3.08 diff - again!
Discussion
It's been a while now since this topic has been aired. So here we go....
I'm in the camp that thinks our cars are too low geared, at least the 5ltrs.
I've also got a clonk from the rear when prompted, so most likely worn pinion bearings and time for a rebuild this winter. And a good time to change the gearing.
I've managed to source a NOS GM Holden 3.08 ring gear set at a considerable saving over the new Spicer versions.
I intend to do my own rebuild, and to that end have been building up some info on the diff. The Holden Commodore VS V6 manual contains the rebuild procedures and there is lots of good information on some of the Aussie forums.
Ratech in the US look like the best source for all the parts. They also do a solid spacer and shims to replace the crush sleeve, which I think is a much better way of setting the pinion bearing preload.
Setting the pinion height on these diffs is a little tricky because rather than using a published figure for top of pinion to centre of carrier BW use a zero height mandrel in place of the pinion, and a dummy axle in place of the carrier to provide this height. The space measured with feeler gauges between these items is the thickness of shim required beneath the pinion bearing race. The inference being that they didn't consider the face of the pinion to be a reliable datum point. And that means that copying the old pinion height would also be unreliable. It also infers that the height that they are looking for is bearing race to centre of carrier. So my current thinking is to use a bearing race extractor, sat in the race and use the top of that to centre of carrier as the setting height carried over from the old to new pinion. Presumably if the same make of bearing is used then the same shim can be used. Then the only adjustment required to the shim is any offset difference marked on the pinion ends. At the end of the day, it's gear tooth position that matters anyway, but getting the initial height as close as possible will no doubt ease the task. Anyone been there?
Carrier bearing preload is easier to set on these diffs due to screw adjustment of the bearing carriers rather than shimming.
Anyway, looking forward to an interesting project and I'll update progress over the winter.
Here are some pics of what's arrived.![](https://thumbsnap.com/sc/UvKfERRc.jpg[/img]</a> <a href="https://thumbsnap.com/EpWFjMio" Target="_blank" rel="nofollow">[img]https://thumbsnap.com/sc/EpWFjMio.jpg[/img]</a> <a href="https://thumbsnap.com/LbcYikTg" Target="_blank" rel="nofollow">[img]https://thumbsnap.com/sc/LbcYikTg.jpg)
I'm in the camp that thinks our cars are too low geared, at least the 5ltrs.
I've also got a clonk from the rear when prompted, so most likely worn pinion bearings and time for a rebuild this winter. And a good time to change the gearing.
I've managed to source a NOS GM Holden 3.08 ring gear set at a considerable saving over the new Spicer versions.
I intend to do my own rebuild, and to that end have been building up some info on the diff. The Holden Commodore VS V6 manual contains the rebuild procedures and there is lots of good information on some of the Aussie forums.
Ratech in the US look like the best source for all the parts. They also do a solid spacer and shims to replace the crush sleeve, which I think is a much better way of setting the pinion bearing preload.
Setting the pinion height on these diffs is a little tricky because rather than using a published figure for top of pinion to centre of carrier BW use a zero height mandrel in place of the pinion, and a dummy axle in place of the carrier to provide this height. The space measured with feeler gauges between these items is the thickness of shim required beneath the pinion bearing race. The inference being that they didn't consider the face of the pinion to be a reliable datum point. And that means that copying the old pinion height would also be unreliable. It also infers that the height that they are looking for is bearing race to centre of carrier. So my current thinking is to use a bearing race extractor, sat in the race and use the top of that to centre of carrier as the setting height carried over from the old to new pinion. Presumably if the same make of bearing is used then the same shim can be used. Then the only adjustment required to the shim is any offset difference marked on the pinion ends. At the end of the day, it's gear tooth position that matters anyway, but getting the initial height as close as possible will no doubt ease the task. Anyone been there?
Carrier bearing preload is easier to set on these diffs due to screw adjustment of the bearing carriers rather than shimming.
Anyway, looking forward to an interesting project and I'll update progress over the winter.
Here are some pics of what's arrived.
There is only one type of M78 7.75" ring gear. The crown wheel is stamped 0578. The solid spacer I'm referring to replaces the pinion bearing crush spacer for setting preload.
I think you are referring to the M75 7.5" ring gear which is a weaker item. Although it could be shimmed to fit in the M76 IRS housing we have, I don't believe it was ever intended, or used in the M76 IRS applications. It had its own application in the M75 axle used in base model Commodores and maybe some other models.
The 0578 crown wheels were also used in the M78 solid rear axle and shared with the M76 IRS unit. However the 3.08 ratio was, as far as I can determine, only used in the Commodore VS and VT V6 n/a. Hence their relative rarity. I think the racing application is a self perpetuating myth.
The supercharged variant of this model and the V8 moved onto the M80 IRS which is visually similar, but larger, and shares no parts with the M76.
I do enjoy a few track days so hope it will work out, can't see why not as pretty much any car can be enjoyed on a trackday. Sprinting may well be different and in fact going the other way to 3.77 maybe.
I think you are referring to the M75 7.5" ring gear which is a weaker item. Although it could be shimmed to fit in the M76 IRS housing we have, I don't believe it was ever intended, or used in the M76 IRS applications. It had its own application in the M75 axle used in base model Commodores and maybe some other models.
The 0578 crown wheels were also used in the M78 solid rear axle and shared with the M76 IRS unit. However the 3.08 ratio was, as far as I can determine, only used in the Commodore VS and VT V6 n/a. Hence their relative rarity. I think the racing application is a self perpetuating myth.
The supercharged variant of this model and the V8 moved onto the M80 IRS which is visually similar, but larger, and shares no parts with the M76.
I do enjoy a few track days so hope it will work out, can't see why not as pretty much any car can be enjoyed on a trackday. Sprinting may well be different and in fact going the other way to 3.77 maybe.
Edited by Dougal9887 on Monday 6th July 07:48
Edited by Dougal9887 on Monday 6th July 08:53
At the moment I appear to be permanently in 5th. 5th in town, 5th on open road, 5th to overtake, 5th on the motorway. My gear changes are 1st, 3rd, 5th. (car doesn't shunt anyway)
On track, it's generally 2nd and 3rd, with usually one change to fourth on the longer straight. Annoyingly with the 3.45, 3rd, at 6000rpm 96mph is often reached just short of the braking point and not worth changing up and back down unsettling the car into braking. With the 3.08, 108mph, hopefully, this will happen less often.
With the 3.08 in fourth, 25-144mph
In fifth, 33-180mph
On track, it's generally 2nd and 3rd, with usually one change to fourth on the longer straight. Annoyingly with the 3.45, 3rd, at 6000rpm 96mph is often reached just short of the braking point and not worth changing up and back down unsettling the car into braking. With the 3.08, 108mph, hopefully, this will happen less often.
With the 3.08 in fourth, 25-144mph
In fifth, 33-180mph
The six speed box may well be the best solution, provided that the ratios are well chosen to suit the weight of car and power available.
My point is that the 5ltr cars ended up being substantially under geared.
As far as I can tell, TVR had no input into the gearing available in the T5 gearbox, but I'm happy to be corrected on this point. What they certainly could choose from, was the wide range of ratios available for the BTR M76 IRS differential. I have no doubt that the 3.45 ratio was a good choice for the 4ltr cars, bit not so for the 5ltr cars. 1st is hooelessly high and has to be got out of just as quickly as possible. 2nd is completely irrelevant. 3rd is a satisfactory 2nd. 4th is useful as a change down for roundabouts or a really fast overtake, but unnecessarily low for about town etc. 5th is the gear used most of the time and that is unnecessarily low for motorway or even A road cruising.
So as I see it, changing the final drive ratio is the perfect way to correct it. As to what the final ratio should be, that is the question. In practice it comes down to 3.23 :1 or 3.08 :1.
I think that 3.23 would certainly be an improvement and who knows, maybe the correct choice. To me, the figures still looked too close to the 3.45 :1, so I have chosen to try the 3.08 and time will tell. It's not a choice set in stone, if I don't like it next year, I'll change it again the following winter.
As I am doing all the work myself, the outlay is not great, so experimentation is viable.
Dougal.
My point is that the 5ltr cars ended up being substantially under geared.
As far as I can tell, TVR had no input into the gearing available in the T5 gearbox, but I'm happy to be corrected on this point. What they certainly could choose from, was the wide range of ratios available for the BTR M76 IRS differential. I have no doubt that the 3.45 ratio was a good choice for the 4ltr cars, bit not so for the 5ltr cars. 1st is hooelessly high and has to be got out of just as quickly as possible. 2nd is completely irrelevant. 3rd is a satisfactory 2nd. 4th is useful as a change down for roundabouts or a really fast overtake, but unnecessarily low for about town etc. 5th is the gear used most of the time and that is unnecessarily low for motorway or even A road cruising.
So as I see it, changing the final drive ratio is the perfect way to correct it. As to what the final ratio should be, that is the question. In practice it comes down to 3.23 :1 or 3.08 :1.
I think that 3.23 would certainly be an improvement and who knows, maybe the correct choice. To me, the figures still looked too close to the 3.45 :1, so I have chosen to try the 3.08 and time will tell. It's not a choice set in stone, if I don't like it next year, I'll change it again the following winter.
As I am doing all the work myself, the outlay is not great, so experimentation is viable.
Dougal.
Classic Chim said:
Over 110 mph you will be accelerating a lot slower unless like Daz you have 360 bhp which 99% of us do not have.
I can't imagine why you think that. My rate of acceleration will depend where I am in my torque curve. In 5th gear:
With 3.08 at 110mph I will be at 3700rpm and well within the torque curve up to 4500 at 135mph.
With 3.45 at 110mph I will be at 4100rpm and shortly it will be in decline at 4500rpm at 120mph.
Obviously, if a lower speed was compared in 5th then 3.45 would look more favourable, but I would have the choice to change down and tip the balance in favour of the 3.08.
Thats what the gearbox is for.
My road test showed I was now running leaner, so about a 12% on average EGO correction. After a few miles I switched on autotune to start correcting this.
Regrettably the clunk from the rear was still present.
Back on the lift it was obvious that the unwanted backlash was in the spider gears.
On hindsight, I should have checked this out before dismantling the hydratrak diff, which was new to me. It is difficult to determine side gear backlash once dismantled.
Anyway, diff back out again, 2 hrs flat this time now that I know all the short cuts!
So now a bit of delay while I source suitable shims for the side gears.
One very interesting side benefit to this unexpected disassembly, I was able to view the actual tooth pattern after a 20 mile drive as some reassurance that my gear paste adjustments were good. It was spot on, so very pleased. (This was possible only because the new gears start with a matt black heat treated finish which is polished shiny in use)
Dougal.
Regrettably the clunk from the rear was still present.
Back on the lift it was obvious that the unwanted backlash was in the spider gears.
On hindsight, I should have checked this out before dismantling the hydratrak diff, which was new to me. It is difficult to determine side gear backlash once dismantled.
Anyway, diff back out again, 2 hrs flat this time now that I know all the short cuts!
So now a bit of delay while I source suitable shims for the side gears.
One very interesting side benefit to this unexpected disassembly, I was able to view the actual tooth pattern after a 20 mile drive as some reassurance that my gear paste adjustments were good. It was spot on, so very pleased. (This was possible only because the new gears start with a matt black heat treated finish which is polished shiny in use)
Dougal.
Derek, probably a bit late for a reply! I haven't been on the forum for a while.
Yes, it's a correct 7.75" set and I got it from Australia. However it was a NOS Holden part, so quite cheap compared to the current Dana price. Still had the cost of transport and import tax however. The shop I bought it from had bought up a job lot of Holden stuff and at the time had 5 of these. It sounds like you are sorted now but I can let you have the contact details if your interested.
The Hydratrak was from a Cerbera diff. As it allegedly had a low mileage, I rebuilt the differential without dismantling the differential gears. Then found I had the dreaded clonk. I took it all apart again and found the play to be in the Hydratrak differential spider gears. Their backlash is set with special conical cast iron shims which, when they wear thin enough, break in half. They are completely unavailable here or in Australia so far as I could find. So that was an end to the Hydratrak experiment and I went back to the shimmed up cone diff. I could of course have chosen the very good Quaife lsd, but decided, for me, it wasn't worth the expense. (However I am fitting one to my Honda Elise track car)
Dougal.
Yes, it's a correct 7.75" set and I got it from Australia. However it was a NOS Holden part, so quite cheap compared to the current Dana price. Still had the cost of transport and import tax however. The shop I bought it from had bought up a job lot of Holden stuff and at the time had 5 of these. It sounds like you are sorted now but I can let you have the contact details if your interested.
The Hydratrak was from a Cerbera diff. As it allegedly had a low mileage, I rebuilt the differential without dismantling the differential gears. Then found I had the dreaded clonk. I took it all apart again and found the play to be in the Hydratrak differential spider gears. Their backlash is set with special conical cast iron shims which, when they wear thin enough, break in half. They are completely unavailable here or in Australia so far as I could find. So that was an end to the Hydratrak experiment and I went back to the shimmed up cone diff. I could of course have chosen the very good Quaife lsd, but decided, for me, it wasn't worth the expense. (However I am fitting one to my Honda Elise track car)
Dougal.
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