Hi all,

for reasons of speed or economy, is it worth changing our diff ratios from the standard C306 7.5:1 diffs to the 5.99:1 diffs found in the C303? We saw this idea on www.HouseofFlyingSpanners.net, and it seems like a good idea. Just wondered if anybody here has done this already on their own vehicle?

(See our other new topic for sale of portal axles from our number two truck!  )

Zitat von: RiffnRaff am 13.Oktober 2015 19:27:51

Hi all, for reasons of speed or economy, is it worth changing our diff ratios from the standard C306 7.5:1 diffs to the 5.99:1 diffs found in the C303? We saw this idea on www.HouseofFlyingSpanners.net, and it seems like a good idea. Just wondered if anybody here has done this already on their own vehicle? (See our other new topic for sale of portal axles from our number two truck!  )

Not if you intend to a) keep the original engine & transmission, and b) come anywhere near the original max weight of C306/TGB20.

Hint: fully loaded, there is no such thing as the common 2nd-gear/low-range climbing of even mild hills. And 1st-gear/low-range is downright scary, because that makes any subsequent hill-start (or slowing down) impossible.
(this is based on C306/civil 7.1:1 diff's, at 4.5-5t (max TGB is 5.7t; max civil was 5.5t formally (Tollarp brochure), but in reality, none had much more than 5.2-5.4t)
(despite the fact that a firebrigade-chief once showed me, with an evil grin, the print-ticket of the weight-measurement from their original 1st deployment: 6400kg....:))))
(this is also what I referred to with Kuppi, about the 4th leaf of the rear springs touching, even without much load....poor springs, 30 years under such a constant pressure....;))))
(no military TGB20 ever faced such an ordeal!....8-))


That said, and give and take the max possible strain between gearbox and diff's: there is/once-was a T-case upgrade/modification to 3.7:1 (from 2.4:1 that means a 50% improvement).

Hmm, that's interesting!

I'm glad to never have been standing downhill of that particular fire engine...! 

We are planning an engine/gearbox swap eventually, but money will only allow a few changes at a time.
The engine we are considering (we don't really want diesel) is a 4.0L straight 6 out of a Jeep Cherokee, converted to bi-fuel/LPG, 190HP, 305Nm torque at 3000 rpm (I think those revs are right), with a 4 speed auto box to limit driver fatigue on long journeys.

As far as the diffs go, it looks from your comments that it may not be worth changing them out - we're trying to keep everything as light as possible but a camper conversion does mean a certain amount of weight is unavoidable.
Do you (or does anybody) have any idea what the back body of the TGB20 actually weighs? We know from when we had ours transported, the vehicle as a whole weighs 3490kg. It would be very useful to know exactly how much of that is the back box.

 

Have a chat to Rijo. He has crown wheel and pinion conversions and data mapped out. Johnny

Zitat von: RiffnRaff am 14.Oktober 2015 08:39:5

Hmm, that's interesting! I'm glad to never have been standing downhill of that particular fire engine...!  We are planning an engine/gearbox swap eventually, but money will only allow a few changes at a time. The engine we are considering (we don't really want diesel)

You do want one, but you can't fit (a decent/tall) one easily, nor run one without massive regearing.
(otherwise I recommend you drive a HDJ-80 or even HJ-61, tweaked, intercooled & free-breathing (3" exhaust))

Zitat:

is a 4.0L straight 6 out of a Jeep Cherokee, converted to bi-fuel/LPG, 190HP, 305Nm torque at 3000 rpm (I think those revs are right), with a 4 speed auto box to limit driver fatigue on long journeys.

The fatique is nowhere even *near* the shifting (not even in the non-servo/non-power clutch pedal).
Fatique, or better ergonomy/multitasking, is where servo-steering/power-steering is making heaps more sense, in particular in combination with a steering knob, for true one-handed steering (stronger & faster/more-reactive/more-control (in *any* position!) than 2 hands without knob!).
(worst-case^2 is the wheel being grabbed/sliding from your 'unwarned' hands when touching a high curb along the street....can get very ugly, *very* fast)
(even more in a front-steer cab with little protection, crashing into solid concrete/brick buildings, with 3-4 tons behind you)

Only argument would be that any gasoline, *all* having lousy bottom-end torque{*}, in any gear, should have auto transmission, for steep hill starts & shifting.
(I'd just as much would like having a double-clutch tranny for that matter though!)

{*} Don't trust *any* graph ending above 1000rpm!....500rpm is minimum, yet only 250rpm *really* tells you what is happening, worst-case.

That said, and you already mentioned it yourself:
Given such a 4-5-6t vehicle, *and* a gasoline, an auto-transmission is *the* absolute nightmare downhill....adding insult to injury, possibly even literally....
(T-case/cardan mounted electromagnetic retarders do exist though, but expensive, and probably affecting belly clearance)

Zitat:

As far as the diffs go, it looks from your comments that it may not be worth changing them out - we're trying to keep everything as light as possible but a camper conversion does mean a certain amount of weight is unavoidable. Do you (or does anybody) have any idea what the back body of the TGB20 actually weighs? We know from when we had ours transported, the vehicle as a whole weighs 3490kg. It would be very useful to know exactly how much of that is the back box.

I can't recall having seen any 6x6, even bare-naked cab/frame, with less than 2,25-2,50t, so 1t for the complete bed + canopy seems a good estimate.

Far better option is to fit Original Toyota 1HZ motor with 5 speed gearbox coupled directly to VTG (transfer case) than crappy american Jeep offering.
Jeeps are thought of as shit in Australia. Only good jeeps are the real Jeeps the CJ series. 
I have 1974 Jeep CJ6 that I swapped out inefficient gas gusling AMC 258motor for GMC V6 aluminium motor and Turbo400 auto gearbox.
For sale at present on Ebay Au.
 

Zitat von: johnnyk am 18.Oktober 2015 08:8:3

Far better option is to fit Original Toyota 1HZ motor with 5 speed gearbox coupled directly to VTG (transfer case) than crappy american Jeep offering. Jeeps are thought of as shit in Australia. Only good jeeps are the real Jeeps the CJ series.  I have 1974 Jeep CJ6 that I swapped out inefficient gas gusling AMC 258motor for GMC V6 aluminium motor and Turbo400 auto gearbox. For sale at present on Ebay Au.

I wouldn't want to run any TLC atmospheric diesels at more than 3000rpm, which requires significant regearing (compared to the stock 4-5k)).

That said, this Dutch guy is advertising with (the option of!) a GM V8-diesel, plus automatic transmission:

http://www.marktplaats.nl/a/auto-s/bestelauto-s/m946848057-volvo-volvo-c03-6x6-5-v-1979.html?c=8c285449651fa109c354bbabe740c1b&previousPage=lr

(including making it road legal, not the least important issue)

No clue what he does with gearing, don't think any GM tranny has decent overdrive, do they?

That said, refitting a totally different manual gearbox, cq create/extend remote shifting, is most likely a way bigger project than the engine itself.
(cq bonus for automatic....but the malus of not having decent engine-braking (even fitting a modern 6-9sp from a German SUV doesn't give you full (torque) lockup in 1st-gear (safety issue))
(odd: these German transmissions come from the same Japanese factory as Toyota, but Toyota refuses to fit them with full-torque lockup (and even the quarter-torque is implemented poorly, see J20 aftermarket upgrade project on Buschtaxi)