Scan Gauge 2 How it works
 

Can someone explain to me how the SG2 works. I'm electronically inclined so a technical explanation is ok also a watered down version wouldn't be bad either. I know the SG2 gets it's info from the OBD2 computer which get's it's info from the various sensors. But how does it calculate MPG and gallons to empty, etc... Thanks for the help

The SG doesn't actually measure fuel used to calculate mpg. It calculates mpg by taking engine size and an assumed 14.7:1 air-fuel ratio and measures the air used. On my SG1, I used to tweak the mpg readout by adjusting the engine size if the SG's mpg was different from the calculated. This is partly why an SG doesn't work on Honda's with lean-burn mode such as the VX or Insight or HCH I with a 5-MT.

I thought that EVERY car went away from a stoichiometric ratio (to a leaner ratio) at certain low load conditions. Is that not true?

The ECU will always try for 14.7:1 even though the engine may want/need something different.

diesels can lean out a lot more.

I thought it read the injector pulse width from the ECU and calculated fuel usage from that.

maybe this will help...

https://www.scangauge.com/support/pdfs/SGIIManual.pdf

Hello -

One thing I have noticed with other OBDII readers (like the CarChip) is that they also only monitor 4 values at once. This leads me to believe that the OBDII "bus" can only place 4 "instant readout" values on the bus at one time.

No proof, just observation.

CarloSW2

A cool upgrade to the SG2's firmware would be the option of it automatically cycling through user-selected gauges, just like some digital watches allow for auto-cycling between date and time. That would save FE nerds like me from constantly cycling between the gauges and trip data screens.
Shouldn't be too hard for the wizards at Linear Logic.

Ron, are you listening?

Reeealy. Wow. I was positive that all cars went a little leaner at low loads in order to bump up FE. Thanks.

I don't believe that is true anymore. The fact is, manufacturers have determined that the old narrow band oxygen sensors which make keeping a perfect stoiciometric ratio an easy target are inadequate for optimal power and efficiency. The use of wideband sensors has opened up the ability to tune more accurately to both ends of performance and efficiency - allowing them to carefully control ratios far leaner than the standby 14.7:1.

I don't believe that all modern cars have a lean burn mode, but there are certainly many more now that do.

Answering the original question, I'd bet that SG actually uses injector pulse width as the primary means of metering fuel. When you first plug it in, it assumes an average rate of injector size for a given engine displacement and calculates fuel used based on the length of the injector on-time. The user calibration that you can do afterward is simply a way to correct for the assumed values of consumption, as it's a fairly linear relationship that does not change significantly with variations in atmospheric conditions.

Thanks guys. Hijack over.

does it do anything to moniter manifold vaccum, so it can figure out what the fuel presure regulator is doing? or does it just figure an average fuel presure?

I agree, the scangauge must use injector pulse's as the primary method of accounting for fuel burn. Otherwise while maintaining the same RPM on cruse you should always see the same fuel burn/mpg rate yet it does not. I flip on the A/C I see a few mpg drop, a truck changes into my lane and my MPG jumps up.

I don't know how it works but I know that on a Saturn the IAT has a huge effect on the reported mileage. I asked Ron about this and never got a reply. You can deceive your self about 20% if you want :rolleyes: .

I have been considering installing an IAT resistor just to get more consistent results.