Opinions on Diff Ratio

[ACT59FC]

I was driving the FC the other day around 60mph and thought this poor old girl is screaming! Even at 45mph it seemed to be revving high. I was wondering if anyone has altered the diff ratio and what effect this had. What ratio would be good to provide some low end power but gave a better top end performance. I was thinking the 3.55 ratio may be a good one? I don't want to have to change the whole banjo, Just provide a different ratio for the standard rear end.

Obtaining tyres with the correct rolling dia is becoming more and more difficult.

If anyone has tried or has an opinion on this matter please feel free to have your say.

Regards
           Rod.

[ardiesse]

Rod,

In my FC I've got a 3.36 with 185/70x13 tyres.  It's a very relaxed long-distance cruiser.  You'd be fine with a 3.55 as well.  The LC Toranas ran 3.08 diff ratios even with the 2250 red motor.

Rob

[RET]

Most 70R13 profile tyres don't have the right rolling circumference, and seriously affect the speedo reading and the revs. You should definitely run the car with your GPS going for a journey to get a feel for how far out the speedo is. You might be surprised.

185x80R13 is as close as you can get with a radial to the original rolling circumference.

cheers
RET

[RET]

Oh, one other thing. The original diff was 3.89:1, and I can only assume that's what you've got. If it's been changed, it can only be longer legged, not shorter.

[mcl1959]

I'm with Rob, a somewhat tired old stock grey would go a 3.55 ok
A good strong grey could get 3.55 or 3.36
A seriously hot grey could go a 3.08

All these diff centres will bolt into the stock banjo except the 3.08. It needs a slight modification to one of the bolts.
You will need to change the rear universal to the later style though.

Ken

[Fraze]

I am running a 3.55 in my stock standard FC (out of an HT sedan). It has made quite a difference to cruising, now much more relaxed. It doesn't pull quite so well on a hill, which is understandable, but is really barely noticeable. Some people are running a 3.08 but in my opinion that is way too tall for a stock engine. Go for the 3.55. Cheers. Fraze

[FCRB26]

Yep i use 3.36 and 3.55 when towing

[NES304]

How longs that take you Pete?

[FCRB26]

the fe i tow with has 3.55 and the wagon has 3.36 but since u asked i can do it in 32mins is my record 18 odd centres i did in the sedan with the rb30 back in the day being an idiot  ;D

[ACT59FC]

Are there any trip ups like different axle splines. What splines do the FC's have as standard? Are all diff splines the same?


Regards Rod.

[NES304]

So Pete what your saying is you start ALL the projects and wow us along the way?

[FCRB26]

Are there any trip ups like different axle splines. What splines do the FC's have as standard? Are all diff splines the same?


Regards Rod.

Hey Rob

Theres coarse spline and fine spline centres fine splines are less common very noticeable difference tho.
They also made LSD banjo centres they range from 400-800 dollars if you can find one they were also coarse and fine spline.

i have only ever seen 3.36 and 3.55 fine splines.
Also a rare one is a v8 banjo the diff pinion yolk (where the tailshaft bolts) is a lot larger on the diff centre  weak as piss behind a v8 hence why there isnt many around.
If in doubt text me a pic of the centre and i will tell you if its right 0410687343

Are you after a 3.36 ? i dont why but i still have a fetish for collecting banjo centres i have them everywhere  ;D

[ACT59FC]

Hi Pete, yes I am after a diff, tho my poor old FC has a very tired stock motor. From the advice given above I think the 3.55 will be the one for me. I hate to hear the old girl revving so high with the standard diff there. So if you have any 3.55's Im interested. Oh you didn't quite mention which spline the standard FC diff is, is it the coarse or fine?

I think what I need is a 3.55 with coarse splines????? I take it that If I change Diff's I will need to put the eh universal joint on the rear of the tailshaft?

Rod.

[FCRB26]

Hi Pete, yes I am after a diff, tho my poor old FC has a very tired stock motor. From the advice given above I think the 3.55 will be the one for me. I hate to hear the old girl revving so high with the standard diff there. So if you have any 3.55's Im interested. Oh you didn't quite mention which spline the standard FC diff is, is it the coarse or fine?

I think what I need is a 3.55 with coarse splines????? I take it that If I change Diff's I will need to put the eh universal joint on the rear of the tailshaft?

Rod.

Personally i would go with the 3.36 i shall check my stash.
You need a coarse spline as fine spline were more for cars with a bit of HP gtr and xu1 type toranas ect.

Yes u will need to fit the EH uni very easy job.


Pete

[ardiesse]

Rod,

Might be a dumb question, but . . .

Are your tappets adjusted correctly?  Clattery tappets add to the general engine noise level at speed.

And if your motor's tired, then putting a taller diff in may (possibly) be asking a bit much, unless you're running lower-profile radial tyres.

Rob

[ACT59FC]

Hi Rob, tappets are all good, One of my first tasks was tappet adjustment and a good tune, replaced Points, condenser, rotor button, distributor cap, leads and spark plugs. The engine is very tired to the point of smoke being visible from time to time as I accelerate. The tyres fitted at the moment are 185/75 R13. The old girl is revving hard at 45mph let alone 60mph. I dont think a 3.55 will be too hard on her.


Regards Rod.

[FCRB26]

Hi Rob, tappets are all good, One of my first tasks was tappet adjustment and a good tune, replaced Points, condenser, rotor button, distributor cap, leads and spark plugs. The engine is very tired to the point of smoke being visible from time to time as I accelerate. The tyres fitted at the moment are 185/75 R13. The old girl is revving hard at 45mph let alone 60mph. I dont think a 3.55 will be too hard on her.


Regards Rod.

with 75 series i think your right the 3.55 would be the go.. might be valve stem seals. maybe a compression test might give you a bit more of an idea..

[ACT59FC]

You didn't locate any 3.55's in your stash Pete? I have a set of 7" Mags Im going to put on her while my orig rims are being sandblasted and powder coated. The tyres are 205/60 R13.


Rod.

[FCRB26]

Just looked 4 3.08s
5 3.89
2 3.36
1 2.78 your out of luck
I have some in container at work unknown condition

[ACT59FC]

Hi Rod (by the way great name champ),

I went through this many years ago and found two grey crash box speedo gears. The standard for FE/FC having 23 teeth and a 22 tooth gear from a 48-215/ FJ for 15 inch wheels.

Please have a look at this thread.

Speedo Gears???

Cheers

Rod

The above was posted on speedo calibration thread by a different Rod.

It seems like you can have some adjustment to the speed readout by changing the speedo drive gear from an FC to a FX/FJ one. I am hoping this will do the trick when I replace my 3.89 with a 3.55.

I dont know how to make the Thread work but its an interesting read.

Rod.