RRC V8 vs Disco V8 fuel economy?

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MikeV8SE

MikeV8SE

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OK, this is not the most accurate comparison in the world, but bear with me!

My RRC 3.9 V8 auto, showing 90k on the clock, seems to be signifcantly less economical than my mates Disco 1 3.9 V8 auto (153k on the clock).

As a case in point, the other weekend 3 of us went greenlaning in my RRC, drove around 90-100 miles there and back, plus the greenlaning in between - used about 3/4 of a tank, or £60 in monetary terms.

This weekend 2 of us went to a pay & play day in his Disco, again around 90 miles there and back plus the offroading itself, and used just 1/2 a tank, around £40!

So, given the cars weigh similar amounts, cruised at around 60-65mph there & back and are both about as aerodynamic as my house, what's could cause mine to use 50% more fuel?? Oh, and tyre pressures on both were checked before we went to each day and were fine.

Since I've had the car I've changed the oil (Mobil 1 10W40), oil filter (genuine LR), air filter (K&N filter element) and coolant change + flush.

Conversely, since owning his Disco he changed the oil (Halfords 10W40) and oil filter (genuine LR) and tha's it.

So, what else could cause such a difference? The engine runs really sweet, but what else could I check/try? New spark plugs? Leads? Thermostat (it is a bit lazy, but surely once up to temperature shouldn't make a difference)??

Cheers! :)
 
juiceyv8 said:
type of tyres thermostat.
Click to expand...

Well mine is on narrower 205/80/16's, his is on 235/75/16, so the wider tyres should be even worse!

I think the thermostat may be an issue, so may well change that - but as I said, once up to temp would a dodgy thermostat really make a difference?

FWIW, once up to temp it stays bang in the middle too, doesn't run cold or hot, just sits perfectly at the halfway point!
 
you could try installing a kenlowe fan if you dont already have one! it is supposed to give 8 miles to the gallon more, suposedly,but look at them in book, on the internet,i think they are around about 150-200 pounds :) hope this helps
 
MikeV8SE said:
Well mine is on narrower 205/80/16's, his is on 235/75/16, so the wider tyres should be even worse!

I think the thermostat may be an issue, so may well change that - but as I said, once up to temp would a dodgy thermostat really make a difference?

FWIW, once up to temp it stays bang in the middle too, doesn't run cold or hot, just sits perfectly at the halfway point!
Click to expand...

Are the tyres exactly the same type n age?
Thermostat doesnt have to be dodgy. Different ones opening at slightly different temps are available.
A hotter engine is slightly more economical. A cooler engine is slightly more powerful (in theory)

Then there's the auto box fluid, oil cooler, main radiator, wheel bearings other running gear and brakes.....all can affect mpg
 
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joe1972 said:
Are the tyres exactly the same type n age?
Thermostat doesnt have to be dodgy. Different ones opening at slightly different temps are available.
A hotter engine is slightly more economical. A cooler engine is slightly more powerful (in theory)

Then there's the auto box fluid, oil cooler, main radiator, wheel bearings other running gear and brakes.....all can affect mpg
Click to expand...

No idea what 'stat is in either car, but all the replacements for mine suggest an 82 degree model - sound about right? Cos I've seen 88 degree models as well!

jamesmartin said:
discos are often low compression
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Really? BHP and torque are both quoted the same - why would they lower the compression? And why would this give better fuel economy?
 
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How would I know? The only visible difference is his has the square airbox and mine has the cylindrical one - mine is a 1991 RRC, his is a 1994 Disco 1. Oh, and he has an LPG kit, but that is currently disconnected.
 
No idea what 'stat is in either car, but all the replacements for mine suggest an 82 degree model - sound about right? Cos I've seen 88 degree models as well!

The 82 degree is a summer stat and the 88 a winter stat. Stats were changed winter/summer in the old days, until the emission control regs caused engines to be run as hot as possible all year round to reduce emissions.
 
wammers said:
The 82 degree is a summer stat and the 88 a winter stat. Stats were changed winter/summer in the old days, until the emission control regs caused engines to be run as hot as possible all year round to reduce emissions.
Click to expand...

Aah ok, good to know! So do I want an 82' or 88' stat to improve fuel economy? It's doesnt have any problems overheating - if anthing, it takes quite a while to heat up, but I guess that means the stat is lazy and needs changing :)
 
MikeV8SE said:
Aah ok, good to know! So do I want an 82' or 88' stat to improve fuel economy? It's doesnt have any problems overheating - if anthing, it takes quite a while to heat up, but I guess that means the stat is lazy and needs changing :)
Click to expand...

The hotter the engine runs within reason and the cooler the air going in the more efficient the burn. Hence hot engine lower emissions. Think the adaption of what is an old design, to run at the present constant 89 degrees summer and winter that seems popular these days, maybe contributing to the cracked block syndrom on the overbored blocks. Just a theory though so don't tell anyone. Or the cleaver brigade will be all over me like a rash.:D:D:D
 
Just a thought as I don't know much about petrols but it might be worth seeing what the difference is on the emissions between the 2 motors. Could you be running a bit rich and wasting fuel ?
Davie
 
wammers - I see! You and your crazy theories!! ;) So would an 88' stat allow the engine to run hotter than at 82', or does it work inversely for some bizzare reason??

Davie - worth a look but probably a bit impractical! If mine was running rich, what could I do to change it anyway? It is fuel injected, after all... :(
 
MikeV8SE said:
wammers - I see! You and your crazy theories!! ;) So would an 88' stat allow the engine to run hotter than at 82', or does it work inversely for some bizzare reason??

Davie - worth a look but probably a bit impractical! If mine was running rich, what could I do to change it anyway? It is fuel injected, after all... :(
Click to expand...

88 degree stat controls the engine temp to 88 degrees or thereabouts. 82 controls it at 82 degrees or thereabouts. Simple as that. Maybe if you fitted a 88 degree stat your fuel consumpion would maybe get a little better.
 
You're assuming that you're comparing like with like and, as far as the engines and aerodynamics you are, but how about gearboxes and rear axles. I know it wouldn't account for the amount of difference that your quick fuel cost check seems to show but it might be a start. Try comparing road speeds to engine speeds on the two cars.
 
I was only thinking about the condition of the injectors, can the fuel/air mix or fuel pump timing be adjusted, as i said I know very little about petrols and probably should have stayed out of this. Also since petrol evaporates quickly, would it be possible to have a leak on the system when pressurised that is not noticable when shut down.
Davie
 
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