If you don't have a vacuum gauge, you're missing out!

[99prizm]

I ordered the Auto Meter # 2337: Mechanical Individual Console Vacuum Gauge. It has the different colored portions on the dial though.

[RH77]

I'm going to need a cheap way to get some feedback on Teggy (moved the SG to the Insight). This looks like a good option...

[4ringcircus]

Just got my Sunpro RetroLine gauge from JC Whitney. The dial is a full face dial, so it is really easy to see. It is also just a two inch face; I was worried it might be a bit small, but I drive a Ranger and now that i've installed it, anything bigger would not have worked. The full face dial is great because you can easily see how efficient you are driving despite the smaller gauge. Finally, it was cheap. I paid about 30 bucks!

[dipstick]

Forgive the obvious lack of experience and knowledge, but isn't FE determined by fuel consumption as determined by volume of air flow through the venturi in the throttle body in the case of a carb, and by computer in the case of injection? And in the case of injection, isn't vacuum pointless because the real issue would be the health and output of a specific sensor? Wouldn't it be better to modify the output of said sensor, perhaps the Throttle Position Sensor??? Trick the computer to believe the throttle is at a lower setting, while allowing greater air flow, thus increasing the chamber pressures and leaning the mixture and increasing power and FE?

Or did I miss something?

[Christ]

Originally Posted by dipstick

Forgive the obvious lack of experience and knowledge, but isn't FE determined by fuel consumption as determined by volume of air flow through the venturi in the throttle body in the case of a carb, and by computer in the case of injection? And in the case of injection, isn't vacuum pointless because the real issue would be the health and output of a specific sensor? Wouldn't it be better to modify the output of said sensor, perhaps the Throttle Position Sensor??? Trick the computer to believe the throttle is at a lower setting, while allowing greater air flow, thus increasing the chamber pressures and leaning the mixture and increasing power and FE?

Or did I miss something?

I think you did indeed miss something.

The vacuum gauge is a beginner modification that can be used to help transition someone into the world of instrumentation available to aide them in achieving better fuel economy.

Determining the negative pressure of the manifold will, in all accounts, be beneficial, as even with OEM EFI systems, the standard practice is that LessVacuum == MoreLoad, hence: more fuel injected to compensate.

[dipstick]

Originally Posted by Christ

I think you did indeed miss something.
...the standard practice is that LessVacuum == MoreLoad, hence: more fuel injected to compensate.

Less vacuum does not = more load, it simply = greater air flow via less throttle plate restriction, or lower rpm's. Though the two happen simultaneously in most cases.

Load is based upon resistance to rotation at the output shaft of the engine whether due to a hill or added weight, or in the case of the generator which was used for the example earlier on in this thread, the added magnetic resistance to rotation due to a greater electrical demands of the load.

When the rpm's of the generator are reduced due to the above, the engine rpm's are reduced as well, which causes the governor to lessen its resistance to the spring which controls the throttle plate in the carb... this allows the plate to be pulled to a more open position and allows more air flow through the venturi, and thus a greater flow of fuel due to the increased air flow at the fuel port. This increase of available fuel and air in the chamber allows for an increase in RPM's, thus increasing the output power of the generator head, in addition to causing the governor to again over power the spring to the throttle plate.

If you plug the opening to the carb with your hand, the air flow drops, vacuum spikes, and the engine draws too much fuel and dies both from lack of oxygen and a flooded condition. This is also why a carburetor engine with a vacuum leak runs too lean.

Dipstick

[slogfilet]

Originally Posted by dipstick

Less vacuum does not = more load, it simply = greater air flow via less throttle plate restriction, or lower rpm's. Though the two happen simultaneously in most cases.
Dipstick

Does less vacuum = more load for a given rpm?

[user removed]

Lock your throttle to a specific position, say 25%.

Then climb a grade that is becoming more severe as you continue.

Your vacuum reading will decrease as the grade increases, until you reach 0 vacuum and your speed drops as the grade continues to increase.

Until you change your throttle position, or shift to a lower gear, you will continue to read 0 vacuum and your speed will decrease as the grade increases.

Increasing your throttle position, as long as you do not increase enrichment, will have no effect on your speed, unless you downshift and get your engines RPM higher where it can produce more power.

Vacuum is a measurement of load.

The existence of vacuum indicates that you are lowering the effective compression of your engine by reducing the amount of fuel-air mixture that enters the cylinders. At 0 vacuum you have max compression. At highest vacuum you have lowest compression.

An engine idles because you are choking off the air and fuel supply until it will only run that fast (idle speed).

regards
Mech

[Christ]

Originally Posted by Old Mechanic

Vacuum is a measurement of load.

Thank you.

[99prizm]

Well, my girlfriend drives the Prizm, and I don't think she cares as much about gas mileage, so I installed the auto meter vacuum gauge on my 2003 Nissan Frontier. It's a 4 cylinder 5 speed, and I've been getting about 22-23 on the freeway. I'll log the mileage for the next couple of tanks and see how much it helps. It's surprising how a little lift of the throttle brings up the vacuum without decreasing speed much. Helps you monitor how steady you hold the pedal as well. Probably will help if only because it almost makes you feel guilty giving it too much gas, the needle goes towards the red part of the dial haha!