Scangauge and VCM

[youbetcha]

Has anyone tried using the "Scangauge II" tool on a VCM engine? Does 3 cylinder operation throw it off?

For example does it calculate MPG as if all 6 cylinders are firing even if they are not?

[tangotango99]

Originally posted by youbetcha
Has anyone tried using the "Scangauge II" tool on a VCM engine? Does 3 cylinder operation throw it off?

For example does it calculate MPG as if all 6 cylinders are firing even if they are not?

very good question.

[A6Pilot]

Off hand, I would guess the mileage calculation doesn't care which cylinders are or are not firing. I would assume the mileage is calculated simply as a function of how much fuel is used to go a measured distance.

Just a guess.

[youbetcha]

I don't think OBD tracks accumulated fuel use. So I'm thinking Scangauge probably gets injector pulse data and counts spark firings, something like that. But the spark plugs actually fire even in the closed cylinder bank VCM mode, for example.

[ctobio]

Originally posted by youbetcha
I don't think OBD tracks accumulated fuel use. So I'm thinking Scangauge probably gets injector pulse data and counts spark firings, something like that. But the spark plugs actually fire even in the closed cylinder bank VCM mode, for example.

Do they? I'd be surprised if there was spark on the deactivated cylinders.

[youbetcha]

Originally posted by ctobio


Do they? I'd be surprised if there was spark on the deactivated cylinders.

"The spark plugs continue to fire in inactive cylinders to minimize plug temperature loss and prevent fouling induced from incomplete combustion during cylinder re-activation."

http://www.hondanews.com/categories/891/releases/4459

[ctobio]

Originally posted by youbetcha
I don't think OBD tracks accumulated fuel use. So I'm thinking Scangauge probably gets injector pulse data and counts spark firings, something like that. But the spark plugs actually fire even in the closed cylinder bank VCM mode, for example.

Here's how it works for normal cars:

http://www.windmill.co.uk/obdii.pdf

The formula:

Here are the steps to make the conversion:
1. Divide the MAF by 14.7 to get grams of fuel per second
2. Divide result by 454 to get Lbs fuel per second
3. Divide result by 6.701 Gals fuel per second
4. Multiply result by 3600 to get gallons per hour
The expression for GPH is: MAF * 0.0805
To calculate MPG divide the MPH by GPH.
The final expression for MPG will be: VSS * 7.718

I think in theory that this would work properly with cylinder deactivation in this scenario. Since the valves are getting shut on the deactivated cylinders, they're not pumping air through the MAF.

Interesting about the spark plugs. That makes sense.

[youbetcha]

Originally posted by ctobio


Here's how it works for normal cars:

http://www.windmill.co.uk/obdii.pdf

The formula:

Here are the steps to make the conversion:
1. Divide the MAF by 14.7 to get grams of fuel per second
2. Divide result by 454 to get Lbs fuel per second
3. Divide result by 6.701 Gals fuel per second
4. Multiply result by 3600 to get gallons per hour
The expression for GPH is: MAF * 0.0805
To calculate MPG divide the MPH by GPH.
The final expression for MPG will be: VSS * 7.718

I think in theory that this would work properly with cylinder deactivation in this scenario. Since the valves are getting shut on the deactivated cylinders, they're not pumping air through the MAF.

Interesting about the spark plugs. That makes sense.

Very interesting -- the formula ssumes that the engine is only run rich or lean for small enough amounts of time that MPG isn't affected. Your conclusion about the airflow not going through cylinders, thus not affecting the calculation, makes sense, too. Thanks.

[ctobio]

Originally posted by youbetcha


Very interesting -- the formula ssumes that the engine is only run rich or lean for small enough amounts of time that MPG isn't affected. Your conclusion about the airflow not going through cylinders, thus not affecting the calculation, makes sense, too. Thanks.

I would say so. It's reasonably accurate to calculate it this way. If you want true MPG, you'd have to measure fuel flow, but in most engine management systems, there's no "fuel flow" sensor, but there's an air flow sensor. Making the leap that you're pulling in air and that you're burning at near-perfect efficiency, it stands to reason that then you can apply the stoichiometric formula to figure out how much fuel you're drinking.