Thread: 3.9 v8 fuel economy

Ah, old Chinese proverb say, "V8 go past everything except petrol station", face it, no matter what you do the fuel consumption will never be "good" because the old rule of physics applies "fuel in equals power out" and you are dealing with a pretty inefficient piece of machinery. Whatever you spend on special fuel injection, exhaust mods, electric fans etc,etc, the cost of all these bits will never over a million klm's be returned in increased efficiency, (less fuel use) but there are plenty who sell the bits who would be happy if you believed that all the "strap ons" would do the trick. If it's economy you want buy Suzuki Alto.
If it's fun you want, drive a V8, and try to forget how much it's costing you!!

Depends, how many kilometres do you get from a tankful?

Whats That

3.9 V8 and fuel economy in the same sentence for a disco WOW.

I average on petrol 17L/100 k and on gas 20L /100K. That works out at approximate 310 kilometers on gas.

If any one can figure out to get better economy out of a v8 disco they will be a rich person.

Originally Posted by Neesaan1via

Ok so as the title says I want to no about people fuel economy on these engines and also what you can do to improve it ?

I've done some searching and what not bit most people just say v8 chew fuel bla bla bla . But I figure the more people with these engine know how to improve fuel economy the better .

I've heard things like removing the mechanical fan from the motor and changing to electric so the motor doesn't have to work as hard ?

Something to do with fuel cap sealing ?

Higher octane fuel

Thought this would be a good topic .

Economy largely depends on how heavy your right foot is, I have never seen anything that will give a dramatic increase in economy. Like others said, economy and old rover V8s just don't go together.

Cap seal, nope, tank has to vent, even if you silicone the fuel cap, the tank still needs to vent

Some say higher octane fuel gives better mpg, but remember you are paying up to ten cents a litre more so I don't think this is a viable option, comes back to how you drive it.

Just sit back and listen to that burble out the back and enjoy every mile

Handy hints to make your disco cost less to run:

1. Advance the timing to 10-12 degrees advance
2. Enable the ECU to run O2 sensors.
3. No raised suspension, no roofracks no roo bars no spotlights.
4. Build a high compression 4.6 motor with a decent torque cam
5. Rebuild the trans to get lockup in 3rd
6. Read the weather forecast, plan trips downwind.
7. Fit a 300tdi engine insead.
8. Park it and get a Corolla as your daily.
9. Fit LPG
10. Don't exceed 80km/h, ever. No hills either.

Some of these methods may cost more than others...

In my dim dark past, I remember coming across a graph that showed the air drag curve taking off (exponentially) at around 50 mph. - 80 km/h...
- Which also makes a nonsense of ultra-streamlining on commuter cars, cars which spend nearly all their life under 80 km/h.

Do O2 sensors make a big differance on a 3.9 ? Enough to justify the $$$? - Easy to fit ?

Having eight cylinders compared to 4 or 6 for the same displacement also makes for a less fuel-efficiant engine, usually. More rotating mass means slower RPM acceleration.


James in Gosnells

'95 Classic 3.9 with EAS, ABS and ABP... and roof-bars

How much better are the d2v8's ??

Originally Posted by superquag

In my dim dark past, I remember coming across a graph that showed the air drag curve taking off (exponentially) at around 50 mph. - 80 km/h...
- Which also makes a nonsense of ultra-streamlining on commuter cars, cars which spend nearly all their life under 80 km/h.

Do O2 sensors make a big differance on a 3.9 ? Enough to justify the $$$? - Easy to fit ?

Having eight cylinders compared to 4 or 6 for the same displacement also makes for a less fuel-efficiant engine, usually. More rotating mass means slower RPM acceleration.


James in Gosnells

'95 Classic 3.9 with EAS, ABS and ABP... and roof-bars

Factoid: The air drag curve is exponential at all speeds, just ask any cyclist. Just that at 80km/h in a vehicle it becomes significantly greater than other mechanical drag.

I don't have data on petrol usage with O2 sensors but apparently it's relatively easy to do, search ye forum and ye should find. On LPG (my field) O2 sensors can increase economy by up to 20% without loss of driveability.

It seems counter intuitive to suggest a bigger motor can lower fuel consumption but some V8's are better than some 6's. If you gear a large capacity motor to run slower (taller overdrive) you can load it up more and decrease its consumption on cruise. 4 Cylinder engines may have bigger rotating mass because of the need for a heavy flywheel to smooth out vibration, think Landy 4 pot engine has a heavier flywheel than a Holden 6 in the same vehicle.

Trivial data point: In the 80's my P76 V8 consumed similar amounts of fuel as a 2 litre 4 cylinder Sigma did, and the Sigma was sold with "4 cylinder economy". Engine economy is more than counting pots.