Diff ratios

[Johns]

Hi all.

I recently changed my perished Olympic Air Rides for 175/13 80 series radials.

Since then, I have comprehensively sorted brakes, front end, etc.,

The upshot of all this is, that after replacing both rear wheel bearings, I now know the diff is noisy. As I intend to take the car to Philip Island, the issue of cruising revs has become an issue.With no technical knowledge, please bear with me from here.

I understand the standard grey diff ratio is 3.89. I have been advised that these diffs use unique bearings and rebuild is expensive. I am told that the 179 Manual diff is a 3.55 and the traumatics run a 3.36. I understand the yoke is different and would require either an EH tailshaft or modification of my existing tailshaft.

My car has a brand new Air Ride in the boot so as a start I thought I should compare the rolling circumference with the radials. This is what I found. Radial was inflated to 32psi for the exercise.

The Air Ride circumference is 216.3cm, the Michelin 184.1cm. This means the difference is 14.9%!!

If I'm not wildly off beam, does this mean I could run a 4.47 diff and have the same final drive ratio as with radials?

If I haven't stuffed up somewhere, am I right in thinking I could put a 3.36 in the car, 13.4% taller than than the 3.89 and still have the overall gearing a little lower than with cross plys?  

Your views would be appreciated

John

[RET]

John,

The reduced rolling circumference has exactly that effect, although I haven't verified the maths.  The 80 series tyres are forcing the car to work harder, since one revolution of the wheel is taking you only 85% of the distance that the Air-ride was.  This means you need a longer diff to compensate.

If you have a look at the CD you got when you joined the club and find the July 2004 issue of Sideplate (Vol. 23 No. 7), you'll find an excellent article by CraigA that explains the effect of low-profile radials on the driveline, along with some recommendations.

You'll probably find you need a 3.55 or 3.36 to get the economy back and reduce the revs you'd be doing at highway speeds.

Hope that helps.

cheers
RET

PS I got your email - I might give you a call tomorrow to discuss if that's OK.

[FB_MAD]

   G'day John. You can fit a red motor diff centre in a grey diff housing.They bolt straight in and the grey axles will fit the red diff center splines.Just make sure you have a course spline red diff centre as from memory holden red diffs had a fine spline from HX onwards and these will not fit the grey course spline axles.The only other modification is you will have to change your existing grey rear tailshaft uni joint to a red motor one to suit the red pinion flange(around $26 or so and easily fitted).Red diff ratios were 2.78,3.08,3.36,3,55 and 4 cyl LC/LJ Toranas had a red diff center with a 3.89 ratio although Torana diff centers will also fit  they do require a slight mod as they had one long bolt that normal red diffs didn't so best to steer away from them.
  I did have an article from an old street machine magazine years ago with a fomula for working out engine revs with different diff ratios and tyre profiles but now can't find it.This may have been handy for you also. If I find it I'll let you know.  
  Hope this info helps

   Terry

[craiga]

Here you go

http://www.wallaceracing.com/gearcalc1.php

http://www.csgnetwork.com/tirerevforcecalc.html

http://www.wallaceracing.com/calcmph.php

http://www.wallaceracing.com/Calculators.htm

http://www.csgnetwork.com/automotiveconverters.html

Knock yourself out ;-)

Cheers,

Craig.

[FB_MAD]

  Gee thanks Craig. A lot better than the fomula I used to have. I now know I can get 166 mph in top @ 5000 rpm using standard grey running gear.........or have I done something wrong???.  

 But seriously thanks for the info its something I'll use regularly now I know where to look.  


 Terry.