gkn diff ratio

[SSPPGG]

on a 93 4.0....what is it likely to be

[SILICONEKID340HP]

SSPPGG said:

on a 93 4.0....what is it likely to be

Which one is the GKN ?early or later one.Ive just found out mine is the early one with the fins ,after speaking to Readman Racing the early one`s ar higher geared..

Ive also been useing the 75w130 oil ,hope that is ok.

[Ant.]

3.31:1.

[SILICONEKID340HP]

Ant. said:

3.31:1.

I have the GKN ,the one with the fins .Is it higher geared..

[K4TRV]

Chimaeras ( and Griffs) appeared from Mar 1993 - commonly called "early ones"

They came with a GKN Diff recognisable with large fins at the back, with 3.31 ratio.

Also came with Rover Gear box until 1994.

Later GKN/early BTR Diff ratio changed to 3.77 (which means more revs for same speed - all else unchanged!!!)

GKN's also used on Tuscan Racers, but they could be had with 3.08 ratio (fewer revs for higher road speed - all else unchanged).

GKN & BTR were recommended originally to use Mobil SC80W/140ID - but equivalent modern synthetic axle oils for LSDs work just as well.

HTH (info from Bible & Advanced search for Diff ratios back 7-years - Chim & Griff!!)

Trev (edited cause I can't spall)

[SILICONEKID340HP]

How do i know if i have the early or later GKN diff .Mine is a 96..I would like to lower the RPM at high speeds.

[K4TRV]

SILICONEKID340HP said:

How do i know if i have the early or later GKN diff .Mine is a 96..I would like to lower the RPM at high speeds.

Jack up both rear wheels, mark the prop with tippex or similar.

If its a standard 3.77:1 then as the prop should turn 3.77 times for 1 turn of the wheel. Best to turn the wheel 10 times and have someone count the prop.

If its a standard 3.31:1 then as the prop should turn 3.31 times for 1 turn of the wheel. Best to turn the wheel 10 times and have someone count the prop.

Then save up and buy a 3.08:1 diff, and that will ruin your Drag racing times, but help enormously your mpg bragging rights. You'll also need to send your speedo off for recalibrating.

Or there again, get your wheel chart out and go for 18 inch rear wheels and 50 profile tyres and you'll achieve something similar - but it might seem a tad tall at the rear?

Don't you just love tinkering??

HTH?

Trev

[dbv8]

K4TRV said:

Then save up and buy a 3.08:1 diff, and that will ruin your Drag racing times, but help enormously your mpg bragging rights.

Not necessarily.

If you have the torque then the 3.08 should still work well enough on the drag strip. I would expect to see the acceleration suffer only marginally on the bigger engined cars.
I am swapping out my standard BTR to a 3.08 this winter. I expect to see 1/4 mile times the same on nitrous but not so close to the redline in 4th. Improved mpg driving to and from the tracks and i want to run a top speed event.... maybe vmax.
170+ Chimaera that runs low 11's and achieves over 30mpg when required

[SILICONEKID340HP]

K4TRV said:

Jack up both rear wheels, mark the prop with tippex or similar.

If its a standard 3.77:1 then as the prop should turn 3.77 times for 1 turn of the wheel. Best to turn the wheel 10 times and have someone count the prop.

If its a standard 3.31:1 then as the prop should turn 3.31 times for 1 turn of the wheel. Best to turn the wheel 10 times and have someone count the prop.

Then save up and buy a 3.08:1 diff, and that will ruin your Drag racing times, but help enormously your mpg bragging rights. You'll also need to send your speedo off for recalibrating.

So your saying there are two different ratios in the

Or there again, get your wheel chart out and go for 18 inch rear wheels and 50 profile tyres and you'll achieve something similar - but it might seem a tad tall at the rear?

Don't you just love tinkering??

HTH?

Trev

So your saying there are two different ratios in the in the GKN diff ,How much would it drop the rpm on cruise ?say 70 to 80mph.
If i find mine is the lower geared diff can it be changed when i get is referbished or do i need to look for another diff.

What is a rebuild anyway ..just replace a few worn parts or remove evrything and start again.

Edited by SILICONEKID340HP on Tuesday 29th November 18:58

[K4TRV]

SILICONEKID340HP said:

So your saying there are two different ratios in the in the GKN diff ,How much would it drop the rpm on cruise ?say 70 to 80mph.
If i find mine is the lower geared diff can it be changed when i get is referbished or do i need to look for another diff.

What is a rebuild anyway ..just replace a few worn parts or remove evrything and start again.

Edited by SILICONEKID340HP on Tuesday 29th November 18:58

Daren nothing is exact re-TVRs, I just checked the search facility and there's little about ratios in the Bible.

What change to revs on ratios ??? it's about maths! Just to give you a comparison - I repeat, just for comparison

Using 3000 rpm to give you, for arguments sake, 80 mph on a 3.77 ratio. Equals 26.6 mph per 1000 revs in 5th

At 3000 rpm with a 3.77 ratio changing to a 3.31 will be equivalent to 12% reduction or dropping revs to 2640 rpm at 80 mph Equals 30.3 mph per 1000 revs in 5th

At 2640 rpm with a 3.31 ratio changing to a 3.08 will be equivalent to 7% reduction or dropping revs to 2450 rpm at 80 mph.Equals 32.5 mph per 1000 revs in 5th

So if you have a 3.77 at 3000 rpm changing to a 3.08 will drop the revs from 3000 to 2450 rpm to stay at at 80 mph. Or change from 26.6 mph per 1000 to 32.5 mph.

Now don't go and say your car does something different at 3000 revs - I just chose 80 mph for illustration. Also, I have read there are 3.64 ratios about too - do the wheel/propshaft check to confirm yours!!

Also.......the figures are for comparison only............plus T5 boxes changed the ratio of 5th gear in later years, probably at the time they went from 3.31 to 3.77 and/or to BTR and a diff ratio again!!

As Dave said above, if you have the torque, a change to 3.08 is a no-brainer for the 5 Ltr - but it may feel a bit less peppy, and it will be less revvy for higher speeds and probably give much better mpg - don't blame me if it doesn't though!!!

Why refurbish, just replace?? Go for the GKN used on Tuscan racers or equivalent??

HTH's ??

Trev

[SILICONEKID340HP]

K4TRV said:

Daren nothing is exact re-TVRs, I just checked the search facility and there's little about ratios in the Bible.

What change to revs on ratios ??? it's about maths! Just to give you a comparison - I repeat, just for comparison

Using 3000 rpm to give you, for arguments sake, 80 mph on a 3.77 ratio. Equals 26.6 mph per 1000 revs in 5th

At 3000 rpm with a 3.77 ratio changing to a 3.31 will be equivalent to 12% reduction or dropping revs to 2640 rpm at 80 mph Equals 30.3 mph per 1000 revs in 5th

At 2640 rpm with a 3.31 ratio changing to a 3.08 will be equivalent to 7% reduction or dropping revs to 2450 rpm at 80 mph.Equals 32.5 mph per 1000 revs in 5th

So if you have a 3.77 at 3000 rpm changing to a 3.08 will drop the revs from 3000 to 2450 rpm to stay at at 80 mph. Or change from 26.6 mph per 1000 to 32.5 mph.

Now don't go and say your car does something different at 3000 revs - I just chose 80 mph for illustration. Also, I have read there are 3.64 ratios about too - do the wheel/propshaft check to confirm yours!!

Also.......the figures are for comparison only............plus T5 boxes changed the ratio of 5th gear in later years, probably at the time they went from 3.31 to 3.77 and/or to BTR and a diff ratio again!!

As Dave said above, if you have the torque, a change to 3.08 is a no-brainer for the 5 Ltr - but it may feel a bit less peppy, and it will be less revvy for higher speeds and probably give much better mpg - don't blame me if it doesn't though!!!

Why refurbish, just replace?? Go for the GKN used on Tuscan racers or equivalent??

HTH's ??

Trev

So rearly the 3.31 is the one to have or the 3.31 with the later higher geared 5th gear ..
If you replaced and not referbished you could end up with the same problem..
The gear ratios feel right but a 6th gear would be the answer bringing cruising down to around 2000rpm .There is enough torque to cope with it.

[Goaty Bill]

SILICONEKID340HP said:

<snip>
The gear ratios feel right but a 6th gear would be the answer bringing cruising down to around 2000rpm .There is enough torque to cope with it.

I am just guessing (a little), and everyone's engine would be slightly different, but my bet is that 2000 rpm cruising will too low and actually use more fuel as the engine is no where near the 'sweet spot' in the torque band.

[dbv8]

Goaty Bill said:

I am just guessing (a little), and everyone's engine would be slightly different, but my bet is that 2000 rpm cruising will too low and actually use more fuel as the engine is no where near the 'sweet spot' in the torque band.

Like those stupid taxi drivers that labour their engines in 4th everywhere.
I would think closer to 3000 rpm is a very economical place to sit on the motorway. But achieving that cruising speed with as little as throttle as possible is the key.

[wycoller]

have a 500SC griff a bit down on power at 356bhp similar torq. My GKN diff was a bit noisy , input spline shaft worn.
Installed a John Reid recon 3.77:1 diff which has just failed after 2k miles so it,ll be going back , that sorted out all my weak spots on my chassis , mount brakets etc all upgraded. well this beast would pull away from standing in 5th gear etc , i,ll be rebuilding my 2 spare 3.31 GKN diffs.
Problem is the Crown and pinion gear wheels are no longer manufactured and are like hens teeth, thats why i just threw in a 3.77:1 diff as only home once in a blue moon and need to be on the road,
Best of luck for road use 3.31 standard is the one to go for.
Hate to say what my consumption was with the 3,77:1

[Graham]

Mmm interesting I have a later chim with the btr 3.45 diff, the LSD has an issue. Now I have a spare 3.73 btr from the Tuscan racer, and I'm tempted to drop that in the chim.

I changed to a gkn 3.3 in the Tuscan ( possible with a special back cover and different mounts)!

The 3.73 means i don't have to have then3.45 rebuilt just yet, would give more acceleration, although it would loose around 10 mph top end.


Then3.45 would be a better spare for the Tuscan too..

[SILICONEKID340HP]

I can`t wait to see how Derek gets on with the 3.08 ratio .

Whats involved to change it the 3.08 ratio ..And is it just a simple speedo re calibration..

Edited by SILICONEKID340HP on Sunday 4th December 12:44

[spend]

SILICONEKID340HP said:

I can`t wait to see how Derek gets on with the 3.08 ratio .

Whats involved to change it the 3.08 ratio ..And is it just a simple speedo re calibration..

Edited by SILICONEKID340HP on Sunday 4th December 12:44

BTR has speedo drive on the output drive shaft so ratio doesn't affect it, GKN's having the trigger on the input shaft are in need of work if the ratio is changed.

Can't see the point in having short diff's TBH, it's only going to break traction most of the time & torque-multiply shock loads on all the transmission?

[Graham]

I have to admit part of the attraction of dropping the 3.73 in is that I have a fully rebuilt one sat in the garage,and a buggered 3.45 hat came out of the car..


Also the first chim I drove was an early one that would have had the 3.77 gkn diff in it and I always remember it feeling a bit more urgent.

At the end of the day it's fairly easy to swap back

[SILICONEKID340HP]

Graham said:

I have to admit part of the attraction of dropping the 3.73 in is that I have a fully rebuilt one sat in the garage,and a buggered 3.45 hat came out of the car..


Also the first chim I drove was an early one that would have had the 3.77 gkn diff in it and I always remember it feeling a bit more urgent.

At the end of the day it's fairly easy to swap back

Im getting confused now "Later GKN/early BTR Diff ratio changed to 3.77"

your saying the opposite!

[Graham]

Sorry my bad not sure how early crept into that sentence!!!! It was a later early car with the lt77 box..

It may of course have been the box that made the difference as the gearing is lower I the lt77 box..