Sierra Diff

[cabrioman]

I need to change the diff in my Hunter to match the swap to a Rover V8 engine.

I was thinking of using a 3.38 ratio but wondered before I take the plunge if anyone had practical experience of running the very rare 3.14 compared to the 3.38.

Looking to end up with something that has sensible acceleration but with lowish revs at motorway cruising speeds.

The engine is a stock 3.5 which I intend to run on carbs so circa 150 - 160 BHP, driving through a 5 speed LT77 SD1 Vitesse box on 15 inch wheels.

If anyone has a spare diff lying around in these ratios I would be very interested in hearing from them.

cheers

John

[Mike]

Re: Sierra Diff

John
I looked for a 3.14 diff for 18 months, and saw only one come up on ebay, and it made over £330: so unless you want to fork out some serious money for one don't get too set on a 3.14.
There is only 7% difference between it and a 3.38, so its fairly easy to take the tyre profile up to achieve the same effect, and probably at a lower cost.
I've gone down this route with my BMW 2.5 M50, and although I've not driven mine, I have driven Chris Cunliffe's Cabrio with the same set up. Chris feels it would make his car too long geared going to a 3.14.
I know you will get a lot of low torque from a V8, but what revs will you be doing at 70mph? - will you ever get out of fourth?
What speed do people cruise at on the Motorway with a Cabrio?
Mike

[lizbillings]

Re: Sierra Diff

Hi Mike. We normally cruise at 60 with the top down but its quite happy at 70 and if in a rush thats what we do.
Regards Liz

[chris_cussen]

Re: Sierra Diff

I'd like to publish my usual cruising speed but I will claim the 5th amendment.

[keith_h]

Re: Sierra Diff

I've got a diff (I believe to be Sierra) that I was waiting to put on ebay but I needed to find out exactly what it is before doing so. It has Ford castings as follows:

On one side

70094 & 83B64025BE

The other side

21 7 88

On the back

FE/2 and D04BA and 83BG-40J3-BA (not totally sure about the J)

Any idea what it is and is it what you want?

I am also about to put a Cabrio Hard top on ebay if anyone is interested (see classified)

[pdm_engineer]

Re: Sierra Diff

Keith
Those numbers are only casting numbers.
The only way to identify the diff ratio is find the code letter/number that should be on an aluminium tag, which is normally bolted to one of the rear cover screws.
The Ford info that I found years ago is as follows.
M=3.14, V=3.38, 3=3.64, D=3.92, A=3.36, U=3.62, B=3.77.
Or of course you can count the prop-shaft turns to complete one revolution of a drive shaft.
The other thing is that John will need the later bolt on type, known as Lobro, drive shafts for his V8.
My data list only shows the 3.14 as being used on 2.3 diesel models, which is why they are rare.

My cruising speed is about 65mph, any more that that and my hat blows away!

[keith_h]

Re: Sierra Diff

Hi Peter, thanks for that. There was no tag so I resorted to counting and it was almost 4 revolutions so I am guessing it is D = 3.92. Just had a thought as I am typing, I didn't check to see what was happening to the drive shaft that I wasn't watching, I'll re do the exercise in the morning

[Mike]

Re: Sierra Diff

Keith
An accurate way to check your diff is to stop one side of the diff rotating, mark the other at TDC, and then see how many turns of the prop are required to complete two revolutions of the single diff.
More controlled and more accurate than letting both go round
Mike

[greyV8pete]

Re: Sierra Diff

Hi John. In the past on a Cortina I have removed the filler plug and put a red chinagraph pencil mark on one of the teeth. It's foolproof for checking one revolution of the crownwheel exactly. Not sure if the filler hole on the Sierra will permit this but you never know.

I think my Hunter diff is 3.62 and I have low profile tyres and according to the sat nav the speedo reads about 10% fast. Good ego booster! I have always thought that the 3.9i engine could pull lower gearing and with a lower ratio diff the speedo reading would probably be nearly correct, so I am following this thread with interest!

I wonder what is the availibility of a matched 3.38 crownwheel and pinion set and if the bolt holes PCD and location diameters are the same? Ford do tend to standardise on these things. I have rebuilt diffs in the past (one of the most satisfying fitting jobs in my opinion) but as my Hunter has a Quaife L/Slip diff this may be a job too far! Peter.

[cabrioman]

Re: Sierra Diff

Hi Peter

Interesting you are running a 3:62 diff, it may be I can get away with that ratio given that the V8 I am using is a stock 3.5 putting out about 150-160 BHP which I am happy with. I may have to do some proper calculations based on the gearbox ratios etc.. If I can use a 3:62 and still get a good combination of acceleration from standstill and relatively relaxed 5th gear motoring that would be easier as the 3:62 diffs are easy to find and a lot of 2wd cosworths had the 3:62 LSD.

John

[dave_kitson]

Re: Sierra Diff

Hi All
Foumd the tag on my diff but it's completely illegible.
I've just jacked up one side of the car, so one wheel stays put, and turned the free one and counted prop revolutions, and it is nearly 2. Am I missing something, as I seem to be out by a factor of 2?!?!

More precisely, for 6 wheel revolutions I got 11.75 prop revs, a ratio of 1.96. Applying the fudge factor of 2, this is 3.92, which is what I expected. I believe the diff was from a 1.8 Sierra, and that's what they were fitted with according to Haynes.

The car has a 3.5 V8 with fast road cams and a 4 SU set up, so good for about 200bhp and plenty of torque (grunt). SD1 5-speed box. Wheels are 15" with 195/65 tyres. This equates to a rolling diameter of 25", which is more relevant than the actual wheel size.

You don't really need 1st gear to get going! At 70mph the engine is doing 2800rpm in 5th, so not revving that high to my mind. On the trip from Bradford to Exmoor, then Spain and back, I got 25mph average over 1200 miles. Not bad I thought, but the biggest chunk was motorway down and back.

Hope that is helpful. My feeling is that it's not worth going less than 3.62 diff, especially if you go for bigger wheels/tyres. Meanwhile, where does that factor of 2 come in?
Cheers - Dave

[cabrioman]

Re: Sierra Diff

Thanks for the info Dave, saved my cashflow. If you are getting that with a 3:92 diff I'll stick with my 3:92 until I've had chance to drive it and then decide if I need to go down to 3:62, from the figures you have quoted I can't see me needing or wanting to go to 3:38

John

[Mike]

Re: Sierra Diff

Dave

Your factor of 2 comes from holding one wheel still: the diff forces the rotating wheel to rotate twice.

[dave_kitson]

Re: Sierra Diff

Thanks Mike. That's a relief!!!
Dave

[marks]

Re: Sierra Diff

I have a very good 3.62 LSD available. Make me a reasonable offer?
Jonathan