another econometer question

Post by **Dusty** » 23 Jul 2006 08:28 am

Hi all!

I have a question that might sound silly, but so far, I have been unable to find an answer anywhere on the net, so maybe some of you guys can figure it out - I have an '86 360GLE 2.0 that has been imported to Croatia some 10 years ago. Among other things on the dashboard, there is an econometer which shows the fuel consumption, that I'm sure you're all familiar with. I find it a very convenient tool, especially now with the rising petrol prices. However, there are no numbers on the instrument, which would indicate exact consumption in litres, just a "white" and red field. I am aware that some other cars, like BMWs from the eighties have had the same instrument, but what they had were actually numbers in litres on the instrument.
When the engine is well warmed up, the needle sits at the third mark from the left-any idea how much is that, converted to litres? And what's the value of other marks as you approach and enter the red (meaning uneconomical) field?
I would have included a picture, but I'm not sure how to upload it

Post by **redline** » 23 Jul 2006 10:46 am

Hi Dusty , I dont think the volvo ones are that accurate

its basically a case of

when your in the white your doing alright

when your in the red your a petrol head

Post by **mac** » 23 Jul 2006 12:08 pm

Hi Dusty,

The gauge you refer to (above the fuel gauge) doesn't actually measure fuel consumption as such it's only a vacuum gauge measuring depression in the inlet manifold. It's a general indicator only.

Redline put it exactly right - the further into the red the gauge goes the more fuel you are tending to use. The reading is very much influenced by gear position, throttle opening, engine load etc. It is probably of more use on a carburettor car where throttle opening can influence fuel useage more than in precisely metered fuel injection. Unless you understand the exact relationship between manifold depression and fuel consumption you cannot use such a gauge as anything more than an indication of how economically you are driving.

Mac.

Post by **jtbo** » 23 Jul 2006 12:27 pm

Engine rpm affects quite lot to fuel consumption too.

Maximum consumption could be around 20-25l/100km that would come if you keep throttle on floor whole time.

For 5th gear you can make rough guess when you watch where needle sits as average when driving normally on road and checking how much car consumes fuel driving that way, but something near 8l/100km might not be too far away I think, even some have got extremely low consumption.

But indeed it is at others say just quite rough indicator.

Accelerating too slow is using more fuel than accelerating faster, for example when you join to highway and accelerate to 80kph you would do that so that 1st and 2nd gears you rev to near 4000rpm, keeping throttle quite well open, then rest with 3rd and skip 4th gear completely.
Also looking more forward to road and avoiding accelerating if you might have to stop soon saves really a lot of fuel.
This is at least what they teach here, but I'm not sure if all applies to carb engine, maybe avoiding 2nd throat opening might be better?

Post by **Dusty** » 23 Jul 2006 02:25 pm

Thank you all for your answers!
However, does that mean that other econometers (i.e. on other cars) are equally unreliable? If so, I'm beginning to wonder what's their practical use, apart from being able to distinguish between "economical" and "uneconomical" ride, or merely of being able to monitor fuel consumption over a certain period of time...Mine hasn't been changing for years now, which, I guess suggests that things are more or less ok with the engine, in terms of petrol usage?
I have also noticed that, if accelerating slowly (meaning really, really slowly), I am able to reach cruising speed of about 80-100 km/h without practically going into the econ's red field, which made me believe that slower accelerating is more economical than the fast one. Say, when doing 60 km/h in the 3rd gear, according to the econ, the needle is at the same position as if doing 80 km/h in the 4th. In both cases, the engine spins at about 3000 rpm-which, if we rely on those data, can be claimed as the most fuel efficient rpm range. I've read in a car magazine not so long ago, that older cars, like ours, performed more economically when in certain rpm range, whereas today's modern cars, with their new concept engines are more economical at lower rpms, which is one of the reasons why quick gear shifting is advocated today. Frankly, I can't imagine myself doing 50 km/h in the 5th gear

Post by **mac** » 23 Jul 2006 03:04 pm

Hi again Dusty,

Unless 'other' econometers actually measure specific fuel useage and compares that with distance travelled (and most do not) then yes, they are all equally unreliable, and in my opinion , of not much practical use at all.

The only resonably accurate Volvo system thst I know of is that of the 'infomation centre' fitted to 480s and high spec.400s (that has a fuel flow sensor, a speed sensor and distance sensor, and a fuel tank level sensor - it processes the data in its' own ECU and can present a dashboard readout of instantaneous fuel consumpion, consuption over journey, or range remaining in tank based on average journey consumption.

However if you're watching all that lot - you're not watching the road!!.

Most of these gauges were a product of the emissions legislation and fuel consumption publications of the 1980s - Salesmen wanted to say to customers - look, our product helps you drive economically - buy our product! (But maybe I'm just cynical having been in the trade for so long).

Drive the way you are comfortable - if you don't go everyware on full throttle and don't 'labour' the engine at low rpm in a high gear don't worry about 'best efficiency ranges' and where the gauge needle is - at the end of the day the difference you would make is very small. Particularly with fuel injection engines the fuel consuption tends to be what it is! Keep an eye on it by all means as sudden large changes can indicate faults - but don't loose sleep - just enjoy your car!

All the best - Mac.

Post by **Dusty** » 23 Jul 2006 03:49 pm

Mac,

thanks for the clarification, take care...

Post by **jtbo** » 23 Jul 2006 04:00 pm

Hmm, how much is fuel flow sensors?

I think it is needed to have two as fuel goes and returns need to calculate difference and we have consumption/time which we can use to calculate consumption over distance if we fit hall sensor to wheel (from bicycle computer) so it would not be too hard to have electronics kit to process data and output it to two segment led display, we should then have 3 options l/100km MPG and kilometers/litre for display.

I don't know but, total expenses might be quite low and it would be possible to install rather easily to instrument panel.

If I would know how to design this I would do it, but eh, Chris

Post by **Chris_C** » 23 Jul 2006 04:15 pm

It's possible Jani, you should remember this

More pictures here:

http://www.ecs.soton.ac.uk/~clc202/panel/

Thats a 400 info centre in Kar, my 1.4. The fuel flow sensor is fine in a carb car, you put it between the pump and the carb, as then you don't have to worry about the return line.

I'm currently fitting a Jag XJS computer to Fake, they are basically the same thing, I'm hoping to have a kit I can tell people to get if they want one soon.

Post by **jtbo** » 23 Jul 2006 04:26 pm

Yes, I remember now that, it is quite bit more than I was thinking actually, that is full computer thing

I'm sure there is someone already designed this kind of diy fuel consumption meter thing, just need to find.

Oh yes and naturally you are right about sensor, there is indeed no flow away from carb, except what engine burns.

I might know one 480 that is going to be disassembled, so needed parts would be flow sensor, speed sensor, instrument panel and that switch from it?
Price just may be bit more than I'm ready to pay as 40 would be quite bit all I can spare for this, maybe not enough

Post by **Chris_C** » 23 Jul 2006 11:03 pm

Jani, you'd need the dash (not sure how much as off yet, I can't remember how I hacked things together off the top of my head..) and I can't remember what the 400 speed sensor looks like, but would probably work. It doesn't have a fuel flow, as they are all injection, and it's done on injector pulses.

If you are keen long term though, I'm making up some B200 LE jet injector boxs that went with the taxi meter, which might work with 'squirt, else when I finally fit 'squirt to Kar, I'll just let you have a mirror copy of my stuff.

Post by **jtbo** » 24 Jul 2006 12:02 am

Sounds great.

With megasquirt it should be possible as you can get custom inputs to megasquirt, there is already support to wheel speed sensors for traction control in msextra code, however fuel flow might need some code to be done and it is not easy as msextra uses already all memory on cpu.

So I was thinking external solution because of that.

Post by **foggyjames** » 27 Jul 2006 08:16 pm

Surely you haven't forgotten the most kitch of all 300 series accessories, Mac?

cheers

James

Post by **mac** » 28 Jul 2006 10:15 am

Oh! woe, I am undone - my reputation dissolves before my eyes.

Quite right foggy, I had indeed forgotten that wonderful device!
(oh the joy of wiring little magnets on to a driveshaft - and I'm very sure that John (990) could think of a place to put that long flexible mounting - in those charming little muppets that trashed his motor)

Chastened (and rightly so) - Mac.

Post by **Chris_C** » 28 Jul 2006 01:42 pm

Doesn't have all the options of mine those does it Foggy

And I don't look like a minicab driver