im in two minds weather to swop my diffs for disco one`s is it about 10 mph you gain? ill sit at 60mph now but it sounds like your nailin it so if i fitted these diffs it would lower my revs at 60 but is it enough to make it worth the time buyin them and doin the work and are they any tougher than series ones any one done it thanks.

Are you still thinking about a coiler axle conversion?

OK, how much its worth, or what it will do depends on a few things, like wheel-size and the engine you have.

What sets your top speed, is NOT gearing, but available power; you can gear a moped to go 200mph if you like, but it wont go that fast unless you give it enough power to overcome the drag at that speed

(for a motorcycle sized shaped vehicle, takes about 50bhp to crack a ton; with super stream-lining, and 1/3 the frontal area, from building a cigar shaped 'kneeler' you can get to 200mph with about 100bhp.... ish. So practical engineering says, you are NOT likely to make a moped do 200mph, as you simply cannot make an engine that small make enough power to go that fast, nor make the frontal area small enough and the streamlininbg good enough to go that quick, with what power you might make such a tiny engine provide AND still fit a real person in the vehicle to drive it!)

For a Land-Rover, takes about 100bhp to crack a ton; its just doable with a tuned four pot or V8; a TDI can JUST squeek it, with its 110bhp, if on form.... But, series Landies dont 'quite' have the gearing to go that fast, as standard.

Going through the figures:-

Driven Wheel;

205R16 is 16" + 80% of 205 x 2 = 734mm give or take a noggin. multiplied by Pi gives a rolling circumference of of, 2.3m per turn.

235/85R16 is 85% of 235x2+16" works out at, 783mm diameter, 2.46m per turn / circumference.

Series Diffs; 4.7;1 reduction, so one turn of the prop would give 1/4.7 turns of the wheel, or 76 degrees of rotation.

Working back thru the box;

High range in the X-fer gives 1.148:1 reduction

Main-Box gives 1:1 reduction or straight thru drive in 4th

So crank speed into the main-box comes out crank-speed into the X-fer, comes out 1/1.148 to the prop.

Overall reduction from crank to wheel, works out at:-

1/1 x 1/1.148 x 1/4.7 = 1x0.87108x0.2127 = 0.185336

Or 1 crank rev = 0.185336 wheel revs.

The series engines, make max power at 'around' 4,000rpm, give or take a bit, and rev out to 'around' 5,000rpm.

I did the sums for 'cruising' based on 3,800rpm, which is about peak power on a TDi engine; and it works out like this:-

3,800rpm at the crank = 0.185336 at the wheel, or 704rpm.

Multiply that by 60, gives us 42,256 revolutions an hour.

Multiply THAT by the wheel circumference, gives us:-

97Km/h or 60mph MPH on 205's
104Km/h or 65MPH on 235's

Do the same sums to get theoretical top speed, IF you can rev the thing to the limiter, at 4,700...... you get

120Km/h or 75mph MPH on 205's
129Km/h or 80MPH on 235's

Chuck in an over drive or 3.5 diffs, that 'raise' the gearing by around 25% (or more precicely 'reduce reduction!) and you get:-

124Km/h 77MPH at 3,800 on 205's
133Km/h 83mph at 3,800 on 235's

158Km/h 98MPH at 4,700 on 205's
170Km/h 106ph at 4,700 on 235's

But, whether you actually SEE those speeds depends on the power you actually have available from the engine at those crank speeds.

Said TDi has about 110bhp, and that SHOULD be enough to propell a series shed to around 100mph; and a TDi WILL rev, just about to 4,750 rpm... not happily, but it can be made to turn that quick; so on 235's and with 3.5 diffs or an over drive you MIGHT just get one to crack a ton.... but, peak power is a lot lower down the rev range, and it drops of very quickly, so practically, even a 'good' TDi tops out at about 90mph; you could gear it even higher, to get peak power closer to top speed, but you'd need about another 25% again, so 3.5diffs AND an OD unit, on 235's.

On a series 2.25Diesel, torque curve is a table top, doesn't mnatter how hard you rev the thing, it just doesn't like to make anything move, and really its 60bhp is good for about 60-65mph no matter WHAT gearing you give it.

Series 2.25Petrol, bit more willing, and makes a bit more power, around 70bhp... still only as much as a 1300 Metro, but better than the Mini 1000 poke of the deseasil! Thats JUST about good for 70mph..ish.

Which the standard gearing allows.

Gear up, and you dont have the power to go much if any faster with those motors.

Stick an Over drive on a 2.25D and it will cruise a little more comfortably at around the 55060 mark, but it still takes a lot of encouragement to make the needle head north of 60.

2.25 Petrol, you get a LITTLE more effect from fitting an OD. Knocks back the revs at 'cruising' around 55-60ish to something a lot more relaxed, and puts peak power somewhere more useful in achieving a top speed of about 75mph.

Note that these speeds are still lower than the theoretical top speed for the gearing if the engine would pull to max revs in top.

they are infact a lot closer to the theoretical 'cruising speed' at 3,800rpm.

Reason being as standard, Landies are 'over geared' for the power they have..... not much..... but a bit!

Reason gearing up makes them ANY faster is simply becouse that little extra gearing knockes the engine revs at road speed back to a place in the power curve where the engines delivering more power; but the difference is small. 10mph 'more' is very optimistic; nominally 3-5mph is what you'll actually see, unless you have the favourable conditions of a tail wind and or a long hill.

But, observing taht the real world top speeds are achieved a bit UNDER the engine speed where peak power is claimed for them, suggests that they would actually go faster if the gearing was LOWERED not raised, so that peak power actually coensided with max speed.

Right; so the cars over geared to begin with, and gearing it up nmakes a small difference to top speed, but more notably knocks the engine revs back to something more comfortable at 'usual' road speeds.

Now, gearing up by an OD, gives 25% less reduction, in top; but its an extra gear-box, and doubles the number of gears you have; so dissengeged, you still have the 'normal' reduction offered by 1st, 2nd, 3rd & 4th; engage the OD, and you get a 'fith' gear above 4th, but also 'inbetweenie' ratios between 1st & 2nd, 2nd & 3rd & 3rd &4th.

This is REALLY useful on a Deseasil that rund like a Brigs and stratton lawn mower engine..... ie happiest at ONE engine speed! Means you can maintain road speed by varying the grar ratio, playing with the OD to find something suitable, rather than trying to get the engine to do something it doesn't wat to.....

Most useful actually is 3rd Od, which I found a huge boon to save changing down to third, where it would have revved its nuts off, and loosing road speed on longer hills, becouse even though it would rev out in third, changing up to 4th would have knocked it back so far as to make it bogg......

Not SO significant on a petrol, but still useful.

Now, 3.5 Diffs; give the same gearing increase as an OD, 25%, BUT its not a gear-box, once fitted, you lift ALL the gears by 25%, and the gaps between them, are actually bigger; more road speed for same revs means more road speed to be accelerated through between gear changes.... Oh, and you are starting from a higher first, so a bit like doing a hill start every time you pull away from a stand still; not QUITE as bad as trying to pull away in second, but the same sort of added strain on the engine.

So where the OD was 'useful' by giving the facility to knock back engine revs by engaing it, as well as the ability to use it to 'split' the gaps between gears.... with 3.5's you have bigger gaps, and no chance to fill in between them.......

Some people reckon that's 'OK', but most I've ever talked to, unless they have had a big motor or a tuned motor in there and had the power to haul-tall, its made them a lot harder work, and a lot less 'eager' to do anything, and demanded a lot MORE thrashing between the gears, for the little they save at speed.

I do NOT reccomend 3.5 diffs for increasing speed, improving ecconomy, or making driving 'more relaxed' as basically, they dont do that; yes, they will be a bit less frenetic trying to hold 60 on a duel carriageway, but you'll be thrashing them harder every where else; so its simply a shift of compromise, and I dont think a very good one.

The diffs aren't any stronger than the 4.7's they replace; basically the same assembly just a different sized crown wheel attached; they ARE stronger than an OD, so on big diesel engine conversions or V8's that make enough torque or power to stress OD's they were a better way to lift the gearing, but then those engines had the power to pull the ratio.

As a 'cheap' alternative to an OD, they dont really work, and certainly dont do what an OD does; but then they are cheaper...... but then if the advantages of an OD are important to you, then that money is worth it, 'cos 3.5:1 diffs 'ent an OD or substitute for one!

to be honest i havent read all that but im just now wanting to keep my series axles and fit the hight ratio diff`s powers not an propblem as im runnin a 200 tdi init wich leads to my other concern about breakin half shafts so easly as i mainly use it for off road use wich the other thing i was thinking buy useing higher ratio diff would it not be putting quite as much torque through the shafts?

mickdj said:

to be honest i havent read all that but im just now wanting to keep my series axles and fit the hight ratio diff`s powers not an propblem as im runnin a 200 tdi init wich leads to my other concern about breakin half shafts so easly as i mainly use it for off road use wich the other thing i was thinking buy useing higher ratio diff would it not be putting quite as much torque through the shafts?

Click to expand...

200TDi has the power to pull taller gearing that Stock series; if you HAD read through though, I say how fastr and what gearing gets you that fast with a TDi. Becouse it makes its peak power so low down, less that 4,000 revs, it needs very tall gearing to get it to the sort of speeds its power might permit; a lot more than 3.5 diffs on thier own would give....

But you have found the dilemah in the situation. 3.5 diffs aren't stonger than 4.7's, again, in test, exactly the same unit.

And, making them turn slower WOULDN'T reduce the 'torque' or forces on them, it would actually increase it.

Power = torque x revs

So, lifting the gearing curtecy of 3.5s would reduce shaft speeds, hence raise shaft loads and you'd actually be more inclined to break them, or whatever is the weakest link in the transmission... on a series with a TDi in it, usually the gearbox lay-shaft, way the blower ramps load on the shafts induces HUGE 'stand-off' pressure between the cogs, and in that box, main shaft will flex as the presure wears out the main-shaft bearings, but rigid lay-shaft has no flex, so usually snaps!

But 50/50.... becouse you are still putting twice the power Rover inteded be put through thier diff, and half shaft, whether 3.5 or 4.7, when running in high range, becouse you have two wheel drive and ALL the power is going through that one cog, not being 'shared' between two, as with permenant four wheel drive coilers.......

Further you get into this, closer you get to the conclusion that you are better off with an LT77 transmission....... then you ought to have CV joints in the front axle........ and then you want those coiler axles again!

There aren't any 'easy' answers to this one; sorry.

Personally I'd not stick a TDi in a series; I dont think its a very good match; but 'dream rover' time; series II 88', V8, LT77 with ashcroft adaptor to series X-fer; coiler axles on leafs, with four gear diffs, lockers, uprated halfers, and drive line dampers and cushdrives on the flanges and props...... but cheaper to buy a brand new defender

so now i think ill try and find a salisbury axle for the rear and hope i dont brake out on the front , i was gonna fit an lt77 not long after i fitted my engine but was advised against it because of the uj`s on the front.

if you have a 200tdi in it then I would recomend the 3.54 diffs in it. I have a v8 and put those disco diffs and it makes the gearing a lot nicer to drive. It will also affect low ratio by making them slightly higher but I havn't found it a problem....and no, i didnt read all that either!

GO FOR IT!!!

do you use yours for off raodin or just on the road? my bigest concern is i dont want to make stuff even easyer to brake as teflon thinks im more likley to brake half shafts with them diffs in. iv been out in it a few time since fittin the tdi and havent broke out due to that yet but upon swoping a diff for an other reason found a half shaft was was twisted on the slpines and looked as if it was ready to go

I use mine for both on road and off road. Have given the driveline a hell of a lot of abuse and have had no problems. They do drive so much nicer. Just start by changing the rear diff, which can be done in less than the hour and drive it round in 2wd. You'll see what a difference it makes

I say don't do it as it will spoil your Land-Rovers off road ability as the gearing will be too high in low ratio first which is good for crawling over very rough ground or down steep hills. I think an Ian Ashcroft high ratio transfer box is the best option as it should be stronger and more efficient than an overdrive.

Ascroft Hir-Rat X-fer is useful, essentially he's switched the drop-gogs to reverse the ratio.
Maintains low rario, but increases high ratio by 33%.
But; PTO becomes unuseable, so you cant fit a pto, winch pump or over-drive, if that was a feature needed.
Ratio is a lot higher than 3.5's or OD, and like 3.5's spreads the gears out.
For a low reving disel engine, its a useful mod; but for anything more 'spritely' or less powerful, it can be a bit too much.
Attached to a 2.5NAD, reports suggested it was just too high, and that it was like trying to pull away in 2nd all the time, while the motor just didn't have the 'oomph' to hold top gear very often, so 3rd got over-used, and saw little extra speed and no added ecconomy.
On a Perkins/Mazda conversion, original box was fitted with an OD, which the motor ate within a fortnight; running without it, couldn't be coaxed above 45mph, BUT could go green-laning in Hi-Range!
3.5's were fitted; coaxed top speed up to 55mph. Ascroft HiRat X-fer was fitted, coaxed top seed up to 'near' 70, but rear shafts started doing the toffee twist, and the gearbox colapsed!
Running in hi-range, was deemed more of a killer than off-roading. In low-box power is shared between axle, so the rear axle only sees half the strain; running in Hi, back axle has to take twice the load, and that's when the half shafts start twisting.
So gearing a series up, is actually LESS of a 'risk' to the drive line when used 'off-road' where you would think the drive line would actually have a harder time; it doesn't, mainly becouse you aren't going so fast, and the loadings aren't actually as high as on the road, accelerating between traffic lights, and sharing forces between axles.

Teflon said:

But; PTO becomes unuseable, so you cant fit a pto, winch pump or over-drive, if that was a feature needed

Click to expand...

I'm considering this conversion in the future to go with my 200tdi (currently being fitted complete with intercooler and turbo) with SIII gearbox and dowty hydraulic pump. It says on the Ashcroft website that an overdrive cannot be fitted, but that the rear PTO can still be used, so I guess the hydraulic pump can probably still be used too with a bit of luck.

I noticed your comments about running a 200tdi through a series gearbox. I must admit I am a bit concerned that I'm going to break the drivetrain so I've been thinking about having a system to be able to adjust reduce the boost possibly to 0 when using it off road as I immagine that's the situation that's most likely to break the gearbox. Any thoughts on that?

i know a few people with tdi series and have only had problems off road, thing is untill you drive it after its a totaly different motor off you id say most of the time your not even workin the turbo just tickerling the pedle, but im still thinking of haveing my pump done but in two minds because of the extra power but my mates when his was done watchin him go up hill it only gave out the odd puff off black because you dont need your planted like with the 2 1/4

I think I'll play it by ear. I might fit a Salisbury rear axle if it's busting too many halfshafts, and perhaps try a SIIA gearbox if the SIII box gives trouble. If it gets too much of a pain in the ass, I'll take the turbo off it.

Difficulty with disbling the turbo is that theres no easy way, the only thing I could think of is to mod the linkage, maybe a mini heater control cable, one of those you pull and twist, you might be able to connect this the control rodfor the butterfly valve and permanently hold it in the open or low boost position, you'd have to look at a tdi turbo on the bench, probably isnt so hard to do.
Dumping the boost air to atmosphere is not really an option, I believe (never tried it) if you run a turbo open ended compressor side the extra load on the impellor due to the lack of pressure increasing the volume knackers the hydrostatic bearings, must twist the spindle to one side or something. Probably why boy racer 'dump' valves screw up turbo's.

or just take the turbo off. saying that my mate i was on about when he did his tdi conversion he put a 2a box in had some propblems with the clutch had to make a spacer or somethin , first time out in it on a failed hill climb it jamed in between first and second gear and stalled the engine and had to put it in nutral with high low leaver.

I'm with Fenby to a degree,

You will spoil your offroad driving changing the diffs, (I know all about that).
But you need a higher ratio for road use cos flat out at 55Mph Also Sucks with a TDI (I also have been there)

I havent got a series but a 90, I converted it to a 200Tdi then uprated the transfer box to a disco one much better gearing me thought on 750's. Fitted a salisbury rear axle and detroit etc. Then I got some tall 900 tyres. On road its massivly over geared. Offroad its horendus! Far too fast for anything decent. We have just come back from Derbyshire and it really showed up as far too fast in lowrange 1st. Now I have spent along time and I mean along time working out my options with regard to ratios.

I am now fitting 4.7 ratio axles front and rear lower my lower my overall gearing so thats low range and high range. I suspect this will be pretty good offroad ratio much more controlled. For road use the overall ratio works out faster than when I had my old transfer box in and lower than with my 750's and disco box. I am considering the fact that high range may be to low and might have to get a 1.00 transfer box. but at least the low range will be suitable.

Offroad driving with the gearing so high was horrible Where I would use 3rd low for getting around a pay and play site I was using 2nd low, when I needed 2nd low I was in 1st and so on its just really wrong and horrible to drive. Jai

mi heads spinnin just like your wheels.