Isuzu Gearboxes calculations ??

[Lotz-A-Landies]

I've been doing some ratio calculations to determine the LT230 ratio and gearbox selection but am not sure my figures are working out.

The first table is the gear ratios of a number of Isuzu 5 speed boxes published by Dougal some years ago. The four RH columns are the inter gear ratios.

http://www.aulro.com/afvb/attachment.php?attachmentid=65782&d=1379508452

The second table is the final drive calculations using 265/75 R16 diameter according to Explor Oz tyre calculator; 3.54 diffs; revolutions / km + RPM at 100KPH. by the various LT230 ratios at 1:1 irrespective of gearbox.

http://www.aulro.com/afvb/attachment.php?attachmentid=65783&d=1379508513

The third table is the various LT230 X Isuzu GB X RPM in 5th at 100KPH

http://www.aulro.com/afvb/attachment.php?attachmentid=65784&d=1379508543

The fourth table is the various LT230 X Isuzu GB X RPM in 5th at 110KPH

http://www.aulro.com/afvb/attachment.php?attachmentid=65785&d=1379508565

The RPM seem very low for all but the very rare in Oz 12D 1.667:1 LT230.

What has gone wrong?

[isuzurover]

Looks about right. Why do you want to be doing 3000 rpm in 5th at 110km/h? 2000 sounds much better!

For 0.776 top gear with 1.003 high, 3.54 diff gears and 31" tyres I get 2042.03 rpm at 110 km/h.

Do not fit a 36D (Q) case. It will not last.

[Lotz-A-Landies]

somewhere I found a reference that the max BP was developed at about 3200RPM.

Will my planned use of the Disco 1.211 LT230 be too high?

[isuzurover]

...

Will my planned use of the Disco 1.211 LT230 be too high?

See my edit above. You do not want to fit an LT230Q (1.211) case. The ratio is good but they cannot hold up to an isuzu.

https://www.aulro.com/afvb/
http://www.aulro.com/afvb/attachments/members-rides/63430d1374758473-isuzu-county-tourer-20130725_205509.jpg


However, a 1.222 would be fine. Despite what you have read, you do not want to sit on 3200 rpm on the highway.

[Lotz-A-Landies]

Is it all the 211 boxes or just the "Q" suffix?

I feel that the 003 ratio will be too high for the 6x6.

[isuzurover]

Is it all the 211 boxes or just the "Q" suffix?

I feel that the 003 ratio will be too high for the 6x6.

All 1.211 are Q cases.

As I said, I would use a 1.222

[Lotz-A-Landies]

Sorry I thought some of the 41 and 42 boxes were "T".

It all gets very confusing, the LT230 comes out of a pre-ABS Disco 1, so there is a possibility that it has a 28D 222, but wilh my luck it will be the Q.

Do we know when the D1 changed boxes/ratios, it had a 3.9 V8/R380 so I'm not hopeful. :BigCry:

[Blknight.aus]

Ignore max HP, pay attention to max torque....

An R series produces peak torque at 1400 Rpm and max HP at 2100.

Youd think that when you hit a hill it would sit at 2100 in third with a load on but they dont they creep down to 1400 Rpm and then just sit there.

or if youd like that in rice boy terms...

Hp lets you work out how fast you can hit the wall Torque tells you how far the wall is going to move when you do.

where ever peak torque is for your engine is where you want to get your desired cruising speed to be, you might want to even aim it a little higher so peak torque occurs just below your desired speed

[Bush65]

As Dave and Ben have stated, be guided by where maximum torque is. Also, it is not a coincidence that minimum fuel consumption and maximum volumetric efficiency occur close to where maximum torque is made by diesel engines.

This pic (sorry not so clear) for a 4BD1 shows power, torque and specific fuel consumption. Maximum power at 3200 rpm as you said. Maximum torque at 1900 rpm, minimum SFC at 1400 rpm.

The same year (1989) 4BD1T has max power at 3000 rpm, maximum torque and minimum SFC at 2200 rpm

[Bigbjorn]

The operating range of an automotive diesel engine is the speed range between governed RPM and maximum torque, the period of torque rise. Now in heavy duty trucks the governed rpm is usually the truck's maximum speed. Many small diesel engines used in cars, 4WD's, light commercials are not sufficiently powerful to achieve governed rpm in top gear. Heavy trucks are designed and expected to cruise at governed rpm. The small automotive diesels usually have a quite high governed speed and are not expected to cruise at these engine speeds nor are they sufficiently robust to do so. There would be rods, pistons, and cranks littering the highways if you attempted to cruise these engines at full throttle and 4,500 rpm.

Do a gear split chart and you will clearly see the speed range in each gear and the overlap between gears and thus identify the shift points. My County-Isuzu with LT85 pulls 2000 rpm at 100 k's and 2100 at 110 k's in fifth high. It runs out of puff at 125 k's.

[Dougal]

I'd think somewhere between 2000-2500rpm at 100km would be good for a 6x6. You will need a better turbo than factory (especially if factory was no turbo) to give you more low down boost and torque.

As Brian said, the gear split and making sure you have a suitable gear for each forseeable occasion is the most important thing.

[Lotz-A-Landies]

Bloody Microsoft re-booted my machine overnight and excel didn't autorecover the spreadsheet, now I have to do it again! :mad: :mad: :mad:

At least I have the image files.

[ScottW]

Here's a spreadsheet I made some time back.

Change the orange cells and it should fill out the rpm's for you.
There are three tables to compare the LT95, LT85 and Isuzu/LT230 combo.

I think I got the ratios from ashcroft.

[Bush65]

My gearing calculator (http://www.aulro.com/afvb/attachments/isuzu-landy-enthusiasts-section/58244d1364371191-spreadsheets-isuzu-gearing-calculator.zip) is in the spreadsheets thread - in zip format.

It has a summary sheet that compares many options using the popular/useful combinations of gearboxes and transfer cases for the 4BD1/T, which are on individual sheets.

[Lotz-A-Landies]

Thanks guys.

In following up on Ben's comments about the Q box, I wend to Graeme Cooper's on the way to work. They looked up one of their training manuals which specifies the Discovery 1 had 28D (? LT230T) suffix up to "F" 1.222:1 26 tooth input gear.
suffix "F" 1.214:1 28 tooth input gear.
suffix "G" 1.211:1 38 tooth input gear.So perhaps I'll be lucky. :(

[isuzurover]

Thanks guys.

In following up on Ben's comments about the Q box, I wend to Graeme Cooper's on the way to work. They looked up one of their training manuals which specifies the Discovery 1 had 28D (? LT230T) suffix up to "F" 1.222:1 26 tooth input gear.
suffix "F" 1.214:1 28 tooth input gear.
suffix "G" 1.211:1 38 tooth input gear.So perhaps I'll be lucky. :(


I can't see how that could be correct and it conflicts with Ashcroft's info.

38 tooth input gear should be the Q case. You cannot make a 38-tooth input gear with T-size teeth.
My broken Q case posted above was a 36D from a late model Disco 1.
The case I replaced it with was a 28D 1.222


28D = 1.222

29D = 1.192

32D = 1.222

36D = 1.211

38D = 1.211

40D = 1.211

41D= 1.211

43D = 1.410

47D = 1.667

57D = 1.410

59D = 1.211

61D = 1.211

62D = 1.211

68D = 1.211

69D = 1.211

70D = 1.211

If you already have a t-case, just pull the pan off and you can see instantly if it is a Q-case or not.

Q gears are on the right of the equivalent gears in a T transfer case in this pic.
https://www.aulro.com/afvb/

[Lotz-A-Landies]

Hi Ben

Only suggesting that is the information sheet from Jaguar Rover Australia's training manual. It also talks about the currrent Defender at that time being a 22D.

The text states: " ... In its original form it was designated as LT230R. With the replacement of the radial intermediate bearings with tapered roller bearings the designation was changed to LT230T. This specification is still being ustilised in present versions of Discovery and Defender and also for Pre-3.9 litre Range Rovers. ... This makes the information early 1990's and not necessarily contradictory to Ashcroft's or your information on later boxes or the ones in the 3.9 litre RRc.

It is possible the LT230 I have will be one of the earlier 1.222 or 1.214 boxes, but I'm still not convinced (given the 3.9 and R380 in the D1).

[isuzurover]

...

It is possible the LT230 I have will be one of the earlier 1.222 or 1.214 boxes, but I'm still not convinced (given the 3.9 and R380 in the D1).

Is your case a 28D or 36D prefix?

[Lotz-A-Landies]

Is your case a 28D or 36D prefix?Don't know yet, it is located at Leppington 40 Km from home and 50 from work.

I can only get out there on weekends.

[Lotz-A-Landies]

My gearing calculator (http://www.aulro.com/afvb/attachments/isuzu-landy-enthusiasts-section/58244d1364371191-spreadsheets-isuzu-gearing-calculator.zip) is in the spreadsheets thread - in zip format.

It has a summary sheet that compares many options using the popular/useful combinations of gearboxes and transfer cases for the 4BD1/T, which are on individual sheets.John

Do the Speed/RPM graphs update when you change the data? Not seeing it happen.

Diana

[isuzurover]

John

Do the Speed/RPM graphs update when you change the data? Not seeing it happen.

Diana

You probably need to "enable macros" or similar...

[Lotz-A-Landies]

Sorted the problem. I copied the whole sheet to a new one to have comparative sheets for dirfferent Isuzu 5 speeds and the graphs lost their linkages.

[Bigbjorn]

Here are a couple of gear split charts for an Isuzu engine, 3.55:1 diffs and 4 or 5 speed trans. Note that the NA Isuzu won't pull theoretical top speed, 3200 rpm in top.

[Lotz-A-Landies]

Is your case a 28D or 36D prefix?Hi Ben
First chance I've had to check the transfer box. Its a 28D with "E" suffix so likely 1.222:1 with 26 teeth on the input gear.

I guess l'm lucky for once!

Diana