M84 fuel pressure reference input

[the_bluester]

OK, I have had a search though help and a look through the ECU manager software and I can not find a way to do what I am after.

Is there any way to have the M84 ECU reference fuel pressure against manifold pressure? Given that the only fuel pressure reading of importance is the differential between manifold and fuel rail pressures it would be very useful to be able to have the ECU output that figure via CAN rather than raw fuel pressure. But unless I have gone blind I can not find a way to do that and my friends car (With an M84) sends raw fuel pressure over CAN to his C127. I wrote some maths for him to generate real fuel pressure in i2, but it would not be much consolation to work out why he blew his engine up after the fact by looking at a log file. Real fuel pressure output would permit immediate logging but more importantly, easy display and alarming for fuel pressure problems.

If anyone can point me to anything I have missed I would be happy to be proven wrong.

[AdamW]

I don't think you can do it in the ECU but you could do it in the dash.
Set up a 3d table, call it say, diff FPress. MAP on one axis and fuel press on the other. I haven't thought this through in great detail but the values in your table are differential fuel press and would probably look something like below. You can then display diff FPress on dash or set up as an alarm.

[the_bluester]

I was hoping to avoid doing something like that but I will set it up if I have to.

It is one thing Haltech have got right, there is a tick box in the software to reference an ECU fuel pressure input either against the manifold or absolute (Or it might be against zero PSIG, Cant recall)

[David Ferguson]

The M1 does this as well. You can use either absolute or gauge fuel pressure sensors, and it will calculate the differential fuel pressure using the MAP value

[the_bluester]

It would be great if the M84 and M400-600-800 did the same. Hopefully something to be seen in a future update.

For now I will have to do the custom table route, though even that is a little tricky. For some reason his car has been fitted with 2000 PSIG sensors for oil and fuel pressure rather than more appropriate ones like 100 PSIG, and the 2000PSI ones do not appear to have a calibration for KPA due to the size of the number produced. I may have to get a bit more creative in the table in his dash, or create a custom calibration for the ECU in KPA. The problem with that being the resolution will be a bit poor (As it is already) when we have his highest oil pressure producing about 0.7V with 0PSI being 0.5V and 2000 being 4.5V! Not the sensors I would have chosen..

He plans to change the sensors over for something more sensible and recycle the ones has has as brake pressure logging, but it might be a while. For now we have oil and fuel pressure coming in as PSI into a template looking for KPA and you have to ignore the units saying KPA in the dash and remember that it is PSI. Caused him some headaches with oil pressure alarms as he set them in PSI in the alarms page, so it faithfully converted a reading of 50KPA (PSI in reality) into about 7PSI and promptly alarmed every time his RPM threshold was met.

[the_bluester]

Just to pose the question directly for the Motec guys if they happen to look in here.

Has Motec considered adding a selection to reference fuel pressure against manifold pressure in the M84? If not, can you consider this a feature request?

It would be useful in my friends car and I am looking more and more likely to fit an M84 to my own car next year, so useful to myself as well. For anyone with an electronic display this would allow extra engine protection by easy display and logging of a relevant fuel pressure. Even if display units for manifold and fuel pressure were converted to PSI in a dash like the C127 and displayed in adjacent positions I would not fancy making the decision to continue or retire based on mental calculation of the difference between two moving figures on the dash in the middle of a lap.

[Stephen Dean]

Hi,

Development for the M84 and Mx00 family of ECU's has been finished, we are going forward with the M1 series.

Support and sales of these products will continue for the foreseeable future, but the actual product will not change.

With the differential FP on the dash, the simple table posted by AdamW will generate the value that you are after, as long as the data that is provided to it is correct. By default, the CAN transmit messages from the M84 are in the units that you have the sensor set to, when they come into the dash they then have to be converted in the receive template back to kPa and then you can display the units that you want on the actual display.

[the_bluester]

Oh well. It is always worth asking.

I am not going to look at it until he gets his sensors sorted out, currently the resolution is poor given he has 2000psi sensors using around 5% of their range. Too much cobbling together of work arounds for my liking.

[the_bluester]

For interests sake I thought I would post again.

While my dash is not in the car yet, I set up a 3D table as per the above and tested in the simulate function, it all appears to work like it should.

I also did the same last night for my friends car though he still has the 2000PSI sensors that were fitted to his car originally with no calibration in KPA. For the time being I have set up a 2D table to convert his "PSI sent as KPA" from the ECU into KPA, which is then used as an input to the 3D table to calculate the differential fuel pressure. I assume that some accuracy will be lost given we are talking pressures that will be maximum of about 5% of the full scale reading of his sensor then multiple interpolations in the tables to calculate a final value, but it should work OK until he changes over his sensors to more appropriate ones. Once he does that I can delete the 2D table from the workflow.

[Scott@FP]

M84 is pretty limited for changing table axis, but Mx00 is a simple 3D table in one of the fuel comps to do what you want.