Fuel 'Economy'

[ukkid35]

£54 at 112p/ltr for 220 miles round trip, all motorway or dual carriageway, works out at 21mpg - comments?

(4.5 lightly loaded, almost all 5th gear but not driven with economy in mind)

[waveydavey1000]

I get anythng from 18 to 24 mpg from my 4.2. I don't particularly drive with fuel economy in mind so your figures seem about right on a run?

[jamieduff1981]

I get 200 miles between fill-ups normally (I fill up when it indicates just below a quarter remaining usually) or 250 miles on a run. This was before my current running challenges in case you're tempted to invalidate mine!

Yours seems to be in the right ballpark OP

[ukkid35]

jamieduff1981 said:

This was before my current running challenges in case you're tempted to invalidate mine!

You need to fix that and get back on the horse.

[scotty_d]

Seems normal, I range from 15mpg-31mpg ( once on a steady run to the ferry to Portsmouth I managed 295miles to a tank before I bottled it)

But for mixed light driving 21mpg sounds normal, steady motorway see's me close to 30mpg most of the time. I use a fill up app to track all my cars now, intreasting to see how all my cars performe.

[aide]

I’ve been asked to add instant MPG estimates to RS-AJP.
I haven’t even looked at how to do it but, based on Injector Durations from the ECU and distance travelled from the GPS sensor, it might be as simple as keeping a running total of fuel injected over the past mile travelled?

[Stunned Monkey]

The trouble is that both flavours of TVR engines avoid the manifold vacuum connection to the fuel pressure regulator meaning that you can't directly infer fuel flow from injector cycle.

I don't know why they did that but there you are...

[Stunned Monkey]

The trouble is that both flavours of TVR engines avoid the manifold vacuum connection to the fuel pressure regulator meaning that you can't directly infer fuel flow from injector cycle.

I don't know why they did that but there you are...

[spitfire4v8]

Stunned Monkey said:

The trouble is that both flavours of TVR engines avoid the manifold vacuum connection to the fuel pressure regulator meaning that you can't directly infer fuel flow from injector cycle.

I don't know why they did that but there you are...

Yep the effective pressure differential across the injector increases on light throttle on the 4.2 so fuel flow increases over and above what you'd think from the injector duration alone.
the 4.5 is always spraying into atmospheric pressure because its injectors are above the butterlfy, so you could make a stab at approx mpg on that system maybe.?

[jackwibble]

After Jools remapped my 4.5 it would return 30mpg on a motorway run at 80mph or lower speeds but it doesn't do that very often!

[aide]

spitfire4v8 said:

Stunned Monkey said:

The trouble is that both flavours of TVR engines avoid the manifold vacuum connection to the fuel pressure regulator meaning that you can't directly infer fuel flow from injector cycle.

I don't know why they did that but there you are...

Yep the effective pressure differential across the injector increases on light throttle on the 4.2 so fuel flow increases over and above what you'd think from the injector duration alone.
the 4.5 is always spraying into atmospheric pressure because its injectors are above the butterlfy, so you could make a stab at approx mpg on that system maybe.?

Ahh I get it now, since the injectors on the 4.2 are below the butterfly's, on light throttle, the intake stroke effictevly sucks fuel over and beyond what would normally be injected for the corresponding throttle site because the pressure isn't regulated. That's interesting. Calculating instantaneous MPG in that case would mean that that extra fuel needs to be accounted for. When I reverse engineered the throttle positions in the ECU one thing I noticed was how granular the steps became as it tended from open to closed. Since the position of the butterfly's is available, and is at its most precise at those small butterfly openings, then it could be used to infer how much extra fuel is being consumed.
That would be a good starting point but since every engine setup is slightly different, in that the idle position of the butterfly's isn't always the same, then it would be difficult to line up the inferred additional fuel with the corresponding injector duration in order to calculate the total amount. For a 4.5 it's a little more straight forward to make an approximation, but I'm guessing the length of the induction pipes would have to be factored in too?
To be able to add a mpg feature that covers all variants correctly is probably too much work, especially since it's not as straight forward a as I initially thought. Thanks for the info, I'll stick that idea on the nice to have list!

[Jubag]

I'm glad this has been brought up as my car seems to be really uneconomical. Over Christmas I drove from Norfolk to Sussex and back and only returned 19mpg and that was mostly driving like a saint (or though maybe not the on the A3, it's that tunnel I just can't help myself...) so I was rather disappointed. Is there any way a 4.2 can achieve better fuel consumption? I know it's not the type of car that will ever be brilliant but I'm quite envious of the chaps here getting 30mpg. Just as an aside, the car was almost faultless apart from when leaving Liphook the car has started beeping the MIL for a split second and no sooner there it's gone off. Any ideas???

Edited by Jubag on Wednesday 14th January 22:00

[jamieduff1981]

When was its last proper 12k service Jubag?

[scotty_d]

Jubag said:

I'm glad this has been brought up as my car seems to be really uneconomical. Over Christmas I drove from Norfolk to Sussex and back and only returned 19mpg and that was mostly driving like a saint (or though maybe not the on the A3, it's that tunnel I just can't help myself...) so I was rather disappointed. Is there any way a 4.2 can achieve better fuel consumption? I know it's not the type of car that will ever be brilliant but I'm quite envious of the chaps here getting 30mpg. Just as an aside, the car was almost faultless apart from when leaving Liphook the car has started beeping the MIL for a split second and no sooner there it's gone off. Any ideas???

Edited by Jubag on Wednesday 14th January 22:00

Don't be too envious these returns are only seen when the car is in GT mode heading down south to Le Mans and so on, mixed driving see's mine around low 20's. One thing that improved my mpg a bit was getting my injectors cleaned, mine were very dirty car is 16 years old now.

[jamieduff1981]

Who cleaned your injectors Scott?

[scotty_d]

I used this crowd Jamie, the ones based in Paisley, was quite expensive at just shy of £250 but done correctly and filters replaced as needed as well as o rings, I know penny items but it has probably paid for its self on fuel savings this year alone.
http://www.asnu.com/centers/scotland.htm

[jamieduff1981]

Thanks Scott. Did you just send the injectors off to them?

[scotty_d]

Yes my mate who owns a local garage sent them with a few of his customers ones as I was heading back offshore, I just collected when I was back.

[Jubag]

jamieduff1981 said:

When was its last proper 12k service Jubag?

That's the million dollar question. I don't know when it last had the 12k service. I purchased the car as some what of a gamble last year as a non runner. After fixing some niggles, it has, so far, been remarkable. I think with the flickering MIL I should think about getting the plugy-in-thing and seeing if there are any codes being stored. It might just need the throttle pots setting up.

[jamieduff1981]

Jubag said:

jamieduff1981 said:

When was its last proper 12k service Jubag?

That's the million dollar question. I don't know when it last had the 12k service. I purchased the car as some what of a gamble last year as a non runner. After fixing some niggles, it has, so far, been remarkable. I think with the flickering MIL I should think about getting the plugy-in-thing and seeing if there are any codes being stored. It might just need the throttle pots setting up.