I have a 1985 KP36. I've made a lot of mods on it mainly on the mechanical part.

1. Changed the stock 2K for a 4K  motor (Brand new cylinder block)

2. Put a Weber DGV 32/36

3. Switched the T40 gearbox with a T50. 

4. Put new Headers and new Radiator with a low temp racing thermostat.

 

All these are great. The problem I have is on the freeway. My Differential is a stock KP36 (picture attached) one with a ratio of 43:10 (4.3) so travelling long distance is a pain and hard to reach top speeds. I need to get me a 3.9 ratio differential in so I will be able to cruise at 100-130 KMH with the engine at low RPMs.

Is there a way I can only switch my Final Diff Gears without removing my entire rear axle? 

Any suggestions ???

Can you run a taller wheel/tyre combo....?

3 hours ago, coln72 said:

Can you run a taller wheel/tyre combo....?

No. The factory tire settings were 165/65/12. The ones I have are 175/70/13 with extra 8 mm spacers in order for the front tires to not touch the shock absorbers. At the rear when I get some extra weight in, the tires escape the wheel arches just a few millimeters. So taller tires are not an option.

Its a jap style diff so you can remove axles, then the centre, and put a complete taller ratio centre in if you like.  Or you can remove the one you have and have different gears fitted to it if you can find them.

I am not familiar with how many diffs there are out there, if I were to buy this one which seems to be the only one available, would it fit? https://www.ebay.co.uk/itm/Toyota-corolla-AE86-t18-4age-m-t-T-series-diff-gears-3-7-ratio/263714728652 

The link is for T series diff gears.  They are 6.7 inches.  The KP diff is 6 inches. 

Hi Michalis,

                    Off memory, the JAP 4.31:1 ratio diff was fitted to Corollas with auto transmission.  Manual transmission models, were often fitted with 4.11:1 ratio diffs.

The  3.89:1 ratio diffs were not fitted to any Corollas as standard, from memory.  However, I am led to believe, that the Datsun 1200 & 120Y series, used the 3.89:1 ratio diffs, of the same origin as Toyota, & that if you can get hold of a 3.89:1 crown wheel & pinion set new, a good diff reconditioning company should be able to swap them over, & lap them in for you.  A lot od Datsun 1200 utes had 3.89:1 ratio diffs.

Others on this forum may be able to confirm or correct my thoughts, as I haven't actually done it myself, but I do have it on my "bucket list", as I have a KE30 with a 1.43:1 ration diff in it, that I would love to fit a 1.389:1 diff into it.

Here is a listing I found, that has a number of 3.89:1 & 3.9:1 ratio diffs

http://www.difflap.com.au/ratios/gearratios_ad.htm

Cheers Banjo 

Edited November 14, 20187 yr by Banjo