What engine to install, 4.6 vs. 3.9, to my 1986 RRC

Budaah · Apr 13, 2020

Good day to all,
I am a newbie here and have very limited knowledge about Range Rovers. I have recently purchased a 1986 4 door Range Rover. The engine is in a poor condition and the gearbox is manual. I would like to upgrade the engine to a stronger engine with an automatic gearbox.

My mechanic has suggested that he could install a 3.9 V8 or a 4.6 V8 petrol engine for this vehicle. My mechanic is trustworthy and he does not sell either engines. I would have to source the engine from an independent dealer. I have been given the following quotations:

1. A low mileage (42 thousands kilometer used) 3.9 engine plus automatic gearbox, plus the ECU for the sum of approximately 1400 pounds.

2. A low mileage fully rebuilt P38 4.6 thor engine plus automatic gearbox, plus a remapped ECU for better mileage and acceleration for the sum of approximately 2200 pounds.

What I would like to find out is to understand in your experience:
A. Which engine is more reliable in terms of maintenance;
B. Which engine gives me better mileage;
C. And which engine provides me with better acceleration;
D. Are the quoted prices for the said engines are fair for the described specifications?

Given my limited knowledge about engines and how to maintain them, I would have to rely on my mechanic for the upkeep and maintenance.

I will look forward to your kind responses.

mikieman · Apr 13, 2020

I have a 92 it has the 3.9 ....
A good strong engine
As far as gas mileage.... don’t get stuck on that.,. They don’t have great gas mileage...,
The thing to focus on is a good engine
And you will have a Range Rover Classic
That is enough....also learn as much as you can about them so as to not need a mechanic.... but once it is running good
They are low maintenance....
Good luck with you Rover...

wammers · Apr 13, 2020

3.9 is a safer bet than the liner dropping 4.6. Although technically they both use the same block. But rumour has it 4.6 blocks had bad batches at some stage.

Datatek · Apr 13, 2020

A VM would be more reliable and give you better gas mileage

oakey · Apr 13, 2020

Welcome to site.
With a Rangie, mileage is irrelevant, they are all pretty poor.
I would go for the 3.9 as it is my belief that the 4.6 is over-bored altho I have one lol.
The 4.6 does have more power tho.
The Thor engine is better than the older GEMS engine or so I am led to believe.
Price wise I would guess they are both reasonable if in condition stated.

DanClarke · Apr 13, 2020

The 4.6 as folks have said above is more powerful, but at the cost of fragility. It isn't the most robust lump.
The original 3.5 V8 engine was pretty bulletproof but woefully underpowered, the 3.9 seems about right.
Forget economy and MPG, Rangie ownership means accepting expensive motoring in style & comfort.

200tdiforever · Apr 13, 2020

Agree the 3.9 is the way to go, ive had plenty of 3.5 & 3.9s over the last 30 years and no engine problems with either.

brianp38dse · Apr 13, 2020

From what i can see the 4.6 thor would need a lot more electrical messing about.

Western Slope Rover · Apr 13, 2020

Marshall8hp · Apr 13, 2020

Despite what the experts might tell you, the 3.9l, 4.0l and 4.6l all have exactly the same bore and liners fitted. The 3.9 and 4.0 are essentially the same engine. The 4.6 has a longer stroke.

Henry_b · Apr 13, 2020

The 3.9 and 4.0 are the same unit both 3960cc.

The 4.6 is a 3.9/4.0 with a longer stroke.

Liner issues effect the 3.9 and 4.6 if you cook it neither are better or worse.

The reason they crack is the coolant galleries are offset in the block thanks to the castings being on the píss, look down the valley for an example
some tappet bores have machine marks above others don't etc etc..

They can be reliable units if the cooling system is looked after, on another website an ex GMP P38 4.0 had 287,000 on it before going to Turners for a rebuild, so they can rack up mileage if maintained.

The 4.6 would need the dizzy drive timing chest, which is available from a few places, Turners and RPI do them IIRC..

Personally i'd rather the 4.6 much greater low end grunt..

norseman · Apr 13, 2020

Marshall8hp said:
Despite what the experts might tell you, the 3.9l, 4.0l and 4.6l all have exactly the same bore and liners fitted. The 3.9 and 4.0 are essentially the same engine. The 4.6 has a longer stroke.

Plus one on that

A friend & myself both had 3.9 autos that fell victim to the dreaded liner slip.
Historically 4.6's suffered the most from this fault, though they were the most powerful of the factory produced RV8's.
The most reliable unit is the original 3.5, the lowest performer but can be tuned or even stroked to greatly increase torque without altering the bore size.
Decisions - decisions

nb. If I were a younger/wealthier man I'd go for the last option.

Henry_b · Apr 13, 2020

4.6 and 3.9 are the exact same unit, different stroke.

i didn't think of stroking the 3.5..

4.3 is possible IIRC

Marshall8hp · Apr 14, 2020

Leyland produced a version of the RV8 in Australia for one of its cars. It was 4.4 litre being a square engine. The 3.5 litre is 3.5" bore x 2.8" stroke. The Leyland engine was 3.5" bore x 3.5" stroke. They were a very good unit. Leyland also produced a heavy duty version for their Terrier trucks. It is a much sort after engine in this part of the world.

norseman · Apr 14, 2020

Henry_b said:
4.6 and 3.9 are the exact same unit, different stroke. i didn't think of stroking the 3.5.. 4.3 is possible IIRC

Just as the 4.2 in the LSE is a stroked version of the 3.9. The cynic in me suspects that LR used the LSE as a test-bed for the forthcoming P38, same wheelbase, air suspension & a stroked engine (allegedly using left over cranks from the cancelled Perkins/LR Iceberg V8 diesel project) which LR then sold to buyers knowing that warranty claims would feed back data that could be analysed before rolling out the new model

nb. 'Real Steel' used to market a stroker kit for the 3.5, taking the capacity to 4.3 as you say.
Don't know if the product is still on the market.

romanrob · Apr 15, 2020

I have a 4L GEMS and a 4.6L GEMS. The 4L runs about 7 degrees cooler - which may help longevity - certainly I have never had to touch the 4L engine in its lifetime. The 4.6L got replaced by a PO 60k miles ago and has had at least two new HGs since. The 4L GEMS is in my Defender ie the control systems are not super-sophisticated and so should be do-able DIY.

Budaah · Apr 16, 2020

I would like to thank you all for your kind responses, sharing your experiences and opinions. A quick recap of your discussions lead me to believe:

1. I should not worry about the millage;
2. 3.9 is preferable to 4.6;
3. The quoted prices to me seem to be fair.

I did not notice any comments about the acceleration of 3.9 engine vs. 4.6. Is this because the difference is negligible, or as a Range Rover driver, it is the last thing on anybody's mind?

Budaah · Apr 16, 2020

One more thing with respect to the common concerns expressed with respect to 3.9 and 4.6 engine is that of thinner cylinder sleeves. The solution that you have discussed is to ensure sleeves have been tophatted to avoid the slipping sleeve syndrome and burned out motors. Although, 3.9 can also suffer from this issue but given their thicker walls, they are less susceptible. Have I understood this correctly?

Budaah · Apr 16, 2020

Henry_b said:
The 3.9 and 4.0 are the same unit both 3960cc.

The 4.6 is a 3.9/4.0 with a longer stroke.

Liner issues effect the 3.9 and 4.6 if you cook it neither are better or worse.

The reason they crack is the coolant galleries are offset in the block thanks to the castings being on the píss, look down the valley for an example
some tappet bores have machine marks above others don't etc etc..

They can be reliable units if the cooling system is looked after, on another website an ex GMP P38 4.0 had 287,000 on it before going to Turners for a rebuild, so they can rack up mileage if maintained.

The 4.6 would need the dizzy drive timing chest, which is available from a few places, Turners and RPI do them IIRC..

Personally i'd rather the 4.6 much greater low end grunt..

Thank you Henry_b for sharing your views especially with respect to "coolant galleries". Does anybody else have comments on this matter? What more can be done to look after the cooling system to avoid "crack" in the motors?

Mark Piercy · Apr 16, 2020

Budaah said:
Thank you Henry_b for sharing your views especially with respect to "coolant galleries". Does anybody else have comments on this matter? What more can be done to look after the cooling system to avoid "crack" in the motors?

Toyota V8 diesel...

What engine to install, 4.6 vs. 3.9, to my 1986 RRC

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