Relationship between vacuum and FE? New questions

[cRiPpLe_rOoStEr]

Originally Posted by mpg_numbers_guy

The lowest speed in the highest gear generally nets the highest FE....what's with trying to complicate things with a vacuum gauge?

Decreasing the pumping losses in order to allow the engine to operate more efficiently not just due to the RPM range but also due to the fewer pumping losses. It makes some actual sense.

[mpg_numbers_guy]

Originally Posted by cRiPpLe_rOoStEr

Decreasing the pumping losses in order to allow the engine to operate more efficiently not just due to the RPM range but also due to the fewer pumping losses. It makes some actual sense.

Would this then apply only to acceleration?

[cRiPpLe_rOoStEr]

Originally Posted by mpg_numbers_guy

Would this then apply only to acceleration?

Not just acceleration, it's good for cruising too.

[Hersbird]

Ironically I have an old "fuel economy" gauge that runs on vacuum. I think it's for a 64 Pontiac Bonneville console, it's really a pretty thing. Anyway it goes from 24 to zero with a secondary color bar code under that goes from economy to power, economy being the 24 side and power being zero. So on a 64 big block Pontiac they figure the more vacuum you have the more economy you are running, based on the throttle being closed, and the less vacuum the more power you are making. It doesn't really consider that the motor is operating with better thermal efficiency with the throttle open because with the throttle open on a big block carbed Pontiac you will be going 100mph in a 1/4 of a minute or so and you can probably watch the gas gauge go down. In that case high vacuum, throttle closed as much as possible is where you get the best mpgs.

[Ecky]

Another thing to consider: while pumping losses do change with the amount of vacuum, probably more important is the ratio of fixed parasitic losses to power output. You're sapping the same amount of power spinning A/C, alternator and water pump at 3000rpm regardless of how much power you're asking of the engine. An engine closer to peak output at a given RPM will have a better ratio of useful work to parasitic losses.

[cRiPpLe_rOoStEr]

Sure the parasitic losses are another thing to consider. Well, maybe some improvements could be achieved with an electric fan setup and electrically-assisted power steering (which by the way is now standard in the overseas Ranger).

[hat_man]

Driving the lowest speed in the highest gear isn't possible for my drive to work. I can hold 35 mph in 5th gear before the engine starts to lug. My drive to work is 65 miles on mostly 2 lane blacktop roads with very few passing zones. So if I want to spend 2 hours in the truck instead of just over 1, and become a pariah to the other drivers trying to get to work, and possibly become the catalyst for an accident or a road rage incident I could get incredible mileage. But I would rather share the road and get the most out of the tank as I can at the same time. Beings I don't know much about vacuum at this point, I am taking anecdotal evidence and trying to develop a plan to help do this. Trying to find a way to be in an optional FE range at the speeds I drive the most. I am hoping that the vacuum gauge will give me some insight.

I really don't mean to sound so snarky but it's early and I haven't had my coffee yet and it's shaping up to be a very trying day.

[Ecky]

To put it more plainly, an engine spinning at half the speed has half (or less) of the frictional and parasitic losses for a vehicle traveling at a given speed. You can't just keep halving engine speed though - eventually the engine won't be able to produce enough power. This is a major part of why high load driving is most efficient: the lower the engine speed and closer to its maximum output at a given speed you get, the better your economy. Vacuum is a good indicator of how close you are to this.

[hat_man]

I'm beginning to understand the function of vacuum and I think I'm on the right track. That is unless someone tells me that I'm way off base.

So I think I'll look for a decent vacuum gauge that has the color coded sections. I still think getting into the throttle to hard and getting the rpm's too high isn't as efficient as cruising in a "comfortable" rpm range.

Now to research gauges and how to hook them up temporarily. I really don't want to hook it up permanent. I want to leave room for an UltraGauge someday and a permanent vacuum gauge will eat up a bit of dash space. Besides, if my idea pans out, I won't need a permanent vacuum gauge anymore.

I see the vacuum gauge as the rough tuning and the UltraGauge as the fine dialing in.

[MeteorGray]

I have installed vacuum gauges in my cars since I got my driver's license in 1958. I still have one in my 2003 Jeep Wrangler. It's a great analog way to know what condition your engine is in. For instance, an abnormally low vacuum level under various conditions could indicate a vacuum leak, or worn cylinders resulting in low compression or clogged catalytic converters, none of which might be otherwise noticed at the time.

Regarding fuel economy, the vacuum gauge can be useful in keeping the driver informed of how hard the engine is working. Under cruising conditions, the lower the vacuum, the lower the fuel economy, provided, of course you're not revving up the engine to keep the vacuum high. When cruising, higher speeds come with lower vacuum levels which might indicated lower pumping losses, but wind resistance and tire and engine frictions quickly reduce fuel economy to overwhelm any benefits of lower pumping losses.


I've got an automatic tranny and usually let it do its thing in regards to shifting strategy, even when climbing hills. When accelerating, I know the reduced pumping loads as indicated by a low vacuum value can help engine efficiency, but I also know that the extra energy needed to provide rapid acceleration uses more fuel, so it's a pretty fine line to walk in that endeavor.

I've concluded that for twenty bucks, a vacuum gauge is a bargain. Besides, I'm a gauge nut and love the looks of an instrument panel full of needles and dials.