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_R_J_K_

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then comes the cost of a tune from Mace to allow the different ratio (regardless of the diff changing the gear ratio will require a tune) that increases the cost by about $250 minimum.

As for an M80 factory LSD will that be predictable for doing skids and keeping an angle without one wheel gripping like the open centres do? A mechanic recommended the Harrop TruTrac for burnouts because of its strength and reliable whilst also maintaining the ability to be a quieter and easier alternative to welding the stocky for a daily. From what I’ve read I need a 1.5 or 2 way diff specifically for this application and the only decent one I can find for a VX V6 auto application is https://www.harrop.com.au/shop/truetrac-m78-28-spline

You don't need a tune for this. You're probably thinking of changing the PPK values for the speedo? You can do this with a Tech 2 or a box that intercepts the speedo signal, it's more of an LS thing and I'm not even sure if it's a value stored on the memcal anyway. Mace say they can't do anything for it anyway, see here - I need the speedo recalibrated as I changed the diff gear ratio from 3.08 to 3.46 and I run 235/45/17 tyres. - (maceengineering.com.au)

So what exactly is the application here? Are you going drifting or just ripping skids? The 1/1.5/2 way refers to the diffs ability to lock on acceleration and deceleration. If you're just doing burnouts a 1/1.5 way will be fine, wouldn't really recommend a 2 way for daily driving (or you might as well weld it). It should be reliable enough (It's behind an LS engine with far more displacement and torque), it could be exactly what you're after, and if it's not just wait until it wears out and rebuild it then.

IMHO there's very little point in spending that kind of money on a diff for an N/A Ecotec (until you know you need it) when a stock M80 swap is so much easier and cheaper, will be super friendly for normal driving too. Even then rebuilding the M80 you swapped in with a Truetrac centre will be far cheaper/better than doing the M78 even with the initial buy in of the diff. The Eaton M80 Truetrac centres are like $600 or something, the only thing comparable to that price in M78 world is the Torqueline/ Torque Lock diffs, then finding diff gears for them can be difficult depending on what you want.

I think you're focusing too much on the car being a VX V6 auto instead of looking at the diff itself. The Harrop M78 Truetrac really only exists to cater for the VL-VS live axle diffs that can't swap to an M80, pretty sure Harrop licensed the design of the Truetrac from Eaton to build it.
 
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Immortality

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On a VX you can fix the speedo in the dash with software from Envyous Customs (site sponsor), Tazzi has that problem solved. No need to alter the tune in that respect.

I have both the Eaton Truetrac and the Harrop version for the M78 and I'll say the Harrop is definitely a nicer bit of kit but at double the price it should be. I'd do the M80 swap if I didn't already have the Harrop unit.
 

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Okay so I have found a solid M80 to swap out the M78, the bloke says the gear ratio is 3.07:1 and my M78 is 3.08:1. Would I need to get a tune to suit this ratio for the auto shift points and fix the speedo or would it be driveable straight away? Also with swapping the diffs, do I need to swap the axles or anything? Would what the swap entail?
 

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Okay so I have found a solid M80 to swap out the M78, the bloke says the gear ratio is 3.07:1 and my M78 is 3.08:1. Would I need to get a tune to suit this ratio for the auto shift points and fix the speedo or would it be driveable straight away? Also with swapping the diffs, do I need to swap the axles or anything? Would what the swap entail?
No just bolt it in providing you have a 3 bolt tailshaft.
 

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No just bolt it in providing you have a 3 bolt tailshaft.

would it nearly always have a 3 bolt tail shaft in the VX V6 with m78?
if it doesn’t have a 3 bolt tail shaft, what will I have to do?
 

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No. VX was the change over model to the newer drive shaft configuration that uses the rubber dougnut type coupling instead of the uni's. If they are both the 3 bolt flange than the swap is straight forward. If it's the newer style than you will need to swap over the diff input shaft coupling hub as the V6 uses a smaller coupling than the V8.
 

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No. VX was the change over model to the newer drive shaft configuration that uses the rubber dougnut type coupling instead of the uni's. If they are both the 3 bolt flange than the swap is straight forward. If it's the newer style than you will need to swap over the diff input shaft coupling hub as the V6 uses a smaller coupling than the V8.
I thought it was only the l98/LS2 VZ that had the larger coupling, and all pre 6L couplings were the same (assuming 3 bolt)? I'm probably wrong though, I've never had much to do with vt-vz v6's.
 

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so I’m certain that it’s a 4bolt? Into the rubber doughnut coupling and can’t find anything on the web for what I will need to do to convert the tail shaft to be able to fit into the M80? Also spoke to someone and it seems to be more than just converting the tail shaft to fit into the new diff?
 

_R_J_K_

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what I will need to do to convert the tail shaft to be able to fit into the M80?

You convert the M80 to fit the tailshaft, you don't change anything on the tailshaft. You undo the big nut in the 4 bolt flange and bolt the 3 bolt one from your old diff in. Others will chime in about if this will effect the backlash or the preload.

Do you know what model this diff came from?
 

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Personally I’d buy a 3 bolt tailshaft (cost next to nothing because they’re everywhere) and fit that. There is the risk of crushing the crush spacer and ruining the bearings and gearset if you overtighten.
 
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