3.7 diff and 3.9 diff pinion gear differences (both 10 teeth)

YouTube · Post by **El_Freddo** » Tue May 15, 2007 2:46 pm

Anyone know what the difference is in the 10 toothed pinion gear between the 3.7 and 3.9 ratio diffs?

I'm wanting to know if there's any visible difference or if anyone knows how the two interact with the ring gears (when swapped - eg: the 3.7 pinion with the 3.9 ring - is it noisy, will it break a tooth off something? etc etc)

Any input of experience with this would be greatly appreciated. I'm wanting to know the difference, not a flat out "it can't be done" without some reasoning...

Cheers all.

vincentvega · Post by **vincentvega** » Tue May 15, 2007 4:41 pm

the pinion gear is machined to match the crown, whats why when you swap a diff ratio you always put the crownwheel you want to use back in the diff housing it came from.

I would imagine it would be noisy and would wear very quickly. dont know - never seen anyone silly enough to try it

AlpineRaven · Post by **AlpineRaven** » Tue May 15, 2007 7:46 pm

if you used same number of teeths but 3.7 and 3.9, it won't match and when you turn it around, it'll go "cluck cluck" but wont turn smoothly.
I've done it before, it has to be matched set ie. 3.7 with 3.7 and 3.9 goes with 3.9.
Cheers
AP

YouTube · Post by **El_Freddo** » Wed May 16, 2007 2:33 pm

Cheers guys, that throws out my idea of swaping the 3.7 diff for 3.9 while retaining the 3.7 drive gears (1st to 5th). Anyone know how to do this using the 3.9 pinion and the 3.7 drive gears?

legacytt · Post by **legacytt** » Fri May 18, 2007 1:23 pm

Use your 3.7 input shaft as is, get all the gears pressed off the 3.9 pinion shaft and have the 3.7 gearset pressed back on. This will give you 3.9 front diff with 3.7 gear ratios.

YouTube · Post by **El_Freddo** » Fri May 18, 2007 1:47 pm

legacytt wrote:Use your 3.7 input shaft as is, get all the gears pressed off the 3.9 pinion shaft and have the 3.7 gearset pressed back on. This will give you 3.9 front diff with 3.7 gear ratios.

Hey legacytt, i'd thought about that a long time ago but its never re-surfaced... I'm taking it this will mean some big $$$$ or just some $$$?

Cheers

legacytt · Post by **legacytt** » Mon May 21, 2007 11:34 am

If you take the shafts in to a GB place with a press and get them to do it it won't cost much, just a bit of Labour. If you take two complete boxes in, yes it will cost a bit. Be prepared to pull apart and reassemble and you will save.

YouTube · Post by **El_Freddo** » Mon May 21, 2007 3:33 pm

legacytt wrote: Be prepared to pull apart and reassemble and you will save.

I'd be doing all the work i could if i had the tools and knowledge... now to just get the boxes... fun fun.

I'm looking at doing this to an AWD box (when i get one), if i want to run 3.9 diffs i'd have to get a 3.9 AWD box and a 3.7 PT box, swapping the drive gears from the 3.7 onto the 3.9 shaft, plus the other 3.7 drive gears into the (original) 3.9 drive gear's place. is this correct or have i lost some people along the way again?

Cheers

BlackMale · Post by **BlackMale** » Mon May 21, 2007 3:57 pm

El_Freddo wrote:is this correct or have i lost some people along the way again? Cheers

LOL you have lost me again

I htink that i need to go back and look at what are you wanting to acheive by doing this again?

YouTube · Post by **El_Freddo** » Mon May 21, 2007 4:14 pm

BlackMale wrote:LOL you have lost me again

I htink that i need to go back and look at what are you wanting to acheive by doing this again?

Here it is as i've previously stated - good to read again...

El_Freddo wrote:1st thing i want to do/know is: Can i pull the 3.7 diff out of my gearbox and drop a 3.9 diff in, retaining the 3.7 ratio drive gears (1st -> 5th and reverse). I realise i'll have to change both front and rear diffs to change the ratio in the car. Phizinza has said above that this is not possible without taking the whole 3.9 gearbox internals with the 3.9 diff as the shaft with the pinion gear on it is not a slip on and off application, its cast/ground as a one piece.

The thread from where the quote above (of myself - how bizarre...) can be found here

Same basic thing, just the other way around this time - get a 3.9 AWD gearbox and strip the gears off the final drive shaft to the 3.9 diff. Place the gears from the final drive shaft to the 3.7 diff onto the 3.9 final drive shaft that has the 3.9 10 tooth pinion on the end of it.

This will give me the 3.9 10 tooth pinion on the AWD final drive shaft that will allow the lockable centre diff to mount to the back of it with the 3.7 drive gears (1st to 5th), retaining true speedo reading with larger wheels. Now i have to work out what size wheels i can run with this configuration, i hope its not a piddly up size, otherwise i'll have to look for a different way of doing it.

Hope this helps everyone who has trouble understanding my tormenting way of writing things...

Cheers

Xtreme_RX · Post by **Xtreme_RX** » Mon May 21, 2007 4:30 pm

I was always under the impression that the gear ratios (1st to 5th) were the same regardless if it’s a 3.7 or 3.9 part time 4WD Box.

I will have a look at my FSM when I get home.....

YouTube · Post by **El_Freddo** » Mon May 21, 2007 4:33 pm

I'm looking at doing this not for the ratios but for the same performance as i get with standard size tyres and speedo reading but with larger tyres and a changed diff ratio to compensate for the difference...

YouTube · Post by **El_Freddo** » Tue May 22, 2007 2:14 pm

Xtreme_RX wrote:I was always under the impression that the gear ratios (1st to 5th) were the same regardless if it’s a 3.7 or 3.9 part time 4WD Box.

I will have a look at my FSM when I get home.....

If this is true, does it mean that an L series with the 3.9 diff will travel at 100kmph with the revs at (for example) 3500rpm as opposed to an L with 3.7 diffs travelling at the same speed with the revs at 3100rpm (which is roughly what my car does with standard wheel size)...

Would like to know this too (got a good list of "would like to know this" now...) .
In my mind having the 3.9 box would mean a larger set of wheels and a speedo adjustment unit to bring it back to the proper speed reading and have the same (or near enough) performance as the standard 3.7 box with standard wheel size. This said, it doesn't fit properly with me that it works like this as this would be the very simple option if you could find the 3.9 gearbox and matching diff.

If anyone could clarify it would be helpful.

Cheers

legacytt · Post by **legacytt** » Fri May 25, 2007 9:31 am

Not so easy with the L series AWD setup as these have a different pinion shaft to the Part Time ones. You would need a 3,9 AWD box which came in the XT6 and are rare. May be better just to convert to a Liberty 3,9 and make a crossmember and shorten the tailshaft. The Lib Gearset will go in your L case and the Centre Diff off the Lib bolts on the back but is longer than the PT box.

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3.7 diff and 3.9 diff pinion gear differences (both 10 teeth)

3.7 diff and 3.9 diff pinion gear differences (both 10 teeth)

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