3.5 V8 MPG

landowner said:

I've had V8 3.5 110, two V8 3.9 classics and a V8 Disco 1 , I've never had more than sixteen out of any of them and usually 12 to fifteen on petrol and no more than 14 on gas
Present 4litre disco two is the same, 14 on gas

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I can understand the 110's thirst, but surprised at the 3.9. Back in 2001 I took mine to Shropshire from Essex on holiday & averaged (Full tank - full tank) 20+ mpg. While I was away I chatted to the owner of a V8 Disco who had completed a long journey to the same area & he'd got 19mpg. Both cars were autos & we had used motorways to a large extent. My current Classic 3.5efi auto (also full tank - full tank) returned just under 23mpg during a 600m round trip to Wales a few years ago.
I won't entertain LPG these days so have no comparative figures.

I did very little motorway probably 10 per cent, I do drive economically and cant understand how people get such good fuel returns, maybe it's sitting in traffic and rarely in top gear that drinks the fuel.
My Disco 2 V8 is best on the motorway at seventy plus, get more miles to a tank than when I'm just local

Lower mpg on gas slightly but its cheaper so I get 33mpg equivalent from the Disco

landowner said:

I did very little motorway probably 10 per cent, I do drive economically and cant understand how people get such good fuel returns, maybe it's sitting in traffic and rarely in top gear that drinks the fuel.
My Disco 2 V8 is best on the motorway at seventy plus, get more miles to a tank than when I'm just local

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I think you are right. On the aforementioned Welsh trip I reckon around 80% would have been either Motorway or duel carriage, on which I would have done 70 where road conditions permitted. Very little of the journey was traffic work. The vehicle is factory spec.(no oversize tyres, winches or bull-bars) & was lightly loaded for that trip.

gas them v8s up.

gaza gas said:

gas them v8s up.

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Not in my experience

My 110 with low compression V8 on a longish euro trip (2700 miles) managed a best of 21mpg and an average of 16.2mpg.

The trip included fast and slow driving, off roading, plenty of alpine passses, and quite a few tunnels..

Petrol only.

gaza gas said: ↑
gas them v8s up.

norseman said:

Not in my experience

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I'm interested in your reason(s). Did you have problems with lpg?
I had a Iwema induction system with a Blos carb mixer on a 3.5 90, a 3.9 110 and a RRC 3.9 (transferred between them), and found it very good, once set up correctly, that included an advance/retard box for differential timing. After factoring in the initial costs, it payed for itself over a couple of years (I was doing many more miles incl. motorways than today) and did make running a V8 more affordable. But, the availability of lpg seems to be declining and I was always a little wary of overheating and lack of cylinder lubrication on lpg. I haven't added lpg to my current 3.5 auto 90, my mileage is small so I can't justify the cost.

Edit to add, my 3.5 hotwire efi auto 90 gets about 13-16 around town with a little fast A-road driving, on M-way at 60-65 it goes to about 18-20; I tend to drive gently most of the time, although I do let it rip occasionally.

NewFarm said:

My 110 with low compression V8 on a longish euro trip (2700 miles) managed a best of 21mpg and an average of 16.2mpg. The trip included fast and slow driving, off roading, plenty of alpine passses, and quite a few tunnels..Petrol only.

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I think if I had a 110 I'd be more than happy with that, especially from a low CR engine, your driving style is to be commended.

capo said:

gaza gas said: ↑
gas them v8s up.
I'm interested in your reason(s). Did you have problems with lpg? .

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I'll keep it brief. My 3.9 slipped a liner after around 12k after having LPG professionally fitted, as did a friends. Extensive enquires (this was in 2002) revealed that the higher combustion chamber temperatures produced when running on gas could be a contributory factor. It cost me a new short engine plus labour, my friend opted for a used block then sold the car. Both RR's were correctly maintained with efficient cooling systems.
Consequently my last (3) RRC's have all been 3.5efi's, no 94mm bore RV8's for me anymore & no LPG. The day I can't afford to run the Classic on petrol will see it on the market.

Yeah I don't plan on going on lpg, I know it can be great but I've just hear one too many stories about general reliability issues as well as the long term effects on the engine.
I would say I am good at driving decently economical but probably could do more, especially drafting. I currently get around 15 or 16 on longer runs, dual carriageways etc so not perfect environment but decent, would like to squeeze a little more out, 18 ish is my aim.
The car will be a little heavier but not too much as I'm keeping it all pretty simple.

NewFarm said:

My 110 with low compression V8 on a longish euro trip (2700 miles) managed a best of 21mpg and an average of 16.2mpg.

The trip included fast and slow driving, off roading, plenty of alpine passses, and quite a few tunnels..

Petrol only.

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That sounds like a familiar plan haha, same engine, location and vehicle as I'm planning, distance isn't far off too. What sort of things did you do as prep for the engine? Or didn't bother. Also was that running on twin sus or efi. If carbs did you go off and get them tuned?

On a similar ish note, if I was to go efi I assume hotwire is definitely the way to go, found a really nice flapper setup for £100 locally or a not as nice but okay 14cux hotwire setup off a rrc for £250 (everything I need apart from gaskets for both). Worth going for the hotwire I'm guessing? That sort of money is the same I was going to spend on getting the carbs tuned, might aswell go efi.

Burtie2904 said:

On a similar ish note, if I was to go efi I assume hotwire is definitely the way to go, found a really nice flapper setup for £100 locally or a not as nice but okay 14cux hotwire setup off a rrc for £250 (everything I need apart from gaskets for both). Worth going for the hotwire I'm guessing? That sort of money is the same I was going to spend on getting the carbs tuned, might aswell go efi.

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My engine was an original 84 factory 3.5 V8. It had stromberg carbs in need of a tune, so I elected.to fit an EFI (14CUX) Hotwire Injection system.

I replaced the camshaft and followers, and checked set preload, new and timing chain etc.

I didn't think an expensive cam would be worth it with my low CR engine, so fitted an OEM 3.9 one.

Converting to EFI is a job of work. I did it in about 4 weeks.. I used a hotwire setup from a 3.9.disco.

There is plenty of information available on the 14cux system. Rover guage is a helpful tool.

If I were you I would fit a fresh cam, and make sure your ignition components are giving a good strong spark. Mine was poor, replacing.the distributor with a good second hand unit and new vacuum advance made a good improvement.

I wish I'd done the ignition refurb before changing to EFI - so that I could have seen the differences with each step.

I use my V8 purely for fun, so don't worry too much about economy. I was more interested in reliability and torque. It now starts very easily cruises at 80. (Euro motorways /disco transfer gearing).

I would like a bit more torque, so may rebuild a 3.9 or a 4.6, but for now, it's staying as is.

Improved torque would be my goal, but after putting it off for ten years I've come to the conclusion that factory spec. is probably enough for a pensioner who only does 2k per annum

A good Holley/edelbrock carb and manifold would sort a built 3.5 out.

No flapper injection thank you very much!!

Simpler to maintain and tune!!

Henry_b said:

A good Holley/edelbrock carb and manifold would sort a built 3.5 out.
No flapper injection thank you very much!!

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Each to his own I guess, I've had three such equipped RR's & only had a problem once, sorted with a used AFM for £30 on fleabay.