@Mopar1973Man OBDLink Questions

[AH64ID]

 

I was just looking at your article on the OBDLink and have a couple questions. 

 

You say it gets fuel flow from boost; however, fuel flow at 10 psi and 1500 rpms is a lot different than 10 psi and 2500 rpms. Also 10 psi at 12° of timing is going to be different fuel flow than 10 psi at 8° of timing. 

 

So how accurate is it?

 

What did the SGII use for fuel flow calculations? 

 

Your definition, and how to calculate, volumetric efficiency is based on your fuel tank yet volumetric efficiency has nothing to do with a fuel tank. Forced induction motors often have volumetric efficiency of over 90%. 63% sounds like a 5.9L gas Dodge Ram...again it has nothing to do with your fuel tank, but rather the amount of air an engine moves versus its displacement.  What you described is how to calibrate the fuel consumption. 

 

As for BSFC it's going to vary with each and every timing adjustment, duration adjustment, injector change, turbo change, etc. It's not a fixed number, so are you re-adusting for each mod and each retune? Aside from hours of dyno time I am not even sure how you would re-adjust BSFC. 

 

Or is the OBDLink a wide shotgun blast at what your getting and not really useful in terms of hp or fuel flow?

 

 

 

Edited September 11, 2016 by AH64ID

[Me78569]

Due to the way these types of things calculate MPG and HP it is a best guess based upon inputted variables for fuel input vs load.  This is not a terribly accurate way to do it, but it does help greatly when trying to get an idea of on the fly power requirements.    I would say you need to look at all of this as an average rather than exact, even if the reading is off by %15 at a given point it is still useful for driving style adjustments.

 

I don't know if I would say it is not useful as I see nothing suggested by anyone that does a better job for an on the fly rough guess of power being put down.  By all means if you have a better way to calculate this I am sure we would be all ears.  

 

 

 

If you you disagree with the volume efficenty part of the article would you care to rewrite it to be accurate?  I agree that his use of it is incorrect.   From what reading in the past I have done on VE it will range on our trucks depending on a lot of things.  honestly 73 is pretty close to what I has read in the past.  

 

We would welcome an edit of that section.   We  can say something is wrong all day long, but unless you put the effort into it to fix it it is wasted breath.

[Mopar1973Man]

The whole trick we VE is it operates like the ScanGauge. So using the 63% for the stock truck value I was getting really high numbers. So as I added my offset the numbers come down to close to my log values. So more the VE number goes down the more MPG goes up. The more the VE goes up then the more MPG goes down. Fairly simple.

 

Funny part is I assumed the VE number went down the MPG went down so my first try was subtracting from the base number which gave seriously wild high numbers like 50 MPG. So reverse it and now back on the planet earth again.

Edited September 12, 2016 by Mopar1973Man

[Me78569]

We can't begin to guess how the software people wrote the calculations so sometimes you need to fudge the needs until they result in something that makes sense.

 

 

I had to do the exact same thing to make map read correctly via obd.  The equation I used was clearly wrong in terms of what the obd standard said it was, but that's how I got map value to report a valid address.

[AH64ID]

So the OBDLink has no other way to calibrate fuel flow other than VE? Seem's like we're trying to make lemonade with apples. 

 

Michael, I assume you have been verifying the OBDLink mileage to hand calculated? There is something missing as boost is never going to be an accurate enough indicator of fuel flow to be measurable.

 

What did the SGII use for fuel flow? 

I'm not trying to start any fights or wars... just asking some questions to stuff I see posted here that doesn't make any sense. 

 

[Me78569]

I'd be lying if I said I knew how they calculate that.

 

 

As with any computer for calling mpg you should always hand calcd with GPS verified numbers.  The computer will no be accurate unless you spend all your driving time at whatever point you did the calc for.  I am sure the more you move away from that middle point the farther off it gets.

 

I am pretty sure the scan gauge used a user defined offset for fuel flow also.  Again not terribly accurate unless you are cruising.

 

In theory we could build a very accurate mpg meter off of the quad.  What we would need to know is injector flow and the duration.  We know duration provided we keep it on lvl 3 and below

[Mopar1973Man]

5 hours ago, AH64ID said:

What did the SGII use for fuel flow?

 

Percent offset with MAP sensor data. Just like OBDLink does same thing. Also, SGII is calculating GPH also from MAP sensor data. Same thing... Yes, I check off and on. So either SGII or OBDLink they both work the same. Very rare I see any need to change unless I've changed something physically on the engine like injectors. I'm not going to tweak for minor offset between pumped and reported of 0.1 difference no need.

 

3 hours ago, Me78569 said:

I am pretty sure the scan gauge used a user defined offset for fuel flow also.  Again not terribly accurate unless you are cruising.

1

 

 

 

If you do a lot of idling then the numbers are not correct since neither can track idling flow rates at all. So when I check its basically fill drive to another distant fuel station fill again and check the gallons used against the gallons pumped. Either SGII or OBDLink you end up calculating an offset percent. Adjust accordingly. Nothing fancy. 

 

Cummins engines do not report GPH flow rate at all. So This is why on SGII it set to Diesela which tells the ScanGauge to calculate from MAP sensor info. Same thing on the OBDLink you have the option of GPH value (which is zero on Cummins no PID in the ODB data) or MAP sensor input. 

[Me78569]

using the limited sensor data we have calculating MPG on a turbo diesel is tough.  That is why we always verify with hand numbers if you are really wanting to be accurate.

 

 

The question is how accurate is good enough?   I would say that both the SG and OBDlink get close enough to show a reasonable accurate MPG for a trip.      However it is up to the user to realize when the mpg is not accurate due to the driving style / conditions.   

 

 

[AH64ID]

All Cummins do not report fuel flow? That's a new one, and not what I have seen. 

 

My 05 certainly reports fuel flow. It's wrong, but it does report it. There is an error with how it's programmed since the reported fuel flow goes down from idle to 1500 rpms, but the boost goes up with a stock turbo. Diesela or dieselb. 

 

Based on how much boost can vary for a specific fuel load something has to be reported or the mph wouldn't be as accurate as you see on your 02. 

Edited September 13, 2016 by AH64ID

[Mopar1973Man]

At least that I know from 98.5 to 02. There is no PID in the OBDII stream for GPH flow. I've not dug into 3rd or 4th gen OBDII yet. 

 

Oh dig back on my previous post about the OBDLink and look at the PID hunt I did with the Android terminal search for hidden data. If you look at PID list 5E should be present no on the 98.5 to 02.

 

https://en.wikipedia.org/wiki/OBD-II_PIDs

 

So since its not present on our truck the value is calculated.

Edited September 13, 2016 by Mopar1973Man

[AH64ID]

It may not report GPH but I would guess there is something for injected quantity or load. 

[Mopar1973Man]

No injected amount provided. Load value yes is present.

Here is my hunt results over the OBDII info...

 

[Me78569]

The injection duration message is not on the sci bus.  Only the under hood plug, and I am guessing CCD to do the overhead.

 

So no there is no fuel message on the 2nd gens that can be used by am obd device.

 

 

You might be able to use load but who know how load is calculated in the trucks.

 

 

[Mopar1973Man]

Hence the weird method I have to take to gather engine data and trip data.