Discussion
ChimpOnGas said:
I'm seriously considering a 3.08 diff in 'Ol Gasbag', Ok so I only have 250hp and 260 ft/lbs but I still think it could work well?
Is it not a case that once you're out of 1st the negative effects would be minimal because you're still changing up when the engine spinning at the same speed as it would be if you are running the standard diff?
The box remains the same so as long as the engine has enough power to pull 1st smoothly from a hill start with the 3.08 diff then because the five ratios and spread between them remains exactly the same you just stay in the lower gear for longer.
Currently my gearing looks roughly like this with a 3.77 diff and 225/50 R16 rear boots:
1st - 7mph per 1,000
2nd - 10mph per 1,000
3rd - 15mph per 1,000
4th - 19mph per 1,000
5th - 24mph per 1,000 (Holds an 80mph cruise at 3280rpm)
With a 3.08 diff it would look more like this:
1st - 8mph per 1,000 (Hardly ant difference, doubt I'll notice a thing)
2nd - 12mph per 1,000 (Half way between 2nd & 3rd with standard diff)
3rd - 18mph per 1,000 (Still a fraction lower geared than 4th with standard diff)
4th - 24mph per 1,000 (Becomes same as 5th with standard diff)
5th - 30mph per 1,000 (Will hold an 80mph cruise at 2700rpm)
As far as I can see there are very few negatives, for situations where I normally drop from 5th to 4th to execute a typical A road overtake on my current 3.77 diff I will already be in 4th at the same A road cruising speed, so will simply drop from 4th to 3rd to access the same the overtaking acceleration.
So looking at it this way during normal driving where firm passing acceleration is called for there will be little or no difference with the 3.08 diff other than the fact you'll always be starting from and dropping to one gear lower than I would be with my current 3.77 diff.
I really don't think it'll take any time at all to recalibrate by driving, indeed I would expect the 3.08 will feel perfectly natural and intuitive.
Most importantly the key benefit of the 3.08 diff being 'Ol Gasbag's' engine will be turning a very useful 580rpm slower at my cruising gate of 80mph.
That's going to translate to better economy and lower cabin noise without any loss of real world overtaking performance at all.
TBH I can only see benefits even for my Chimaera because while I may not be in the big horsepower league 260 ft/lbs of torque in a 1100kg (with gas bottles) car is still a very good torque per tonn figure compared with most cars
Then when you look at how low down in the RPM range 90% of the available torque is accessible it becomes quite clear to me even 'Ol Gasbag' could pull the taller 3.08 diff with zero negatives and the one big benefit described in 5th.
Indeed at 80mph with the engine spinning at just 2700rpm 'Ol Gasbag' is already making 93% of her total available torque so the phrase "Sweet Spot" comes to mind, and possibly worth another 2mpg at least
I reckon your car is a 3.45 ,the 3.77 diff is used for the speed six .Is it not a case that once you're out of 1st the negative effects would be minimal because you're still changing up when the engine spinning at the same speed as it would be if you are running the standard diff?
The box remains the same so as long as the engine has enough power to pull 1st smoothly from a hill start with the 3.08 diff then because the five ratios and spread between them remains exactly the same you just stay in the lower gear for longer.
Currently my gearing looks roughly like this with a 3.77 diff and 225/50 R16 rear boots:
1st - 7mph per 1,000
2nd - 10mph per 1,000
3rd - 15mph per 1,000
4th - 19mph per 1,000
5th - 24mph per 1,000 (Holds an 80mph cruise at 3280rpm)
With a 3.08 diff it would look more like this:
1st - 8mph per 1,000 (Hardly ant difference, doubt I'll notice a thing)
2nd - 12mph per 1,000 (Half way between 2nd & 3rd with standard diff)
3rd - 18mph per 1,000 (Still a fraction lower geared than 4th with standard diff)
4th - 24mph per 1,000 (Becomes same as 5th with standard diff)
5th - 30mph per 1,000 (Will hold an 80mph cruise at 2700rpm)
As far as I can see there are very few negatives, for situations where I normally drop from 5th to 4th to execute a typical A road overtake on my current 3.77 diff I will already be in 4th at the same A road cruising speed, so will simply drop from 4th to 3rd to access the same the overtaking acceleration.
So looking at it this way during normal driving where firm passing acceleration is called for there will be little or no difference with the 3.08 diff other than the fact you'll always be starting from and dropping to one gear lower than I would be with my current 3.77 diff.
I really don't think it'll take any time at all to recalibrate by driving, indeed I would expect the 3.08 will feel perfectly natural and intuitive.
Most importantly the key benefit of the 3.08 diff being 'Ol Gasbag's' engine will be turning a very useful 580rpm slower at my cruising gate of 80mph.
That's going to translate to better economy and lower cabin noise without any loss of real world overtaking performance at all.
TBH I can only see benefits even for my Chimaera because while I may not be in the big horsepower league 260 ft/lbs of torque in a 1100kg (with gas bottles) car is still a very good torque per tonn figure compared with most cars
Then when you look at how low down in the RPM range 90% of the available torque is accessible it becomes quite clear to me even 'Ol Gasbag' could pull the taller 3.08 diff with zero negatives and the one big benefit described in 5th.
Indeed at 80mph with the engine spinning at just 2700rpm 'Ol Gasbag' is already making 93% of her total available torque so the phrase "Sweet Spot" comes to mind, and possibly worth another 2mpg at least
Edited by ChimpOnGas on Tuesday 29th March 17:24
ClassiChimi said:
I'm pretty sure mines a 3.08 
Using a sat nav on my phone so hardly scientific but I recorded
Speed mph Gear. Revs on counter
50. 5th 1625
60. 2000
70. 2390
80. 2790
90. 3200
100. 3400
30mph 4th gear = 1200 revs.
As can be seen by these results the sat nav for speed reading is a bit hit and miss,
But that's close to COG calc for 80mph so as I thought I have the 3.08,,, needs to be higher geared still,
That's why my fuel economy figures are pretty good too, 30 mpg on a run easily.
You'll love the difference Daz.
Looks like you have the 3.23 ,better than what 3.45 Using a sat nav on my phone so hardly scientific but I recorded
Speed mph Gear. Revs on counter
50. 5th 1625
60. 2000
70. 2390
80. 2790
90. 3200
100. 3400
30mph 4th gear = 1200 revs.
As can be seen by these results the sat nav for speed reading is a bit hit and miss,
But that's close to COG calc for 80mph so as I thought I have the 3.08,,, needs to be higher geared still,
That's why my fuel economy figures are pretty good too, 30 mpg on a run easily.
You'll love the difference Daz.
Edited by ClassiChimi on Tuesday 29th March 22:27
Edited by SILICONEKID345HP on Wednesday 30th March 22:05
SILICONEKID345HP said:
ClassiChimi said:
I'm pretty sure mines a 3.08
Using a sat nav on my phone so hardly scientific but I recorded
Speed mph Gear. Revs on counter
50. 5th 1625
60. 2000
70. 2390
80. 2790
90. 3200
100. 3400
30mph 4th gear = 1200 revs.
As can be seen by these results the sat nav for speed reading is a bit hit and miss,
But that's close to COG calc for 80mph so as I thought I have the 3.08,,, needs to be higher geared still,
That's why my fuel economy figures are pretty good too, 30 mpg on a run easily.
You'll love the difference Daz.
Looks like you have the 3.23 ,better than 3.45 Using a sat nav on my phone so hardly scientific but I recorded
Speed mph Gear. Revs on counter
50. 5th 1625
60. 2000
70. 2390
80. 2790
90. 3200
100. 3400
30mph 4th gear = 1200 revs.
As can be seen by these results the sat nav for speed reading is a bit hit and miss,
But that's close to COG calc for 80mph so as I thought I have the 3.08,,, needs to be higher geared still,
That's why my fuel economy figures are pretty good too, 30 mpg on a run easily.
You'll love the difference Daz.
Edited by ClassiChimi on Tuesday 29th March 22:27
Mr Haribo said:
I know its rude to ask how much was it all in ? are you using a BTR now ?
Im thinking of do it driving to Le Mans every year the motorway miles can drone a bit !
I`m not sure ,scared to add it up .Could I sell the 3.45 crown wheel and pinion. its only done a couple of thousand miles.Im thinking of do it driving to Le Mans every year the motorway miles can drone a bit !
My cruiser is back 
It sounds alot more sedate at low speed and motor way driving is far more pleasurable ,it feels like a big Yank motor burbling along .
When I hit the loud pedal it picks up quickly ,gear changes are fewer and there is no compromise with acceleration ,you seem to stay in each gear longer.
It was a nice feeling cruising back at 2150 rpm at just over 70mph .
The only thing which needs a tweak is the afr`s ,it now cruises a bit richer lower down .
Very happy with the results.
It sounds alot more sedate at low speed and motor way driving is far more pleasurable ,it feels like a big Yank motor burbling along .When I hit the loud pedal it picks up quickly ,gear changes are fewer and there is no compromise with acceleration ,you seem to stay in each gear longer.
It was a nice feeling cruising back at 2150 rpm at just over 70mph .
The only thing which needs a tweak is the afr`s ,it now cruises a bit richer lower down .
Very happy with the results.
macdeb said:
Good news mate, what size is the crown wheel? [did ask before] Apparently there are two sizes for the 308 ratio [or so I've read]
I forgot to measure it ,I would speak to John Read . I`m aware the 7.5 inch will fit but need a large shim but are not that strong Its all together a far more pleasurable experience....
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