naci Posted April 19, 2009 Share Posted April 19, 2009 dear all As I mentioned before I recenty overhauled 1991 defender and present setup is as follows: 3,9 V8 petrol engine fitted from a discovery series 1 - 1998 model with cux efi LT77 manual gearbox, left the original gearbox on but modified it to fit the V8 transfer case gears now 1,22 - it was originally 1,67 I am using 235/85/16 tires it has 2" lift with bearmach blue springs +++ I am not entirely happy with the gearbox, it makes funny noises when changing the gears and gearchange is not smooth at all... I am therfore planning to change the gearbox to auto, my zf4hp22 options are either from 3,9 discovery S1 or from RR Clasic I am also planning to change the tire size to 35" - 315/75/16 or equivalent, needless to say will have to put heavy duty axels and shafts etc... opinions please... should I go for auto box option or R380 manual box... I dont mind using either manual or auto, do not care about the economy ( if any) of manual the box compared to the auto box all I am concerned which one will be better with bigger size tires... thanks in advance... naci Quote Link to comment Share on other sites More sharing options...
bishbosh Posted April 19, 2009 Share Posted April 19, 2009 I presume that the truck will get plenty of off rod use given the upgrade to 35" tyres? If so, then I would go auto, but be aware that the box is 6" longer than a manual so you will need to move the engine forwards to compensate or have a ridiculously short rear prop (assuming it is a 90 not a 110) Quote Link to comment Share on other sites More sharing options...
Dave W Posted April 19, 2009 Share Posted April 19, 2009 The auto will be a lot easier on your transmission. Just use a decent oil cooler and run a 1.6 transfer box on the back of it if you want to use 35 inch tyres (or even better, use 4.11 diffs with a 1.2 transfer box). 1.6 transfer box is the cheapest way though, any higher than that and you'll be constantly changing gear when you're trying to cruise. Quote Link to comment Share on other sites More sharing options...
naci Posted April 20, 2009 Author Share Posted April 20, 2009 thanks for the replies... sorry for missing info, it is a 110 and its purpose of built is purely offroading... both answers supported my decision to convert to auto... I will also check the option of changing diff gears as I already have detroit no spin in rear and detroit TT in front diff... can you also give little bit mor info regarding oil cooling issue please? Quote Link to comment Share on other sites More sharing options...
Dave W Posted April 20, 2009 Share Posted April 20, 2009 Auto boxes will last virtually forever as long as the oil is kept cool. When you are using them off road or in a "spirited" way on road the oil temperature can rise significantly unless they have a suitable oil cooler. The standard "hedgehog" oil cooler as used on classic Rangies and Discoveries is adequate but IMV it's worth using an aluminium oil cooler instead to keep it as cool as possible. Think auto can supply the coolers, connectors and pipes... http://www.thinkauto.com/ Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted April 20, 2009 Share Posted April 20, 2009 Cheapest way for decent autobox oil cooler is to get 2 RRC items and run the cooling lines through both, they also have good flow, which a lot of aftermarket ones don't give... Quote Link to comment Share on other sites More sharing options...
naci Posted April 24, 2009 Author Share Posted April 24, 2009 hello again thanks once more for the replies, you have been a great help. yesterday I got my new tires of silverstone 35/11,15/15 and my rims ( 8" wide) will arrive tomarrow, so saturday will be a great day... My local mechanic does not recommend me to use auto box + 1,67 tc gears with those rims/tires as he thinks I would not be happy with the cars' acceleration in 1st and 2nd gears... I dont know if you use such expression but he says the car would be "deaf" meaning very "unresponsive" or slow in accelerating... changing differatial gears may be a solution but I can not comment as I have no previous experience... may I have some more comments on this issue please??? my second question is about converting 3,9 efi to 4,6 or to modify it to have more low rev torque... my mechanic says we can use the crank shaft and piston rods of the 4,6 in the 3,9 block and this may help to increase its low range torque... I may also intall rp4 or similar camshaft and some other bits and pieces to achieve my goal... what would you recommend ??? thanks in advance naci Quote Link to comment Share on other sites More sharing options...
Pete Attryde Posted April 24, 2009 Share Posted April 24, 2009 If you use either the 1.4:1 or 1.2:1 transfer box instead of the 1.6:1 box you will accelerate even slower . Changing the diff ratio is the better way to go as it will lower the gearing in both high and low range making for better control offroad. If it was me, and I had the cash, I would go with Dave W's suggestion of the 1.2:1 transfer box and 4.11:1 diff ratios which should give you a reasonable balance of gearing for on and offroad. Pete. Quote Link to comment Share on other sites More sharing options...
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