I race my 1981 Datsun Sunny with an A12 engine in junior speedway in Western Australia. It has standard gearbox and a 4.1 diff (H145 I think).

On smaller tracks I run 2nd gear with large R13 tires and this works great - it pulls about 8000 rpm at the end of the straights and is just running out of steam - perfect !

On larger tracks though it runs out of steam before the end of the straights and everyone passes me. This is not good.

Toyota Corollas run big R13 tires, 4.1 diffs, and 2nd gear at small tracks, and small R13 tires, 4.4 diffs, and 3rd gear at bigger tracks.

There is no equivalent setup to the Corolla's for the Sunny on big tracks.

This year the rules for junior sedans changed as follows:

Gearbox and diff housing to remain standard for make and model) no cutting, grinding, drilling etc allowed) i.e. Corolla must use Corolla gear box and diff. Ratios are free as long as it fits in correct housing and are OEM. (NOT not to borg warner etc)

My dad says if we could find a higher ratio diff or we could change third gear we'd be in great shape.

Everyone says I should switch to a Toyota Corolla cause they have better diff and gearbox ratios, but I love my sunny and wouldn't think of it.

I've read the tech sections, but no answer found. Any help or insight appreciated

BigRed

The original diff in an Aussie Sunny sedan was an H150 in 3.9 to 1, however an H145 in 4.11 to 1 from a Datsun 1000 or early 1200 ute will fit & I guess that this is what you have.
The axle centerline in the H145 diff will be 2.5mm forward of the original & there is no easy way to remedy this, but you probably don't notice it with a welded diff as I presume it to be.

The next standard ratio up from the 4.11 to 1 would be the Sunny's original H150 in 3.9 to 1. This diff, ... in this ratio, was used in all imported Datsun 120Y's [Aussie built sedans used a Warner rear axle assembly] & all Sunny models except the very late, A15 powered Sportswagons which had an H150 in 3,7 to 1 ratio.

There was also an H145 in 4.375 to 1 used in Datsun 1000 Vans & Utes & a rare 4.625 that was an option in these two models.

the diff in a 120Y and sunny is H150...

I have a january '81 sunny sedan... it has a 3.889 diff ratio...

H150's came in 3.7 and 3.889 ratios... 4.11 possibly??

it sounds like they are really tightening up the rules for junior stockys!...

I have the remains of an old 120Y junior stocky... it is set up to go in ONE direction - Anticlockwise...

If you are running 4.1 it may be possible that someone has put a H165 in, out of a Stanza? If that is the case, you will have more choice in ratios.

doubtful as they are pretty stringent on what's allowed...

however the F4W60L* that is in my March '77 120Y is out of a ex junior stocky 120Y

by the rules it SHOULD have a F4W56A!...

*factory A14/A15 gearbox

but if it bolts in and looks stock.........

Quote: The H150 was available in 3.889 [frequently shortened to 3.9] 3.7 & 3.54 to 1. The last mentioned was not offered in Australia, but seems relatively common in the US.
I have not found any listing for an H150 in 4.11 to 1 as a standard offering, but this ratio might be available through Nismo or other aftermarket sources.

thanks for all the helpful responses !

up until this season we could run only 4.1 or 3.9 diff ratios as they were what was shipped with 1979-81 sunnys in Australia.

this season the rules changed for diffs and gearboxes !

the new rule is that for both diff and gearbox all that needs to remain stock is the diff and gearbox housing !

we can do what we like inside the housing. different ratios etc etc so long as they are OEM parts from some vehicle. only restriction is we can't modify the housing and we can't make have gears specifically made.

My dad found a H165 with a 4.875 ratio in a Vanette, and while it could be adpated to use in the car it is obviolsuly bigger and would be seen to be breaking the rules ;-(

The H165 insides are too big for either the H145 or H150.

So unless there is a diff center with a better ratio we could stuff in our existing diffs we are in trouble.

Is it feasible to change third gear ratio ?

BigRed