DamEscort

Hi peeps i've done a search around the forum and cant find the answer.

What i would like to know is how to work out the ratios to match a gearbox to a diff.

My current situation is i've bought a gearbox and diff to fit to my UK impreza, checked the number on the box to confirm it is indeed from a UK spec 1996 Impreza and i was told the diff i got with it was from the same car.

Stripped the backplate off the diff today to double check what i'd been told and when i divided the number of teeth on the crown wheel (39) by the number on the pinion (11) it came out to 3.545.

Now according to the gearbox chart the final drive on the box (TY752VN4BA) is 4.11 so should this diff not also be a 4.11 ratio to match the box or am i just not understanding and working it all out incorrectly

Would really appreciate if someone could take the time to explain how it's worked out if i've got it wrong.

Cheers David

172sport

iTrader: (6)

UK boxes in most of cases have a drop ratio of 1.1 to 1, obviously this one doesnt.

In that case you have the wrong rear diff......

You could take the back off the box and see what ratio the drop gears are. 1.1 to 1 or 1 to 1.

DamEscort

does it show you the drop gear ratios in the spda gearbox charts and which boxes there fitted to etc?

172sport

iTrader: (6)

Some charts do.

DamEscort

Can anyone shed any light on this for me please. Really would like to understand how this works.

Splitpin

Your initial understanding of the concept is bang on. Gearing is just maths at the end of day. So, on that diff you've looked at, you need to spin the input shaft 3.545 times to make the outputs go round once (hence the gear ratio being expressed as 3.545:1).

The Impreza geartrain starts with the input shaft, which has the five (or six, plus reverse) drive gears on it. Underneath it is the output shaft which holds the matching driven gears. Look at a transmission chart and you'll see the ratios. Like with your diff, a ratio of 2.00:1 means the input shaft rotates twice for every rev of the output. 1.00:1 means that the shafts rotate at the same speed (and so the drive and driven gears would have the same number of teeth). A ratio of less than 1 means that the output shaft rotates faster than the input; for example with a ratio of 0.75, the output shaft rotates 1.33 times per 1 rev of the input.

The output shaft is connected to the centre diff which splits the power to front and rear. The front output goes direct to the front diff pinion. The rear output, though, runs through another pair of gears (the dropgears) before reaching the propshaft. On many boxes, these dropgears are 1:1 ratio (and so the front diff ratio and rear diff ratios should match). However UK cars commonly have 1.1:1 dropgears, so...

You're working it out correctly. According to the SPDA chart here, that box does have a 4.11 final drive and 1.0:1 dropgears. You'd therefore need a 4.11 rear diff to match it.

The 3.545 diff you've got is from a later UK car. These boxes have a 3.9 front diff and 1.1:1 dropgears (multiply the 3.545 diff by 1.1 drops and you have 3.9, so making the front:rear ratios match).

DamEscort

Anyone able to help show me how to work this out please?

As far as i've read i do have the correct matched gearbox and diff but i would really like to know how to work it out for future reference.

Thanks mate the time it took me to write this out you had replied lol

Splitpin

No sweat.

DamEscort

Thanks for your knowlege on this Splitpin, it's really got me confused as we had also taken the rear diff off another 96 wagon as the gearbox that had been removed from this car had also checked with the gearbox chart as being a 4.11 box but when we stripped the back plate off the diff to check and count the gears it also was a 3.545 ratio the same as the one i had already. This car was known to have it's origional box and diff from factory and was also a UK model. This made me feel the box and diff combination i had was correct but now reading your above post would make it all wrong again lmao.

Is there anyway you can check the dropgear and ratios of the gearbox without splitting it? I've been told to split the box requires you to destroy a bearing which i would obviously rather not do.

Thanks again for your time.

Splitpin

The random factor is that it's always possible that the SPDA chart is wrong.

You can check the dropgear ratios without splitting the entire box or destroying anything. Just take off the housing right at the back and you'll expose them. Probably an FAQ somewhere on how to do it so have a look in there.

It's slightly more difficult to work out the front diff ratio without splitting, but should be possible with the rear housing off. If you turn both the front diff output shafts together and count how many times you need to rotate them to make the centre diff case turn once. The viscous coupling should keep the diff more or less locked, but repeat the count two or three times for a bit of insurance.

DamEscort

Thanks muchly mate i'll give that a go next time i'm down at the car, it's just caught me off guard a bit with stripping off the diff from the other car and finding the exact same ratios and from the spda chart showing that my gearbox is the same model year as the car we stripped it's been somewhat confusing to say the least lol.

I've just been reading an old thread on here from 2005 about Danny Boy stripping his box but even with the pictures on that i'm struggling, you should see the drawings on the bit of paper i have infront of me hahahahahaha i was never good at art.

Cheers again mate.