Jfrahm

Hi all,
I looked at a supercharged 1990 928S4 on Sunday. I had a question about the MPG indication in the trip computer. The car has large Ford injectors installed. I supposed that the MPG reading would be thrown off by the large injectors, but the owner said the car had a flow meter to determine MPG. I suggested it would need two meters (one on send, one on return) and that it probably used an injector duty signal.

Does anyone know how this meter works?

The car is kinda cool, but has hellacious blower whine and some kludgy stuff on it. Pulls hard though.

-Joel.

Jfrahm

*bump*

No clues on how this works?

BC

The meter, with looking at the workshop manuals too many nights in a row, works like this:

It takes a "consumption" unit number from pin...Oh, I forgot what pin, but it takes that unit number from the LH and sends it to the dash computer for a readout.

The actual information that this unit number is giving is...ready?

The actual pulse width of the injectors! The duty cycle if you will.

So the owner was WAY off in telling you that its a flow meter. Totally wrong. Its a direct value as to how the injectors are being fired from the LH, and since the LH will fire the injectors no matter what size they are, your MPG meter will not work correctly, and will give erroneous readings with larger injectors.

I only know this becuase I think I will be losing this when I install my DTA efi, as there is no way I can us the precious outputs for this purpose.
HTH,

T_MaX

Brendan, is the duty cycle not adjusted via the O2 sensor?

I would think that if you are using up the capacity of the factory size injectors with new mods, and you throw in the next size up injector to match the mods, the duty cycle of the new, now larger injector should be adjusted to the correct pulse via the LH to make up for the difference, yes/no/maybe?

BC

Sure, but with a larger injector, it is my thinking that the scale would be quite a bit off.

Old & New

The pulse width will be narrower with larger injectors, and since there is no data acquisition regarding injector size/flow, then the system cannot integrate the values into an accurate reading. It would seem that the reading would be higher than actual fuel economy. Perhaps?

John Speake

The signal from the LH depends on rpm and "load" - the MAF voltage.

So it should remain a fairly faithful indication of "economy" as its reading has nothing to do with injector pulse widths, but the mass of air going into the engine.

However, I think it would be wrong to beleive it is "calibrated" in any meaningful manner - its output is just a relative indicator of fuel consumption, not an absolute one.

BC

Okay John - Can you tell me where I went wrong with my understanding of the LH pin out for consumption? I know I saw it explained that way in the manual fot my 89.

If the signal is as you say RPM and "Load" or Maf voltage, I will indeed still have no way of getting the correct numbers on my dash with my DTA, but could you go into more detail on this please? There is a possibility that I could get close with the voltage comming from my MAP.

Old & New

On the contrary, Brendan. If they are integrating rpm and the maf signal, then why do you think anything will change with engine output / larger injectors / etc.

If they are measuring air consumption vs. rpm, the results should even be consistent to whatever engine is attached to the maf! (given the same stoichiometric conditions and gearing, of course).

But really, folks, who cares if the fuel economy indicator works in a huffer, anyway?

John Speake

Brendan,

I've observed the pulses on that pin with a 'scope on my test jig, where I can vary rpm, load etc. It's a very crude output, basically 1,2,or 3 , pulses every engine rpm depending on load.

I suppose you could say the value that is presented is "related" to injector on time/fuel consumption - perhaps that's what the info you saw was trying to say ?

Just use it as a relative measuremnt, not an absolute one.

Jfrahm

Thanks for all the great info.

The reasons I was interested was that the seller was pointing to the MPG gauge while we flogged the car as an indication of how well it was dialed in. I also wondered if the seller knew the car as well as he let on. Also, I hate having an indicator in the cockpit that does not read reasonably true.

Based on this info, I would suppose the MPG readings are probably OK as long as you don't get into the boost, but under boost it's probably reading high (MPG-wise.)

The seller also claims very good average MPG on the highway (26mpg at 90mph.) Seems high to me.

-Joel.

John Speake

Hi Joel,
I think the readings would be as "acccurate" under boost as off boost, as the readings are related to mass air flow INTO the engine......

I would be far more impressed if the seller showed me a wide range CO meter readout on the dash, to prove that the mixture was right under all conditions !

Quoting hign mpg figures could make you suspect it was in fact running weak.

Jfrahm

Hi John,
My thinking was that under normal driving conditions, the load data would be about right, but under boost when the engine is putting out 100 extra HP? I thought the load would go to maximum (3 pulses per rev) but the engine would be producing 30% more HP that would be unaccounted for.

-Joel.

John Speake

Hi Joel,
yes, that's a good point...........however it gets a bit complicated.
The S4 engine is volumetrically very inefficient at high rpm. The maximum injector on time at max power rpm and max load (5v from the MAF) is about 5mS.
But at max torque rpm (much lower rpm of course) the max injector time is 10mSec at 5v on the MAF.

So a s/c will make up for that large breathing deficiency at the high rpm/max power point.

I was wrong about the maximum number of pulses given out by the "consumption" pin............ it goes up to a maximum of 6 pulses pre rpm at max torque. Presumably when s/charged, there will be more pulses at high rpm than the "standard" 2-3 pulses I observe on the test jig

So perhaps the "consumption meter" can cope with a s/c engine ?

I wouldn't put any great accuracy on it though.

Regards,

BC

Hmmm. So pulses come from that pin with reference to the load and rpm of the engine. You know I never thought to have it on if I were to put the clutch in and rev the engine freestyle - to see if the numbers changed. As we know, this meter goes to 90mpg when you are coasting in most instances. Hell, mine would go to 4 if I pushed hard enough.

It would just be nice to keep the meter somehow. But no big deal.