I make it more like 33%: If the diff pinion is rotating at y RPM, then the wheels with a 4.7:1 axle will be spinning at y/4.7 RPM, and the wheels with a 3.54:1 axle will be spinning at y/3.54 RPM. The percentage increase in speed will then be 100*(y/3.54 - y/4.7)/(y/4.7) = 100*((4.7-3.54)y/(3.54*4.7))*(4.7/y) = 100*(4.7 - 3.54)/3.54 = 32.8 (approx).
Sorry fenby, cant be bothered to check the sums proffered, we are probably both right, one third original is the same as one quarter of original plus a third........
Gearboxes sap a significant proportion of engine power. A Land-Rover comes with two gearboxes as standard, so adding yet another gearbox (which is what an overdrive is) will sap yet more power. A high ratio transfer gearbox conversion will not sap any more power (as an overdrive will) and will not affect trailer towing or off roading ability (as a diff swap will).
Efficiency losses through the gearbox are very small, those through an OD negligible. When dissengaged the OD is 'locked' and simply rotating as the gearbox output shaft.
Series gearbox has 'lock-out' 4th, the input and output shafts locked, bypassing reduction gears, which are merely idling, minimising efficiency losses.
Running Tall diffs or HR X-fer, you WILL worth the lower gears harder. So, it depends entirely on your driving style and mechanical sympathy whether you get more or less acceleration; taller diff ratio may detur it, but that is as likely to encourage reving out longer in lower gear......
It REALLY makes so little difference its not worth arguing about; but the advantage of the OD unit is that you dont HAVE to use, while having the facility to select it, IF you want, from the drivers seat.
The best savings will be made by keeping your speed down and not accelerating hard.
Oh, the old Commer Walkthru'! What memories that brings back! Made a Land Rover seem 'Fast'!
Yeah, the 4203 was acommon fit, and the commer variant was the better one to use. My Grandad was a factory field service engineer at Commer / Dodge in the 70's/80's, and I remember going through all the old service bullatins he had in his attic a few years ago!
These days, though, you wont find many ex commer variant motors. they are still made by Perkins Japan, I believe, with MANY revisions including a Turbo Charger, and used in Mazda vehicles, and I think another 'eastern' maker... possibly Isuzu?
Had a number of 4204 converted Landies on my Grandads drive when I was a kid, for him to 'sort out'. They were HORENDOUSELY heavy on the gearboxes, and they were even more horendousely under-geared, though Rangie Diffs helped, get speeds back to close to original with the reduced rev cieling.
Doing a conversion these days, reputation / experience of the 'old' 4203 would put me off, before I started looking for adaptor kits, or worrying about finding a commer variant, or converting a plant engine to commer spec. I seem to recall 'Pops' ranting at length about how "it ISN'T 'just' the bloody rev-limiter!" and going on about the injector crack-off pressures, combustion chambering and cam-profiles, before diving under bonnets muttering curses about 'ignorant idiots, messing with things they dont know'........ (he was a miserable old bastard, Pops!)
Looking at Mazda / Perkins Japan, 'stuff', for an SIII, I KNOW it has been done, a chap on another forum I met a couple of times did a Mazda conversion on a 109, but it was a hell of a job, and I think he used an old perkie 4203 adaptor ring he got from Old sod or off another Landy with a 'dead' gearbox and clapped out commer, I cant remember.
I know he went through two gear-boxes getting the thing to work, and there was about a year of met-fab in making all the bits and pieces he needed to make stuff fit and not shake to bits, and FINALLY when he got it working, it did about 45mph flat out, becouse the motor apparently has an even lowe rev limit than the commer-plant spec engines!
Took an Ashcroft HR X-fer box AND Rangie diffs to get it to 'drive' something like, but pulled really well, and did about 70 reletively comfortably.
Continuing with the ideas of LPG, I dont know whether the people you have spoken to that have told you the stuff has been nothing but trouble have had thier kits on Landies or not, or when / where / how they were fitted.
Personally I've had two LPG converted Rangies and they work well, and have given me little or no trouble by virtue of having gas on them. Second one, HAD given last chap grief, and when I collected teh car, gas didn't work, and I suspect that in ignorance they bludgered it up. Using the kit off my old rangie to de-bug the system, I fixed it and got it running sweet, with little hassle.
And on the whole, people I know using the stuff have had little problem with it. Main niggles are that a lot of mechanics are ignorant of the systems, every one is almost a custom system, so they dont always know what they are looking at, dont always see them very often, and dont always know how they work or how to set them up or fault find.
On a series motor, you have a single down draft carburettor, and its a pretty straight forward conversion, with a single mixer.
There are often SIII Kits offered on the small-ads boards, but there are a lot more kits for rangies.
Personally, I'd do a DIY conversion. For the bits needed on a carb motor, I'd look hard at a complet kit, vs piece parts. I'd then be tempted to keep costs down by buying the piece parts or a part kit, and looking on e-blag and the boards for things like the tank and vapouriser.
If not, a pro-conversion can be expensive, in which case, I'd try and find a car pre-converted, and hassle the sel;ler to explain it and show me how it worksed and what to tweek to keep the mixture happy etc.
Stuff doesn't 'wreck' engines, certainly not old low-tech landy ones, unless you are completely stupid, and get the mixture & timing totally wrong AND keep driving it, with the motor knocking like a diesel!
On a Landy, you dont have the complexity of an injection system or even electronic ignition, so the control electrics isn't too daunting, its basically solenoids. Only 'thing' about the system is change-overs and filling / draining the carb float bowl of petrol between running on gas.
Just read up on it..... its worth it!
