dpoelstra

I am stunned at the readings I am getting on the vacuum gage.

Now for the sake of full disclosure, I am not a scientist, mathmatician or journeyman mechanic. I used to overahul small block Chevies and took cues from old hotrodders. I understand that the higher the reading on the vacuum gage, the better efficiency, thus better mileage. The lower the reading the harder the engine is working thus lower mileage.

I have always been taught that lugging an engine at low RPMs is worse for mileage and wear and tear that running slightly higher RPMs, obviously within limits.

As a bench mark, at idle, the reading is 25. I have 4.1 gears and 33 inch tires. I have a CAI, chip and Magnaflow muffler.

With the mph measured by a GPS on a flat freeway at around sea level in Orange County, CA and no traffic, I ran 65 mph. In 4th gear I was running 3500 rpm with a reading of 15.

Shifted into 5th and the reading dropped to 12, shifting into 6th and the reading dropped to 5. A reading of 5 is similar to a V8 engine towing a heavy trailer (from experience) and was stunning for me to see.

I am running through a tank running freeway speeds at 65 and 3500 rpms in 4th gear to check mpg. Higher RPMs but much higher Vacuum readings.

My hypothesis:

I think this engine is not made to run low RPMs in overdrive. It just has to work too hard. I think I may see an increase in mpg while keeping the rpms HIGHER, which goes against the conventional wisdom, but the same concept others have observed may be true with a V8 engine. A larger engine not working as hard, or this engine not working as hard but turning higher RPMs will make more power and run more efficient than trying to keep the RPMs down using overdrive. If this is true I should see a measurable increase in mpg.

I have never been able to get my mpg above 15 mpg (again, as measured by a GPS for speed and distance).

I also think that base don everything I have read here the mpg computers that the Rubis and Saharas have are not accurate.

I will report findings as I get them.

adaycj

The vacuum reading is telling you throttle plate restriction. The difference in pressure between two points, the atmosphere and the end of the line.

This really isn't that well tied to the MPG. I agree that if you can reduce the amount of air going through the engine that you will be able to add less fuel to keep it at lambda (af ratio at cruise), so you would use less fuel. But that only assumes that you don't increase the pumping losses by closing the throttle, and you can make enough power to maintain speed, and you don't go rich from PCM induced loaded engine readings.

Or even worse you pump more air with a faster pumping engine (higher RPM), and have to add more fuel over a given time to keep the engine running.

Plus the drive by wire may play some games with the throttle position, which impacts the readings on your gauge. The pedal isn't really connected to the throttle. While it is likely that the throttle will mimic your pedal inputs, it is not guaranteed.

This is a formula that I used to calculate (with extrapolations) the performance gains for a true CAI since the intake air on the JK is quite warm (.23 HP, BTW).

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)

VSS is in kph and MAF is in grams/sec *100

This is a very accurate formula used in hundreds of thousands of cars, but is very unlike the junk MPG readout in the JK. Since MAF can be calculated from MAP (engine vacuum), you only have a small part of the equation. A scangauge can do the math for you can give you a reading. Or you can read MAF from a hand held tool under various cruise conditions at the same speed. As you can see from the formula, assuming you are at lambda in every case, a lower MAF at a constant speed = better MPG.

You need a MAF gauge, not a MAP gauge.

dpoelstra

I am very interested in what you are saying, and I believe you know what you are talking about, but....Huh?

RJSTJ

I understand what your doing. But why? Sounds like someone needs another hobby besides wheeling.