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.
I'm running a 3.08 which I purchased from Australia .
I'm aware there are two types ,some need a spacer but they aren't very strong .
I was told by the diff builder that mine is a race standard unit
You do loose a little urgency in first gear but then then each gear seems to go forever .
Over a quarter of a mile it is actually quicker but I wouldnt recomend the 3.08 for track days or sprinting .
I think a better way to do it is a 6 speed gear box .
I awopped the GKN for the BTR which was new old stock from TVR Malaysia.
I have a 3.45 crown wheel and pinion with less than a 1000 miles on it .Its for sale if anyone is interested .
There is also a 3.23 available ,it was a factory option for lower rpm cruising .
I'm aware there are two types ,some need a spacer but they aren't very strong .
I was told by the diff builder that mine is a race standard unit
You do loose a little urgency in first gear but then then each gear seems to go forever .
Over a quarter of a mile it is actually quicker but I wouldnt recomend the 3.08 for track days or sprinting .
I think a better way to do it is a 6 speed gear box .
I awopped the GKN for the BTR which was new old stock from TVR Malaysia.
I have a 3.45 crown wheel and pinion with less than a 1000 miles on it .Its for sale if anyone is interested .
There is also a 3.23 available ,it was a factory option for lower rpm cruising .
Edited by SILICONEKID 357HP on Sunday 5th July 23:17
Edited by SILICONEKID 357HP on Sunday 5th July 23:19
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
I’d have thought these old engines are more efficient at slightly higher revs than 2300 odd at 70 mph. Volumetric efficiency and all... thoughts gents.
Shunting can’t be improved with this mod surely though driving at 80 mph possibly is
On a trip through France I found my Chim 450 to be more fuel efficient at 80 mph than at 70 mph for sure. I’d expect air pressure of speed to reduce results but my figures improved which I can only be down to the engine sitting in the sweet spot. 3000 revs ish.
I’ve always liked the idea of higher gearing but not sure it would actually be that useful in reality.
Shunting can’t be improved with this mod surely though driving at 80 mph possibly is
On a trip through France I found my Chim 450 to be more fuel efficient at 80 mph than at 70 mph for sure. I’d expect air pressure of speed to reduce results but my figures improved which I can only be down to the engine sitting in the sweet spot. 3000 revs ish.
I’ve always liked the idea of higher gearing but not sure it would actually be that useful in reality.
Edited by Classic Chim on Monday 6th July 21:05
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
Speaking as a man who drives a 4.0 litre making 250hp and 260ft/bs I feel TVR chose the T5 box & BTR diff gearing with intelligence, the ratios are well spaced offering a good compromise between brisk acceleration and cruising engine speed. If I had a mighty five litre it would certainly be a different story, however saying that even on my baby 4.0HC I could still be tempted to go taller but there's alsways the risk of spending a chunk of money only to discover I miss the acceleration the standard setup delivers.
Before messing with the final drive gearing the better solution would be a sixth ratio, the ideal being the same ratios as the T5 with a taller double overdrive sixth. When I last looked at this the TR-6060 Magnum seemed the best option but it's huge, a Getrag v160/Supra box is smaller than the Tremec and certainly strong enough but our Ricer friends prize this box so will pay £4k for a used one, an Aisin AY6 could work and will support 375 ft/lbs but the ratios aren't ideal.
There are other options but none of them are really strong enough and typically 6th is the same ratio as the T5 fifth so all you get is closer spaced ratios forcing more gear changes with no reduction in top gear engine speed, read POINTLESS! The gear sets in many of the six speed options just aren't in keeping with the way an RV8 makes its power, and then there's the fun and games involved getting any of the above boxes on the back of old Rover. This kinda brings us full circle back to a taller final drive but I'd want a stout 300 ft/lbs plus before it made sense, so in the end there are better things to spend my money on than a taller crown wheel & pinion set.
Its easy to keep modding these cars but it's worth considering if I started with a clean sheet of paper and chose the perfect gearing for my 250hp and 260ft/lb 1100kg Chimaera I may very well end up specifying exactly the same five gearbox ratios in my current T5 box and the exact same final drive ratio already in my BTR diff. There's a lot of evidence to suggest what I currently have is actually the sweet spot, and there's nothing better value than leaving it just as it is.
In my case the equation looks very much like this: Money Spent vs Benefit = Not Worth It Mate!
Before messing with the final drive gearing the better solution would be a sixth ratio, the ideal being the same ratios as the T5 with a taller double overdrive sixth. When I last looked at this the TR-6060 Magnum seemed the best option but it's huge, a Getrag v160/Supra box is smaller than the Tremec and certainly strong enough but our Ricer friends prize this box so will pay £4k for a used one, an Aisin AY6 could work and will support 375 ft/lbs but the ratios aren't ideal.
There are other options but none of them are really strong enough and typically 6th is the same ratio as the T5 fifth so all you get is closer spaced ratios forcing more gear changes with no reduction in top gear engine speed, read POINTLESS! The gear sets in many of the six speed options just aren't in keeping with the way an RV8 makes its power, and then there's the fun and games involved getting any of the above boxes on the back of old Rover. This kinda brings us full circle back to a taller final drive but I'd want a stout 300 ft/lbs plus before it made sense, so in the end there are better things to spend my money on than a taller crown wheel & pinion set.
Its easy to keep modding these cars but it's worth considering if I started with a clean sheet of paper and chose the perfect gearing for my 250hp and 260ft/lb 1100kg Chimaera I may very well end up specifying exactly the same five gearbox ratios in my current T5 box and the exact same final drive ratio already in my BTR diff. There's a lot of evidence to suggest what I currently have is actually the sweet spot, and there's nothing better value than leaving it just as it is.
In my case the equation looks very much like this: Money Spent vs Benefit = Not Worth It Mate!
Classic Chim said:
I’d have thought these old engines are more efficient at slightly higher revs than 2300 odd at 70 mph. Volumetric efficiency and all... thoughts gents.
Shunting can’t be improved with this mod surely though driving at 80 mph possibly is
On a trip through France I found my Chim 450 to be more fuel efficient at 80 mph than at 70 mph for sure. I’d expect air pressure of speed to reduce results but my figures improved which I can only be down to the engine sitting in the sweet spot. 3000 revs ish.
I’ve always liked the idea of higher gearing but not sure it would actually be that useful in reality.
My understanding is the more a cylinder is filled, the more fuel efficient it is due to higher cylinder pressure before combustion. That's why small engines are more fuel efficient for any given output cos they are running under higher load and hence more fully filling the cylinders. Shunting can’t be improved with this mod surely though driving at 80 mph possibly is
On a trip through France I found my Chim 450 to be more fuel efficient at 80 mph than at 70 mph for sure. I’d expect air pressure of speed to reduce results but my figures improved which I can only be down to the engine sitting in the sweet spot. 3000 revs ish.
I’ve always liked the idea of higher gearing but not sure it would actually be that useful in reality.
Edited by Classic Chim on Monday 6th July 21:05
Longer gearing reduces rpm therefore the engine has to run higher load to make the same power therefore runs more efficiently.
Maybe it's the increased engine load (more throttle, fuller cylinders) rather than the increased rpm that's improved your fuel efficiency.
Obviously sweet spot is a balance cos power required goes up with the cube of velocity.
Edited by Beverley Grove on Tuesday 7th July 10:55
Beverley Grove said:
My understanding is the more a cylinder is filled, the more fuel efficient it is due to higher cylinder pressure before combustion. That's why small engines are more fuel efficient for any given output cos they are running under higher load and hence more fully filling the cylinders.
Longer gearing reduces rpm therefore the engine has to run higher load to make the same power therefore runs more efficiently.
Maybe it's the increased engine load (more throttle, fuller cylinders) rather than the increased rpm that's improved your fuel efficiency.
Obviously sweet spot is a balance cos power required goes up with the cube of velocity.
Are you considering exhaust valve opening duration and how leaving exhaust gases in the chamber reduces the amount of fuel required to fill it. It’s a way manufactures create better fuel economy. I’m No expert but slightly faster air flow atomises fuel better in its own right. The difference between the revs might be so small as to make no difference. I think my engine produces more power and thus requiring less fuel at 3000 revs than it does at 2700 but it’s probably marginal. Longer gearing reduces rpm therefore the engine has to run higher load to make the same power therefore runs more efficiently.
Maybe it's the increased engine load (more throttle, fuller cylinders) rather than the increased rpm that's improved your fuel efficiency.
Obviously sweet spot is a balance cos power required goes up with the cube of velocity.
Edited by Beverley Grove on Tuesday 7th July 10:55
All pumps have an optimal operating window, and an internal combustion engine is just a giant air pump. You can move that window about a bit by changing valve timing, ie by changing the lift, overlap and valve duration. You can also increase or decrease your inlet tract length or increase or decrease plenum volume, you can even make your valves larger or smaller.
It will all have an effect, but its always a compromise, using the above methods all you end up doing is moving the window from one place to another, when really what you want is to widen it. The objective of any engine designer is to create the widest possible optimal operating window he can, which is why we have variable valve timing, variable compression ratios, and variable plenum volumes too which is why Porsche gave us their Varioram system.
In all cases these clever innovations were just an attempt by engine designers to widen their engine's optimal operating window, but even the most clever modern internal combustion engine still has a very limited operating window. Gearing needs to take into consideration where engine's optimal operating window exists, but you must also factor in the desired road speed required and the wind resistance at that speed. To a lesser or greater degree all engineering is a series of compromises, as is gearing.
When TVR chose the Chimaera gearing they needed to work within a set of compromises, they needed a fast accelerating car that also wasn't too high revving on the motorway, I still say they got that compromise about as good as they could without adding a further gear ratio which is exactly what modern sports car makers do when the equip there vehicles with a six speed box.
A six speed box is always going to be the better solution compared with messing with the final drive gearing, because the more gears you add the more you can reduce the compromises we are all still forced to work with.
It will all have an effect, but its always a compromise, using the above methods all you end up doing is moving the window from one place to another, when really what you want is to widen it. The objective of any engine designer is to create the widest possible optimal operating window he can, which is why we have variable valve timing, variable compression ratios, and variable plenum volumes too which is why Porsche gave us their Varioram system.
In all cases these clever innovations were just an attempt by engine designers to widen their engine's optimal operating window, but even the most clever modern internal combustion engine still has a very limited operating window. Gearing needs to take into consideration where engine's optimal operating window exists, but you must also factor in the desired road speed required and the wind resistance at that speed. To a lesser or greater degree all engineering is a series of compromises, as is gearing.
When TVR chose the Chimaera gearing they needed to work within a set of compromises, they needed a fast accelerating car that also wasn't too high revving on the motorway, I still say they got that compromise about as good as they could without adding a further gear ratio which is exactly what modern sports car makers do when the equip there vehicles with a six speed box.
A six speed box is always going to be the better solution compared with messing with the final drive gearing, because the more gears you add the more you can reduce the compromises we are all still forced to work with.
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