Upgrade to 4.6

[TRobbo]

I'm looking at upping the go in my 94 disco (auto) from 3.9 to probably 4.6. Are there different 4.6 motors eg high/low compression? Any probs with transferring the heads or should I go a new long motor?
Cheers
TRobbo

[Philip A]

I know the conventional wisdom is to fit a bigger engine in the quest for more power but unless your engine is dead, i think you can get big increases out of your current block and not have to spend bulk dollars.
I believe that you can get more usable power by unichipping the car and fitting a Thor manifold at a much lesser cost.
A 4.6 is only 15% bigger than a 3.9 (actually 3950 CC), so you get 15% more torque, and probably not the HP as you still have the smaller 3.9 throttle body and MAF.
I have 50% more power at low revs than a standard 3.9, with unichip, modified heads, and "Thor " manifold. , in my 92 Range Rover 3.9 auto.

Until I fitted the"Thor" my gripe was that I didn't gain all that much low down torque ( well about 20% but I still couldn't hold cruise control at 115Kmh south up the hill from Moonie Moonie Bridge on the Newcastle expressway, which is a steep hill.) However I do not think a stock 4.6 would have as much as my car now.

Of course if you fit a 4.6 you either have to run an aftermarket ECU or get a unichip anyway, as well as an adjustable pressure regulator, as the 3.9 ECU will not run a 4.6 at full throttle as the injector duration is too short, and the fuel curve will be way out of tune. You either have to go to a Megasquirt, Haltech,or similar or fit a unichip to enable you to adjust the fuelling, and the ignition timing.
I could have done all mine for maybe $2500 if I had not bought all new sensors and injectors when I fitted the manifold.
Regards Philip A

[TRobbo]

Since posting my Q I have stumbled across the following link which people might be interested in if considering an upgrade. http://www.rpiv8.com/faq-conv.htm

BUT PHIL - THANK YOU
your info is fantastic and sounds like a better solution for less $$$. It would be nice if the local RR mechanic could be as knowledgable and free with information as you are.

[bmsa]

G'day TRobbo

I thought I would add my two bob, I went for warm 4.6 build up using parts from the 3.9. No dyno yet because I was planning on adding Haltec before to long but I can’t image its only got 15% torque over the 3.9 off the top of my head I thought a standard 3.9 is something like 310Nm and a standard 4.6 around 410Nm. Its a totally different vehicle to drive now absolute magic, unfortunately the ZF wasn't quite so pleased with the new engine she was smoother then with the 3.9 until she ate the clutch. On the up side she will be better prepared for Haltec now.

But in my case the 3.9 wasn't rebuild able so I figured given the costs I might as well have a new 4.6 oppose to a 4.0 and a much stronger engine then the 3.9 which hopefully means more reliable once I have the teething problems sorted.

[Philip A]

Just looking at 95 US spec 4.0L same capacity as 3.9 and 96 US spec 4.6 the difference is 236 Ft Lb to 280 Ft Lb which is 18%. This is 95 vs 96 model.

So not far off 15% when same injection used, ie apples with apples. I understand the 4.6 has a different cam to a 4.0 so this could provide a little difference.
If you are upgrading its unlikely you will even get the full amount as you will usually be using the old smaller throttle body, maybe MAF if using 14CUX.
Regards Philip A

[bmsa]

G'day Philip

Like you said the cam may have a bit to do with it, the boys at JRT seemed pretty proud of Dave's choice of cam in her. Shes a little bubbly at idle compared to the old 3.9 but she pulls hard all the way. The hole combination of 4.6, choice of cam, and head work along with the extracters and 2.5 exhaust brought over from the 3.9 make it feel and sound the buisness.

[Philip A]

Sounds great and I am jealous. But my 3.9 only has 112,000KM so will probably last oh maybe 10 years at least.!!!!

Of course a 4.6 is more powerful than a 3.9 . All I was trying to convey is that you can get a lot more out of a 3.9 without the expense of a 4.6 if your 3.9 is in good condition, and that the place to start is with the injection setup, then heads. The Thor manifolds are not for everyone because of the fabrication necessary, but for an automatic they further transform the car.

But for torque, all things being equal there is no substitute for cubic inches. Now I would like a full 5.2 litres , but how to justify ?????

Regards Philip A

[bmsa]

Yeh, I was hoping to power up my 3.9 but at 105000 K's she made the decision for me . The full story was back in April I think the subject was "3.9 on its way to Heaven". I thought about the 5.2 or GENIII but given the hassle and the fact I'm not really a lead foot I held myself back. Can always bolt a supercharger to her still but I was happy getting away from that although it seems to work when done properly.

[red90]

A light cam, timing chain, extractors and a chip will do a lot for a 3.9l. Most people have worn out cams and don't realise what they are missing.

A friend that I wheel with has a carbed 3.5l that was rebuilt by RPI with the basics.. It will spin all 4 285/75/16s on the bitumen without any effort and will go much faster than is safe.