig whats she running?

 

Lol, they just have that effect on people don't they! I bet your mikes favourite customer right now lol

I'm good though thanks, gave in with the escort in the end. Doh

The pug runs a pinto with 8 injectors and a t4 making about 450hp

 

sounds fruity

mike actually keeping quiet at the moment letting me see what i come up with before he shoots me down in flames