High Speed Transfer Case
Conversion
Series Land Rovers were not designed to be great
highway cruisers. The overall gear ratio in top gear is fairly low and the engine will
sound very busy as you try to keep up with the traffic flow.
The top gear ratio becomes even more of a problem if
you install a non-Rover engine. Power plants such as the Chevy or General Motors-Holden
sixes do not like prolonged running in the higher part of their rev range and will benefit
greatly if overall gearing can be raised to take advantage of their power characteristics.
You will not necessarily go faster, not with Land
Rover aerodynamics, highway grades and headwinds but you will have a happier and longer
lasting motor.
This article concerns the "High Speed Transfer Case" conversion I installed in
"Mustang Sally" and compares it to other common solutions to the same problem.
Comparison Table
Fairey-Superwinch
overdrive
Alternative
Differential Ratios
High
Speed Transfer Case Conversion
Links to Suppliers
Standard overall ratio in top gear compared with
popular alternatives.
These figures have been calculated for a
Series Land Rover on 7.50R16 tyres. |
Gearing |
Overall
ratio
top gear |
MPH
@1000 rpm |
KPH
@1000 rpm |
Comments |
Standard
|
5.396:1 |
17.2mph |
28.2kph |
A standard Land Rover |
Fairey-Superwinch
Overdrive .782:1 |
4.220:1 |
21.74mph |
35.63kph |
"5th Gear" When disengaged all gearing
is as standard. |
3.54:1
Range Rover type differentials |
4.063:1 |
22.86mph |
37.47kph |
Gearing raised in both Hi-range and Lo-range |
High-speed
Transfer case conversion |
4.089:1 |
22.66mph |
37.14kph |
Gearing raised only in Hi-range |
- This unit was introduced as a Land Rover-approved after-market accessory in 1974 when
the OPEC induced rise in oil prices made gas guzzlers like your Land Rover more expensive
to run. It did make Land Rovers marginally more fuel efficient though you would need to do
about 100,000 miles to recover the cost. However, owners have come to value them since
then for their bolt-on simplicity and the more relaxed performance at speed.
PRO:
- Can be fitted in a few hours by a professional, maybe a little longer if you are not
familiar with it.
- Acts as a "fifth gear". When it is disengaged the overall gearing in every
gear is exactly as standard.
- Can be easily removed for repair and vehicle can be used as standard provided you have
kept the outrigger bearing plate which goes into the top PTO/Overdrive hole in transfer
case.
- Despite the addition of an extra gear lever, simple to use.
- Speedometer readings unaffected.
CON:
- Delicate, and can easily be damaged by abuse which includes
running it low on oil and using it as a splitter in other gears besides Hi-range fourth
and third. Regardless of what some users say, transmitting high torque through it in
Lo-range is not good for it.
- Small oil capacity. Users often invent elaborate breather
arrangements to keep the overdrive from blowing half its oil out on the highway.
- Not cheap considering how easily they can be ruined by
carelessness.
- Unconfirmed rumours as of this date say that Superwinch has ceased
production and destroyed all production tooling. This will probably push up the price of
unsold stock, spare parts and second-hand units.
Back to top
- This is a popular one. 3.54:1 Range Rover differential centres will fit your Rover type
axle casings. You will need two, front and rear. Specialist differential suppliers can
supply you with new units with all sorts of strengthening tricks, locking devices and in a
wide range of ratios besides 3.54:1.
PRO:
- As strong, or stronger than standard ratio differentials.
- Simple to fit apart from the problem of wrestling with the weight.
- Easy to refit standard ratio.
- Cheap if you can source good used Range Rover diff centres.
CON:
- Not cheap if you are buying two new units.
- Speedometer will need to be replaced as existing one will read
slow.
- Both Hi-range and Lo-range gearing will be raised.
- First gear will be higher than before and an early change down to
third can be necessary if adverse highway grades or headwinds are encountered.
Back to top
- This conversion uses a set of replacement transfer case gears,
consisting of one input gear, one intermediate gear and one high range output gear.
Several suppliers (see Links at bottom of page) provide a reworked
bare transfer case to which you fit the studs, covers, intermediate shaft, output shaft
and low range output gear from your old case. New bearings and seals should be fitted now
that you have it apart. Costs can be pushed up if the gears and shafts you intended to
reuse prove to be in doubtful condition.
- The Hi-range transfer ratio changes from a reduction of 1.148:1 to
an overdrive ratio of .87:1 The Lo-range transfer ratio is practically unaffected. See the
end of this section for a table giving actual
engine performance figures.
- My case was reworked locally and it is a precision milling job
involving the repositioning of the intermediate shaft support holes by a small amount.
Larger holes are milled in the case eccentric to the existing holes. The original hole
disappears within the bounds of the new hole. Bronze bushes are inserted in the new large
holes. The bores of the bronze bushes are line- bored and sized to take the intermediate
shaft. The larger holes are machined with a double diameter or "step" facing
into the case and the "stepped" bushes are pressed in from the inside of the
case with plenty of Loctite. They can't fall out or rotate. This reworking is fraught with
possibilities for disaster, as the repositioned shaft must be in precisely the right place
and the line-bored holes in the bushes must be parallel to the other shafts in the
transfer case. Not too difficult for a conscientious machinist with a good milling machine
and all his case holding jigs and measuring procedures worked out beforehand. A machinist
who had never done it before would spend a lot of time setting up so it is probably most
economical to get the complete kit rather than attempt to source the gears alone.
PRO:
- As strong as the original transfer case.
- Speedo readings are unaffected.
- Lo-range transfer ratio changes by about 1%, which is nothing at
all in practical terms.
- Your transfer case will benefit from new bearings and seals.
- Cheaper by half than an overdrive or two new diffs in Australia.
CON:
- Not quick to fit. The transfer case can be removed while gearbox
assembly is in the vehicle but it is a slow cramped job unless you have access to a garage
hoist or are working on a bare chassis with no bodywork.
- Can become more expensive if further replacement parts are needed
after inspection. (Though it is nice to finish up with a totally new transfer case.)
- Not reversible unless you keep a bare unworked case with
serviceable standard parts corresponding to the high speed parts you used in the
conversion.
- First gear will be higher than before and an early change down to
third can be necessary if adverse highway grades or headwinds are encountered.
Performance benefits
of High Speed Transfer Case Conversion. |
Vehicle
type |
Engine RPM@ 60 MPH |
Engine RPM@ 100 KPH |
|
Normal t/fer
case @ 60MPH |
O/drive
t/fer case @60MPH |
Reduction in
RPM |
Normal
t/fer case @ 100KPH |
O/drive
t/fer case @ 100KPH |
Reduction in
RPM |
Typical
109 on 7.50R16 tyres |
3529 |
2675 |
854 |
3705 |
2808 |
897 |
Typical
88 on 6.00R16 tyres |
3880 |
2941 |
939 |
4020 |
3047 |
973 |
These figures
calculated and checked for this article. RPM reduction 24%. |
Back to top
Links, Links, Links
High Speed Transfer Case Conversion.
These firms offer the kit with bare reworked transfer case and the conversion gears at an
attractive price.
From time to time, other Australian firms have offered this conversion but I have no up to
date information.
News of other firms who offer this conversion will be most
welcome.
Alternative diff ratios.
This page last updated on Wednesday, 17 February 1999 10:43
|
