Overdrive unit for 80 series landcruiser

Submitted: Tuesday, May 25, 2004 at 01:37
ThreadID: 13134 Views:19994 Replies:7 FollowUps:1
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Can anyone help with a overdrive unit to reduce revs on a 5 spd manual 1 HDT turbo diesel in a 80 Series landcruiser. 5th gear ratio is 0.88, running a 4.11 final drive which relates to 2800rpm at 110km/h I would like to reduce it to 2100 - 2200rpm.
A 3.54 final drive f +r diff is available ($1700), as is a gearmaster unit from Marks 4wd Adaptors($$$$$$$). Any help would be great. Can the 5th gear be substituted with a lower ratio?
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Reply By: rolande- Tuesday, May 25, 2004 at 08:10

Tuesday, May 25, 2004 at 08:10
Used to be a thing called a FAIREY overdrive, haven't heard of for a while, may pay to type in search engine. Hope this helps
AnswerID: 60019

Reply By: Member - Pesty (SA) - Tuesday, May 25, 2004 at 08:39

Tuesday, May 25, 2004 at 08:39
Marks adapters produce an overdrive unit, sorry thats all the detail I have.
AnswerID: 60024

Reply By: Bob Y. - Qld - Tuesday, May 25, 2004 at 10:44

Tuesday, May 25, 2004 at 10:44

We had one of these 80's and it had 31x10.5R15's on it, and I was always going for another gear.

Why don't you fit 16" wheels with 265 or 285/75R16's?

Seen it all, Done it all.
Can't remember most of it.

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AnswerID: 60047

Reply By: floyd - Tuesday, May 25, 2004 at 14:33

Tuesday, May 25, 2004 at 14:33
Been their done that.

The answer is simple. Just go bigger tyres as above. It is cheaper, easier, gives more ground clearance, is easily reversable and just a hell of a lot quicker to do. Some insurance policies will allow larger tyres. You will probably get lower revs than what you are aiming for so economy will be better and first gear will be more usable (if you have a manual). You should not suffer from worse performance with the turbo. I have done this on 2 turbos and 1 non turbo and the turbos with the larger tyres drive the same.
AnswerID: 60075

Reply By: Member - DOZER- Tuesday, May 25, 2004 at 15:54

Tuesday, May 25, 2004 at 15:54
An auto will reduce your revs down to 2200 at 100kms/hr.(they have an od button lol)
Diffs and transfer mods reduce the Toyota reliability.
Your motor is powerful enough to run 33x10rx15's on your rims (cheaper than 16's)
The down side to bigger tyres is...
each gear gets that much further appart....(not a real problem with a t/d)
low range becomes higher (big problem with t/d take off)
brakes have to work harder to stop you.
A 285 on a 16 will cost 330, plus rims are 100 each minimum. dont know about the 33x10r, but it should be around 250.
Later model brakes can be fitted to your frontend and still keep the 15x7 chromies, therefore keeping the good braking you have with the bigger tyres.
Incidently, if you buy a caravan instead of doing this, you will find the gearing is just right!
AnswerID: 60085

Reply By: bob - Tuesday, May 25, 2004 at 22:05

Tuesday, May 25, 2004 at 22:05
Ask yourself what you are trying to achieve with this experiment. Fuel economy, high speed, quieter cruising?
Altering your gearing to reduce engine revs by 600-700Rrpm at 110kph may sound like a simple solution but it is fraught with risks.
If you "gear up" by say 25%, then you will lose 25% torque in the equivalent gear ie 80 series becomes high speed dynamo downhill but accelerates like a slug.
Acceleration in O/D will be very poor - probably back to 4th to climb the slightest hill.
You may find that you actually use more fuel, because to "pull" this huge gearing you may need a bigger throttle opening to produce the required torque.
There will be huge holes between the gears, particularly in the higher range, gear changes may result in engine speed falling outside of optimum "power" band, or changing down could result in unintentional overrevving $$$$!
Maybe it would be wise to check out RPM/BHP/torque figures before you proceed. (sorry I don't know these figures for your engine). What is the optimum engine speed to get he most out of the turbo?
Modern "high speed" diesels don't have a problem with 2800rpm if maintained properly.

AnswerID: 60167

Reply By: ross - Tuesday, May 25, 2004 at 22:10

Tuesday, May 25, 2004 at 22:10
Wet feet,I agree with the others ,"the rubber overdrive" is the most effective.
Keeps your mechanicals nice and stock for reliability and is the most economical.
My 73 series with .845 5th gear 4.11 diff and 33 in BFG at 110klmh sits on about 2300-2400 rpm.
It throws the speedo out about 6%
It made a huge reduction to cabin noise
I still take off in 2nd gear unless its a really steep hill.
Ive never found 1st gear low range too high with the 33 in tyres and 3F motor,so your turbo diesel will lap it up.
Some of those overdrives are noisy and get hot.
AnswerID: 60169

Follow Up By: wet feet - Tuesday, Jun 01, 2004 at 12:21

Tuesday, Jun 01, 2004 at 12:21
Hi guys,
Thanks a lot to all those that responded. In particular, Dozer, Floyd, Bob Y Ross and Bob. This is the first (well second now)time on explore OZ and I really appreciated the feedback. I have the update model which has the 265/75 x 16 BFG A/T currently and for some funny reason the final drive on the 1HDT is 0.88 cf to 0.81 for say the 1HZ according to my manual (Max Ellery pub'). The "low" final drive makes 200rpm difference at 100kmh.
The reason behind looking for a o/d unit, is in looking for the vehicle, numerous others were looked at and they all seemed to suffer from copious amounts of blowby which I put down to too high revs typically at touring speeds, especially for the turbo diesel. I really appreciated the comments re BHP and torque cf to revs and I will probably go the rubber option as i too were concerned about reliability issues of the o/d unit on the proven toyota product (which incidentally is only supposed to be used at speeds of 80kmh and above in 4 or 5th.
I spoke to my diesel mechanic about high revving diesels and even though the 1hdt is an overhead cam he still thought that 2800rpm on the Hume was too high for the 4.2 litre. Maybe 3litre turbo diesels need more revs, but all the auto's sit on about 2100 - 2300 with a torque convertor though.

Toyota's unofficial comment is the high revs for 5th is that it is made as a 4wd for low ratio and must be useable there????

Anyway thanks a lot for all your comments.

wet feet
FollowupID: 322736

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