Diff centres

[Martin]

Earlier today I was reading a thread about replacing the diff centres.  Now I'm buggered if I can find where it was.

My FC sedan seems to be revving too high.  Probably has to do with both the original diff ratio and the radial tyres on it).  In the thread I was reading, it was stated a HR 3.55:1 diff centre will fit the FC axle housing - so I've got a few questions:

1.  Does the reference to "diff centre" include the pinion and the casing around it (so that it bolts straight onto the banjo housing?

2.  How can I tell what the ratio of my diff is (from the outside)?

3.  Can anyone remember where I can find the original discussion because there was some good stuff in there.

Thanks.

Martin

[nicko]

Hi there is a few discussions on this subject but on June 10 i started a thread titled simply(diff) and there was a couple of answers that included part numbers of needed uni joints,its only down about 5 or 6 from this thread,anyway back to your other question,all holden diffs have the ratio stamped into the crown wheel just turn diff by flange until you see the ratio on edge of crown wheel.there is a few other stamps but number in this case( 3.90 )is followed by the word ratio so you cant make a mistake,also as you can see in photo the whole centre is lift out you just undo your axles and pull them out a couple of inches then unbolt diff and pull it straight out ,you will have oil everywhere that should not be let touch your clothes so i recomend you ware an old shirt and pants and when finished throw in bin as even a couple of drops of gearbox diff oil when put in washing machine will make everything else in wash stink for ever,lol we have all done this trick at some stage,mums love it

[robbzfc58]

hi  guys   the ratio is also stamped on the flange( a flat metal disc) behind the uni joint yoke....clean it up & you should be able to see it.
any centre from any banjo will fit straight in as a complete unit      (3.08  needs a small mod to 1 bolt)
          cheers trev

[Martin]

Great, thanks Nicko and Trev.

It was your thread that I was reading, Nicko, and I've found it again.  Ta.

Thanks, also, for the tip about the oil.  For some reason wives/mums get funny about greasy, smelly things in the wash.  They just don't understand!

Thanks, also, Trev.  I thought there must have been some external reference.  (btw, Trev, a few weeks ago you said I needed to post pictures of my car - well that's still coming, but at least now I've got one here)

Martin

[EJ_Dave]

Martin,

the original 3.89 diff doesn't have the ratio stamped on the flange but all later banjo centres do. If it has the older style yoke (for non U-bolt type unis) it will be a 3.89.

David

[Martin]

Thanks, Dave - you've just identified mine for me; although I figured it is 3.89 by the way the donk spins.

Martin

[Jodstar]

If changing the diff ratio, Can you change the seedo
cable gear in the gearbox acordingly and are they
available from rares? What would be the most economical diff center ratio?
                                                Jodstar

[RET]

No, at least not it a grey crash-box.  It does not have a removeable speedo drive gear, because there was only ever one diff and wheel combo behind it to worry about!

There is adjustment in the speedo head itself that allows for some tuning, but I don't know of too many successful DIY jobs with it.

Martin, you hit the nail on the head, your over-revving is caused by the radial tyres and stock diff.  Our resident technical expert CraigA has written an excellent article on the subject to appear in the next NSW club magazine "Sideplate", and possibly as a Tech Article on this site after that.  But long story short, the most common radial tyres with a 3.55 diff get you back pretty close to an accurate speedo and reasonable RPM ranges.

cheers
RET

[Emu]

O.K.

Having read all that, I'm off the the "Parts Wanted" section to start looking for 3.55 diff for Hilda.

Any volunteers?

Emu - Bird on a steep learning curve.

[RodM]

I've been through this process a number of years ago and somewhere on this site there is a discussion based on it. Through my collection of speedo gears I found that I had two types with a different number of teeth. For memory one had 21 teeth while the other had 23. Ken informed me that one was taken from a 48-215 / FJ gear box because of their bigger wheel diameter. I changed this gearing and while not 100 % accurate it would be within two miles per hour.
Hope this helps.

Rod.

[craiga]

Can anyone confirm the types of plastic speedo gears available? From memory there were different colours for different teeth numbers. Is this right? And exactly what types were avialable? And what were they designed for?

As RET said, in this months Club magazine I have detailed the effect that modern replacement tyres have on overall gearing, and how to overcome this through diff ratio changes. Lots of figures, but I think it makes interesting reading. I would really like to add this info if it can be verified.

And as RET said, we are planning to put all this up on the Tech area of the clubs website for all to see. It will also include some online calculators where, among other things, you will be able to enter tyre type/diff ratio and see what speedo error you have.

Cheers,

Craig.

[Martin]

Thanks, Craig.  I'll have to keep my eye on the "Sideplate" that gets sent to the SA club.  It will be a good reference.

Regards,

Martin

[EJ_Dave]

I had a search and found the thread referred to by RodM- see modification help section 'speedo gears' topic. The FX-FJ speedo gear with a 3.55 should be about right in an FC.

I am looking forward to your article Craig's. To answer some of your qeustions, while there were two speedo gears used in the grey crash box,  several were used in the EJ-HK crash box to cope with different diff ratios, speedo calibrations (50mph/100rpm [pre-EH] or 60mph/1000 rpm [EH-HK]) and wheel diameters. I have a list of colours and teeth numbers somewhere for the later box if you are interested.

David

[craiga]

David,

The document in Sideplate is designed to show the effect of radial tyre replacement on a standard geared FE/FC and does not include any reference to wheel rim size changes. I will however be including this feature on the web based calculator that is currently under development.

The document does offer a conclusion regarding the use of radial tyres and how to ensure your speedo/gearing rmains factory standard. Especially important as my calculations show that there is a VERY significant RPM increase when the more freely available 175/70 or 185/70 radial tyres are used with the stock diff. This information will be included on the web page, which should only be a couple of weeks away.

Could anyone confirm or deny Dave's claim that there was more than one speedo gear used, or available to fit,  the FE/FC crashboxes? My guess is that there was a different type used between FX/FJ and FE/FC to allow for the different size wheel diameters (15" vs. 13")? Am I right in this view?

Maybe someone can advise if gear remained the same but the speedo calibration was changed to make the allowance?

Any help appreciated.

Thanks,

Craig.

[FCwagon]

Craig,
I think the gears could be the same - Many moons ago, I replaced the whole gearbox in my FE with one that came out of an FJ and the speedo didn't change at all.
Leigh

[craiga]

Thanks Leigh, this is what I have heard as well but I obviously need to confirm 100%.

Anyone else with info??

[RodM]

As I mentioned in the post above I can confirm I found two different gears. And there was a difference when I changed them.
Rod

[gp]

Craig,
The Master parts catalogue lists:-
Speedo Gear unit 48, 50, FJ PN# 7405060
&
Speedo Gear unit FE, FC, FB, EK(M) PN# 7405945

I guess you could conclude from this that they are a different ratio  

Hope this helps.

Cheers,

Graham.

[RET]

Leigh

Maybe it was a warmed up FJ with 13" wheels?  

cheers
RET

[FCwagon]

RET,
Hard to know the size of FJ wheels as it was at the wreckers with most of those bits missing. Looked to have been pretty stock otherwise. Can't argue with the parts book but. The FE it went into was a bit hot tho' & unfortunately only lasted 3 months before I headbutted a power pole.
cheers,
Leigh