you'll also need the size of wheels and tyres too... You need to consult your owners manual and dinf the box ratios...

Then google search for a calculator... there are heaps on the net to calculate this for you...

Yeah wheels, tyre profile, tyre width, final ratio and box ratios and speed and it'll work back through the measurements to find flywheel revs...

Just hunt around the net for it...

Cheers...

mine is 2350rpm

http://www.angelfire.com/fl/procrastination/rear.html

This one might be better... Shows a whole line up in go

http://pw1.netcom.com/~sgalaba/mph.htm

Ok...

Discovery... None of these take into account tire buldge (IE because it has weight on it)...

So I have 235 45 17 so to calc radius (I'll work in metric so it ends in km/hr)

45% of 235 = 105.75mm profile

17" = 431.8mm

So calc radius of my wheels are half of mag + profile = 431.8/2 + 105.75 = 321.65mm

Upon measuring (for whatever factors cause the difference) I measure 298mm

So that is 1 inch in radius and 2 in diameter that these equations are out...

If you use this formula to get km/hr (where * is times, / is divide, r is measured loaded radius from ground to wheel centre, R1 is transmission ratio, R2 final (diff) ratio, and RPM is your desired RPM, and remember order of operations (do whats in the brackets first))

(0.00037698 * RPM * r) / (R1 * R2) = km/hr

or to calc RPM when you know speed this way

(km/hr * R1 * R2) / (0.00037698 * r) = RPM

Example:

So in theory (still doesn't measure quite right though)

I think 5th is .63:1? and my diff is 3.27:1 at 100km/hr with 298mm radius


(100 * .63 * 3.27) / (0.00037698 * 298)

206.01 / .11234004

= 1833.81 RPM


Compare with the 322 calc'd measurement of the wheel is 1697.13, so 136rpm of difference. Not much but it makes it more accurate this way...

Hope it helps

hi guys i have 2 questions to ask here !

i own an 1999 au forte 4 speed auto stock ! with the stock diff i imagine 3.08 sd or 3.23! crap!
and here goes i wont some extra punch off the line and this is what i need to know from an expert or someone who has done this.

would a 3.71 lsd centre fit straight into my original single spinner diff housing! without changing anything ! drive in drive away !

if i do need a lsd diff housing : eg from rotor to rotor ! does your nearest diff centre have brand new ones and then they just fit them on. is this how it works.

i am not mechanicaly minded and solely rely on mechanics to do my work. just the way it is.
let the pro's do it i reckon. i dont want the usual 3.45 lsd centre ! i never go the norm.
please help.

Take it to a diff joint..

They will be able to tell you what ratios are made to suit the diff... I get the feeling there is a 3.7, but there is definitely a 4.11...

They get your diff and unbolt the centre, remove the crown wheel from the centre, and remove the pinion...

They install the new pinion and crown wheel and then proceed to reset the preload, blacklash, toe/heel and face/flank between the two gears...

While you're getting all these settings reset you may as well get new bearings put in otherwise if they go in 20,000km you up to get it pulled to bits and everything set again for new bearings (tolerances vary slightly between bearings)...

Best to take it to someone like that to get it right if you're not confident (which I'm not)...

Just be aware that odd sized ratios are worth a fortune too... esp for a brand new ones off a store...

I'd either be trying to find someone selling a good second hand set or a new set they didn't use...

You could be looking at over 1 grand to get a new set installed from a diff place... and you still don't have LSD, its still an open centre...

The better option is to wait for a 9" or something like that to come up... Or a diff that someone has already used... That way its been set...

You have to weigh up over 1k for a new 3.7 install in a single spinner or 300 bucks for a half decent 3.45 LSD out of an XR6...

Ratios like this, due to cost are rarely used in road cars and a left to professionals who fit them for a reason...

Cheers and good luck with your search...

thanks so much tim tim ! i went to my local pick n pay less wreckers this morning and had
a look around !unfortunately there was nothing around for the au !there was 1 au xr6 but someone has already taken the guts out of it ! (wich would be ideal really )
i know nothing about cars but i did notice that the diff was different to mine.
i think it was inderpendent rear suspension! does that open up a new can of worms as the rear would have to be modified to fit that would it not.
the rear of this diff housing had a different look to it.there was two bars running from the back of the diff going over to near the rear wheels. please excuse my mechanical ignorance.

hey thats cool...

AUI models were live axle (same as EA/EB/ED/EF/EL) and these are all the same diff right through...

AUII and AUIII are independent rear suspension...

So you have live axle like this:



Or IRS like this:



Photos curtosy of other peoples for sale threads off here...

as you can see there is a lot more to IRS and I imagine different mounting points yes... (I own an ED so never had to worry about IRS...

Here is an example of 3.89:1 gears:

post1068011.html?hilit=el%20diff#p1068011

I imagine to suit EA to AUI differentials....

Cheers...

Not quite right Tim, All AU low spec (Forte, Futura, Fairmont and XR6HP) got live axle as standard, with IRS optional. XR6 VCT, XR8, Fairmont Ghia and Fairlanes got IRS Standard. IRS gears are the same as live axle. Also, AU got a wider diff to change the offset.

My Au forte v8 1998 has live axle LSD unsure of diff ratio. however the diff is moaning and whining time for recondition...

any advice or rough price's people have paid on there diff rebuilds? am thinking if i going that far going 3.7 gear ratio. should be nice.

good question

just to completely clarify, what woudl be the ideal model car for me to get a diff centre from? 1999 au1 fairmont ghia with irs, pretty sure its a 3.23 or 3.45 single spinner.
would it be wiser just to get an lsd centre? or just to buy the whole housing + centre off a certain model xr6?

what would the price of it roughly be?
cheers

The housings are all identical... 3.08 ratio gives the best final drive for the best fuel efficiency... Remember all cars are designed to be most fuel efficient at around 100 to 110km/hr so knowing this, the manufacturer knows which rpm it must be... Knows the final ratio of the gearbox, and hence picks the diff to suit...

3.08 to 3.45 is not that many more teeth... I have never counted the teeth, and you can't have .08 of a tooth, it must be a whole number... so if you do the maths... the ratio 3.08:1 as two integers is 77:25 at the smallest... So I'd near say there would be 77 on the crown and 25 on the pinion... and 3.45:1 as integers is 69:20 at smallest... Therefore all in all there is actually less teeth in a 3.45 ratio diff This doesn't make the crown smaller, it makes the teeth wider to take up more space... So 8 teeth less on the crown and 5 less on the pinion...

For the sake of it, 4.11:1 doesn't simplify any further than 411:100 and this can't be right... Whereas 4.10:1 is 41:10 which doesn't seem enough teeth, so go one step back and you get 82:20 which might be close...?



If you handy and can change the diff yourself... Get the whole thing... Your fairly limited with regard to model choices... You can only get a diff from AUI with IRS, AUII or AUIII... I somehow doubt it would be a 3.45 ss, never seen a factory ones of these before... It could be a 3.23 or something, but if auto, most likely a 3.08 due to the lower output shaft speed than the manual...

If you just buy a centre you have to a diff place to fit it... It requires a full set of shims to get it sitting right... and the collapsible washer on the pinion... A diff spreader, various scales and tools to set the face/flank, toe/heel, preload, and backlash in the diff... If you have the tools and the know how on how to set these up then get a centre, much cheaper and smaller to post... Otherwise a diff place is likely to change upwards of 500 bucks due to the time involved with chopping and changing shims to get the settings right......

At the end of the day its your call... I live 6hours from Sydney and LSD's are rare out here... I wasn't buying a centre due to not being confident enough to fit it and not having all the proper tools... So I went with a Lokka which goes inside a ss carrier (replaces the hemisphere in the centre), as such the carrier and the ratio don't change and no re-setting of those above mentioned settings needed... Just put it back together exactly as it came to bits...

Goodluck...

alright, are you positive about only being able to get it from an auI/II/III? e series have the same diffs as au's apparantly, the centres will just go straight in. does the donor car have to have irs for it to fit in mine or is that just completely disregarded?