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3.9i 4speed autobox-how much bhp can it take?


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The Overfinch 570Ti Range Rover I had a few years ago (328bhp, 300+ torque) used to get through an uprated HP22 every 4 years or so. A standard one would last even less. If you need it for ordinary road use I would try and get hold of a GM400 4-speed auto box which can handle the power. These were fitted to a large range of GM vehicles in the States. I would assume that Ford of America built a similar box which may also fit. The other problem is the transfer box. Overfinch fitted there own transfer box (with the GM400 or Torqueflite) prior to the intro of the Borg-Warner transfer box.

The Torqueflite will accept the power, but only has 3 gears so the transfer box ratios need to be modified to suit.

If you are not using it on the road very much and don't need to cruise at high speed, I'd go for the Torqueflite box.

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Don't think a standard ZF is going to take more than around 250HP with any reliability although you can, with a lot of money, uprate them. When I last enquired it would have cost me around £1500 to upgrade a ZF.

As above, a TH400 or 4L80E is a good option as either can be bought comparatively cheaply rated at 1000HP although the dollar exchange rate is a killer at the moment. Matching it to a LR transfer box is doable although an Atlas or similar is an easy option for either of them.

The LT230 seems fine with 410 HP 400 lb/ft although the centre diff dies really quickly if you use it on the slippy stuff without the diff lock engaged.

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  • 2 weeks later...

Hi gents, reading this with interest as have an option of a longer stroke crank to take my 3.5L up to approx 4.5L - with no other mods, would this be too much BHP for the ZF 4 speed? (It's a 1988) The guy selling it has advised not to use with an auto box, and I guess he maybe right, but I thought I'd ask. Anyone know how much BHP a 4.5(ish)L Rover V8 would put out?

I'd of thought they'd be able to take 225 at least? As per previous post? That surprised me :huh:



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