deos the MPG indicator need the car to be moving to register an accurate read?
05-29-2009, 10:56 PM
#1
Safety Car
Thread Starter
deos the MPG indicator need the car to be moving to register an accurate read?
How precise is this mechanism, or is it just a toy?
how does it work, where does it get it's data?
is this some fancy vacume gauge?
how does it work, where does it get it's data?
is this some fancy vacume gauge?
05-29-2009, 11:27 PM
#2
Melting Slicks
It can be very accurate. There is a running total of distance travelled and a running total of fuel delivered as well as a display gallons/hr constant that are used in the calculation.
However, in order for the signal to be accurate, the MAF signal must be accurate, the vss signal must be accurate, the display constant and injector constants must be accurate and the injector low pulse width and battery voltage offsets must also be correct for the specific injectors.
Any error in any of these parameters will influence the resulting calculated value.
However, in order for the signal to be accurate, the MAF signal must be accurate, the vss signal must be accurate, the display constant and injector constants must be accurate and the injector low pulse width and battery voltage offsets must also be correct for the specific injectors.
Any error in any of these parameters will influence the resulting calculated value.
05-30-2009, 03:29 AM
#3
Team Owner
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Does the MPG indicator need the car to be moving to register an accurate read?
Your kidding. Right?
Cause the displayed info is about the amount of fuel burned vs distance traveled. If your sitting still, your not traveling. So if your sitting at a light and burning fuel the MPG's will be less than when your traveling at 50 MPH.
Yes the info can be accurate if your speedo is calibrated correctly.
Your kidding. Right?
Cause the displayed info is about the amount of fuel burned vs distance traveled. If your sitting still, your not traveling. So if your sitting at a light and burning fuel the MPG's will be less than when your traveling at 50 MPH.Yes the info can be accurate if your speedo is calibrated correctly.
05-30-2009, 03:33 AM
#4
05-30-2009, 07:26 AM
#5
Safety Car
1) the instantaneous mpg better be 0!!
2) the average mpg should be slowly decreasing
Try it
BTW I have found mine to be very accurate
05-30-2009, 11:37 AM
#6
However, in order for the signal to be accurate, the MAF signal must be accurate, the vss signal must be accurate, the display constant and injector constants must be accurate and the injector low pulse width and battery voltage offsets must also be correct for the specific injectors.
The only inputs it needs are fuel flow rate and vehicle speed.
These should be provided by display constant and VSS calibration, respectively.
05-30-2009, 12:02 PM
#7
Le Mans Master
As said, if you are not moving, and you are consuming fuel, the instant MPG is 0. For checking the accuracy on average, why not compare it to what you compute from the pump?
If you have stock gears, injectors, fuel pressure, etc, then it should be pretty accurate.
If you have stock gears, injectors, fuel pressure, etc, then it should be pretty accurate.
05-30-2009, 12:20 PM
#8
Melting Slicks
The actual fuel rate is also dependent upon the low pulse width and battery voltage offsets, since they can and do contribute to actual fuel flow but are not considered in the calculated fuel flow rate. Fuel pressure is also a factor if not considered in the injector constant setting.
The offsets should effectively zero the injector but they will often result in too much fuel. The display constant can be tuned to over compensated to a point (up to 8 gallons/hr) to handle this kind of error.
For example, if the offsets are too big, then there will be extra fuel delivered that is not considered in the running total. Actual mpg will be lower than indicated. This is likely to be the case anytime larger injectors are installed. 42# injectors are about the practical limit for tuning the displayed mpg.
Every input needs to be accurate or tuned to compensate for systematic inaccuracies.
*Added*
If the injector constants and display constants are set correctly for the actual injectors and fuel pressure, but the actual mpg is still way off, this is a good indication that the offsets are adding too much (or to little fuel) and need correction.
Last edited by tequilaboy; 05-30-2009 at 12:36 PM.
06-01-2009, 12:08 PM
#9
Team Owner
How do I change those constants now that I have 42pph injectors?
