3.5 carb to 4.6 conversion advice

Technical questions and answers
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Ant109
Posts: 5
Joined: Wed May 27, 2020 4:58 am
Location: Mid Canterbury, NZ

3.5 carb to 4.6 conversion advice

Post by Ant109 »

The opportunity has arisen for me to get my hands on a 4.6 with ecu from an old range rover and the idea of dropping it into the stage 1 is enticing.

However, I am aware of my own ignorance and therefore must ask for advice on the matter before I get too excited by extra displacement.

What is involved? Whos done it already? Ive got an ex nz army stage 1 and would like to keep the manual 4 sp. transmission.
map1275
Posts: 1012
Joined: Mon Jul 21, 2008 2:48 am
Location: Brisbane, Australia

Re: 3.5 carb to 4.6 conversion advice

Post by map1275 »

Displacement isn't the issue, the torque difference is. Using a vehicle in which the standard engine has to be fitted with inhibitors.
Plenum is probably too tall for the bonnet and all the wiring has to be extended to mount the ECU in the tool box. Plus making the exhaust fit and mounting Lambda sensors.
Glen
Posts: 34
Joined: Mon Jan 23, 2012 11:45 pm
Location: Sheffield, UK

Re: 3.5 carb to 4.6 conversion advice

Post by Glen »

First off what type of 4.6 is it? They came as GEMS with a black square plenum chamber (which will fit under the bonnet, it was used in the later V8 defenders like the 50th and the NAS) or the Thor with a curly banana manifold. GEMS uses Lucas injection, Thor is Bosch. I don’t think either are that easy to tune or fiddle with on a stock ecu and of course it needs every sensor wired up and working or it’ll be unhappy. Both systems also run the ignition via a coil pack so there’s no distributor on the front of the engine and no provision to drive one from the camshaft so you do need the ECU.

Generally when people fit injection on a carb V8 (or an injected engine in place of one) they use the older Lucas 14CUX (“hotwire”) system from a 3.9 RRC as those systems lacked knock sensors, often didn’t have lambda sensors (or if they did it didn’t do much unlike later vehicles) and they still used distributors so the ignition system is basically the same as a carb engine (especially if it’s been converted to breakerless ignition). Waterpump on a Hotwire is usually the same as a carb engine too so that helps with similarity’s/plumbing.

Doesn’t mean the later system can’t be done but it would be more involved and it’s worth remembering in all cases you’d be delving into ageing electronic systems that aren’t that reliable and are crude by modern standards and may be difficult to program if they need adjusting (14CUX for example does not self tune, even with lambda sensors so if you use non standard air intake/exhaust you run the risk of leaning it out and the larger the Rover V8 the less tolerant of poor tune they get - fixable with a tune if someone has a computer that can talk to it). Other options (expencive) are to throw the ecu in the bin and fit a modern aftermarket one like mega squirt, Link, Haltech or something so you can program it yourself with a modern laptop and use what sensors you want - probubly way OTT but it’s none the less an option and theoretically you could make a system way more advanced than LR ever did because even Thor is 25 year old tech these days.

Final option which seems to be an equally popular way off retrofitting a 4.0/4.6 into a series/90/110, at least in the UK, is to fit carbs. For that you’ll need an ‘interim’ front timing chain cover from a late 3.9/4.2 as that lets you run a distributor (will also need a long nose camshaft) whilst retaining the oil pump which is on the front of the crankshaft on the 4.0/4.6 and ‘interim’ engines (interim engines are basically a 3.5 or 3.9 dizzy engine machined from a 4.0/4.6 block casting as they retooled everything when the 4.0/4.6 came out - they were usually fitted to ‘300 tdi era disco 1/RRC). Much simpler setup and easy to tinker with but of course being carbs it’ll need constant tweaking and generally never run perfect so that’s the trade. Some people run them on LPG though so at that point the carb setup doesn’t really matter as it’s not really used.

Exhaust wise the stage 1 manifolds will fit but they strangle the 3.5 so it’s only gonna get worse. Don’t think P38 manifolds will fit but the cast ‘efi’ manifolds will but you’ll probubly need a custom system from there back. Adding lambda sensors would not be a significant issue at that point if needed.

The flywheel end should be simple. The 4.6 is probubly an automatic but the flywheel and housing off the 3.5 manual should still fit on the block/crank.

As to if it’s a good idea, there will be much more torque from the bigger displacement but the drivetrain should take it fine as the stage 1’s were built stronger than many later models, and peak torque from a 4.6 isn’t that different to a later diesel only a lot smoother. The restrictors were to reduce the original engines top end power, not low end torque, though they did lower that a bit as a side effect. I don’t think many stage 1’s are still fitted with their restrictors but if they were a 4.6 would more than double peak power so you would notice it and should probubly check if that’s considered legal where you live - in the UK this would not be an issue but in some countries it definitely would be. The extra power won’t increase your top speed without changes to the gearing but will let it get up to speed quicker esp if loaded/towing and may permit slightly higher cruising speeds so brake upgrades would not be a bad idea as well as making sure the steering and suspension are in perfect condition - by modern standards though a V8 land-rover will still be one of the slower cars on the road even with about 200 PS, will still be one of the best sounding though!
udo1-ton
Posts: 32
Joined: Fri Nov 28, 2014 3:59 pm

Re: 3.5 carb to 4.6 conversion advice

Post by udo1-ton »

In my opinion the 4.6 should be fitted with the original 3.5l periphery. There are only a few improvements to be done.
1. the oil sump has to be dented out 5mm
2.a distance ring for the crank pulley ( 4.6 crankshaft is 1“ longer)
3 the alternator bracket needs one hole to be drilled to 10mm
4. a new camshaft with dizzy gear ( kent180 or others) and lifters
The engine should have 200-220hp. I recommend to change the high range gear in the transfer box to 1.113. The engine revs at 65mph are 900 less as before with Stage1 gears.
The fuel consumption is round about 5l less per 65 miles.

The injection systems are far more complicated than a carb system and the look is a little bit too modern in my opinion.
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