Thread: Is a Getrag MT82 a possibility for a 4BD1-T

I'd expect it would be more effort tha it was worht due to the bell housing setup on the 82 .

and if you got that sorted the suzi would devour an 82 faster than a drunk on a kebab.

Why the frown on the Isuzu box lotza? There's plenty of info on how to make it and do it right now, just build one.

I've just come back from a couple of months driving a couple of rental 2.4 Puma's around southern Africa. Was only meant to be one vehicle but the first one crapped an engine in Central Kalahari (Botswana) so we ended up with a replacement.
Gearbox in the first one (~150K km's) felt really nice - smooth and precise. The second vehicle was around 220K km's and the box was like stirring a jar of water with a skewer. Selecting reverse was a matter of trial and error (I'd regularly have 3 or 4 goes) as you never knew whether you'd selected that or first until you started to let the clutch out. I hated it.
Not sure if that was just an isolated bad box, but I'd definitely be apprehensive about a 4BD1 shaking one to bits.

Steve

Originally Posted by Vern

Why the frown on the Isuzu box lotza? There's plenty of info on how to make it and do it right now, just build one.

Not frowning on the Isuzu boxes, just scared of getting the adapter housing and shaft wrong then having problems in the bush or elsewhere away from tools and spares. As you've already experienced.

On the Getrag the thought of a simple bellhousing adaption for a six speed box was an interesting prospect.

I like that this thread proposed a gearbox based on it's ratios, after all what good is fitting a gearbox if it doesn't drive well at the end? That got me thinking what the ideal gearbox ratios would look like.

My 4BD1T is happiest between 1500-2500rpm i.e. rev to 2500, change gear and end up at or above 1500rpm. 1500rpm is 60% of 2500rpm, so we are looking for a gearbox with ratios in the progression that only differ by 40% or less (constraint 1).

A stock 300tdi Defender idles in first gear at about 6-7 km/h, so our first gear will ideally operate similarly. With Isuzu conversions, the usual choice is to use a 1.003:1 LT230. A 3.39 (r380 first gear) with a 1.41 LT230 equates to a 4.76 first gear on a gearbox in front of a 1.003:1 LT230; any shorter than this is getting fairly pointless with the torque of the Isuzu. So having a first gear around this point is our second constraint.

As an endpoint, let's say we are aiming to sit at 2000rpm at 110km/h. With a 1.003:1 LT230, this equates to a ~0.77 top gear (constraint 3).

As our fourth constraint, let's say we also want to cruise nicely along somewhere between 1800-2000rpm at 60, 80 and 100km/h. 100km/h is already taken care of with our 0.77 top gear, so we need good gears for 60 and 80. In most gearboxes, second from top will be direct drive, which gives us a solid 1870rpm at 80km/h. The gear below that can range from 1.29 (1809rpm @ 60km/h) to 1.42 (1995rpm @ 60km/h).

The only question that remains is, can we comfortably get from our 4.76 first gear to our 1.29-1.42 gear with one intermediate gear (bearing in mind constraint one) or do we need two?

0.6 (40% ratio change) x 4.76 = 2.86
0.6 x 2.86 = 1.72

No. So ideally we will need a 6 speed transmission with ratios something vaguely along the lines of:

1st: 4.76 - ratio change: 34%
2nd: 3.14 - ratio change: 35%
3rd: 2.03 - ratio change: 33%
4th: 1.35 - ratio change: 26%
5th: direct - ratio change: 23%
6th: 0.77



As I say, this is a theoretically ideal gearbox; plenty of others will work well, but this will be better in at least some circumstances.

How seriously you take the requirement for such a low first gear will depend on how much towing you do and how much traffic you face; people with MSA-5G gearboxes report that they regularly take off in second (a 2.998:1 gear). As such, you might be able to live with a 5 speed of ratios approximately from the 2nd to 6th gears of the above.

So now some comparisons (all with 1.003:1 LT230s, except LT95 - 0.996:1).

LT95



MSA-5R



MSA-5G



MT82



R380



FS5R50A (Nissan Patrol TD42)



Spicer ES066-7B (ridiculously short first gear not shown, yes it's a 7 speed). At 205kg it is too big to be practical, but nice ratios.



Ratios and ratio changes:



Other hopefuls - ZF S6-650? G56? Can we find the perfect gearbag, or should we all just fit Allisons?

I think the r380 first gear ratio u have quoted is the v8 petrol one defender uses 3.692 ...

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Excellent work but ..I'm glad I'm not the only one who thinks about this stuff way to much☺

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What about an Isuzu 6speed double overdrive like the MXA6S?

1. 4.987
2. 2.870
3. 1.684
4. 1.000
5. 0.728
6. 0.626
7. 4.774 Reverse