Diffs on auto and manual FDs

[mike01]

I\'ve got an FD type x that was converted from auto to manual before I bought it and I want to know what the diff ratio is. I think it was 3.9 on one and 4.1 on the other (can\'t remember which is which right now).

Does anyone know if there\'s a quick and easy way to tell the difference between an auto and a manual rear diff?

Thanks,
Mike

[turbotoaster]

3.9 is auto, 4.1 is manual.

had an auto and a manual infront of me the other day and from the outside i couldnt see any differences.

best way to tell is rpm/mph

if you have satnav tell me what tires you have on and what rpm your at when your doing 50mph(go off satnav, not speedo), from that i can work out what diff you have

[mx1311]

one has 39 teeth on the crown wheel the other has 41, that\'s the easiest way to tell

[mike01]

Thanks for the help.
I\'d rather not open the diff up, so will give the sat nav method a go. Just got to get it back on the road first!

[leefitz]

Sorry for the we hijack, how much of a difference is there in the different sizes? As in what\'s your speeds at in 2nd and 3rd ant say 6k rpm?

[turbotoaster]

Sorry for the we hijack, how much of a difference is there in the different sizes? As in what\'s your speeds at in 2nd and 3rd ant say 6k rpm?

assuming your running new 255/40/17s on the rear tyres at 6krpm you would be doing

4.1 manual diff

2nd gear  54mph
3rd gear   78mph

3.9 auto diff

2nd gear 57mph
3rd gear  82mph

obviously the faster your going the bigger the difference is, for example in 5th at 8krpm the difference is 10mph

[leefitz]

Thank you man. What other crown ratios is there in the fd series? What about the fc crown? Fitment the same as on an fd?

[turbotoaster]

what are you trying to achieve, shorten your gears to help with accelaration or lengthen them to get a higher top speed?

[leefitz]

It would be to shortin them. It\'s for my drift car. The track I\'m currently driving on, i struggle to get 3rd, partly clutch, partly lack of power but prob mostly me. When the car picks it\'s heels up enough to go for third I\'m normally too close to the corners so not really an option. So my thoughts are that if I was to up the ratio it mite help with her trying to open up after the slow hairpins an maybe allow me to get 3rd on the following conner, and ofcourse up my angle and maybe speeds I can achieve. I am only a beginner so this could be just silly thinking.

[leefitz]

Can anybody shed some light on other available crown wheels?

[TRex]

The correct answer to you question is,

 It is highly unlikely you will have an Auto Diff still in there as it simply won\'t work with the Manual Drive shaft...
And the Auto drive shaft won\'t fit in the Manual Box!
 So money is on you do indeed have a manual diff.......



BTW, The only real visable difference is in the Drive shaft connection...  

[AtomicRex]

^^^^^^^ what he said!
 
The output flange is slightly different.  I meant to take a picture of both the otherday....but if you want to take a picture of yours and post up we will be able to tell you if that is an auto or not.

[tinker-27]

turn the wheel one revoloution and the prop will turn 3.9 times or 4.1 times, you can easily change the prop flange so its not a given that it has what the flange is , or turn the prop 4 times and if the wheel turns more than one turn its a auto diff and if it turns less than one turn its a manual.

[tinker-27]

if its a auto i will swap you for a manual if you like,i would do it labour free !!

[mx1311]

you can just change the input shafts too, i have done in the past


but if craig is offering, take it up, always a top service

colum