Identifying diff M78 or m80 in VX V6

[_R_J_K_]

Is there much work involved in converting an M78 to M80? Regarding tail shaft, Auto trans, gear ratio etc?

If you can find a manual LS diff it will have 3.46 gears factory. Nothing has to change in regards to the tailshaft or gearbox, but you may have to change the flange on the front of the new diff to your old one depending on what model Commodore the M80 comes from. For the most part it's just M78 housing out, M80 housing in.

TBH, if you can just find a good condition second hand M80 LSD from a manual LS it should tick all of your boxes if it's a daily, Truetrac will be overkill for it The factory M80 LSD is a fair bit better than the M78 ones and will save you a fair bit of cash on the centre and having the backlash set if you changed the gears. Speedo will be out if you change the ratio so you will need to correct it somehow, there's a couple of ways to do it.

 

[vr304]

Don’t be afraid of lower ratios if fuel economy is your concern, it will actually improve in normal daily driving circumstances if your not going silly with ratio choice (like 4.11) as for cost yea I hear ya but if you wanna change your ratio at some point anyway the diff is gonna have to come apart again which is another lot of labour it’s best to suck it up now and do it once if it’s at all possible

 

[EYY]

I wouldn’t even hesitate to go 4.11 personally. Anywhere from 3.45 to 4.11 is great. I was going to go 3.9’s again, but found new 3.7’s cheap and they’ll do the job just fine.

 

[losh1971]

I kinda wish I had gone 3.7 on my VR ute, rather than 3.45. 3.45 has been awesome but I wonder now if I'd been happier with 3.7s, plus 3.7s would have meant 17s without going 30 or 35 AR.

 

[EYY]

I kinda wish I had gone 3.7 on my VR ute, rather than 3.45. 3.45 has been awesome but I wonder now if I'd been happier with 3.7s, plus 3.7s would have meant 17s without going 30 or 35 AR.

You don’t need to change tyre sizes. The lighter the rim/tyre combo the better. Better acceleration and braking

3.45 is still good - means you still drop down to 2nd at wot with the auto at approx 100kmph and good for overtaking at highway speeds.

 

[losh1971]

I was more thinking if I had 17s I could then go VE upgrade and maintain a 50 AR tyre to handle the gravel ok. Whereas 17s on my bus now means 35 or maybe 40 at a push or I lose the benefit of the 3.45s. 16s only allow VT upgrade, and when you have near new PBR ute MC, it's a shame to waste it, when you buy the second to last one that will ever be available. For the OP he has decent calipers anyway, so for him yeah less of an issue. Apologies for the hijack

 

[losh1971]

BTW EYY, not meaning to offend you, as yep you have a lot of great knowledge but when I was looking at regearing you told me that I would be better to go 3.7s as 3.45s were not going to make enough difference, I'd have to go searching to find your exact words but "such a small difference" was what I believe you said.

 

[Bawwx]

If you can find a manual LS diff it will have 3.46 gears factory. Nothing has to change in regards to the tailshaft or gearbox, but you may have to change the flange on the front of the new diff to your old one depending on what model Commodore the M80 comes from. For the most part it's just M78 housing out, M80 housing in.

TBH, if you can just find a good condition second hand M80 LSD from a manual LS it should tick all of your boxes if it's a daily, Truetrac will be overkill for it The factory M80 LSD is a fair bit better than the M78 ones and will save you a fair bit of cash on the centre and having the backlash set if you changed the gears. Speedo will be out if you change the ratio so you will need to correct it somehow, there's a couple of ways to do it.

finding a second hand M80 where I’m at that isn’t slogged is a mission in itself, and for a good price at that. Also when it comes to changing the gear ratio finding an M80 out of a V8 with 3.46 would be great, 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

 

[EYY]

BTW EYY, not meaning to offend you, as yep you have a lot of great knowledge but when I was looking at regearing you told me that I would be better to go 3.7s as 3.45s were not going to make enough difference, I'd have to go searching to find your exact words but "such a small difference" was what I believe you said.

Yep depends on what you want. Personally if I were spending the money on having a diff built and buying all new gears and bearings, I’d go 3.7’s or shorter because I’d really want to feel the difference.

But since you can easily find m80 lsd centres with 3.46 gears for reasonable prices (<$300), it’s sometimes easier and cheaper to settle for 3.46 and still net noticeable performance gains for a daily driver.

A mate went from 3.46 to 3.9‘s and it went from a nice quick takeoff to wheelspin through the first two gears. Very worthwhile change in his case.

 

[losh1971]

Yep depends on what you want. Personally if I were spending the money on having a diff built and buying all new gears and bearings, I’d go 3.7’s or shorter because I’d really want to feel the difference.

But since you can easily find m80 lsd centres with 3.46 gears for reasonable prices (<$300), it’s sometimes easier and cheaper to settle for 3.46 and still net noticeable performance gains for a daily driver.

A mate went from 3.46 to 3.9‘s and it went from a nice quick takeoff to wheelspin through the first two gears.

Yeah and in hindsight, I maybe wish I had, because after a while you get used of the 3.45s and start to think, hmmm what would 3.7s be like??