What is the deal with Vacuum reading ??

[ST2008]

Hi all, I'm new here, so please be kind with my ignorance.

I try to find a explanation about the vacuum reading and the mpg. But there are so many data that I'm lost.

So, can someone explain to me what is the deal about the Vacuum and the mpg ? and what is their relationship ?

Thx

[oil pan 4]

The relationship is dependent on the engine and drive line.
Vacuum developed tells you how inefficient your engine is.

The idea is a vacuum reading too close to atmospheric pressure on a normal engine means you are wasting fuel.
I think the idea is you put the car in top gear and the vacuum gauge acts like an engine load indicator.

[redpoint5]

The power and speed of regular gasoline engines is controlled by restricting air to the cylinders by varying the throttle, which is a valve that restricts airflow to the engine. Whenever the throttle is in any position other than fully open (floored), it is restricting air into the engine. This restriction causes a difference in pressure between the atmosphere (about 14.7 psi) and the combustion chamber as the intake stroke tries to draw air into the engine (a vacuum is created). The less the throttle is opened, the more vacuum there is (difference in pressure).

Visualize the engine as a syringe with no needle on it. As you pull the plunger back, a vacuum is created which is immediately filled by the surrounding atmospheric pressure. If you were to restrict air from entering the syringe by placing your finger over the opening, you would feel the resistance of the vacuum as you pull the plunger back. It takes extra effort to pull the plunger against the vacuum created by blocking the opening to the syringe.

In the same way, energy is lost in an engine when the piston intake stroke works against the throttle, which is partially blocking air from entering. High vacuum (not pressing the accelerator pedal much) is an inefficient way to run an engine since it is working to overcome the vacuum.

This is a big reason why powerful cars are not fuel efficient. If you have a 500 horsepower car, but only need 20 horsepower to steadily cruise down the street, the throttle is barely being opened to supply the needed power. The other 480 horsepower is being restricted by the vacuum created, and the engine is working against that huge vacuum.

This is also why the "pulse and glide" technique is good for fuel economy. It maximizes fuel economy by more fully opening the throttle valve, which reduces the vacuum that the engine has to work against. The vehicle accelerates during the efficient "pulse" phase, and then coasts in the "glide" phase.

As an aside, electric vehicles don't suffer such a huge efficiency penalty for being powerful. A 500 horsepower EV can be nearly as efficient as a 100 horsepower EV. No vacuums to overcome!

People refer to the accelerator as the gas pedal, but it is more accurately an air pedal. The fuel is just added in proportion to the amount of air that is allowed into the engine.

In summary; high vacuum is inefficient (accelerator not pressed very much), and low vacuum is efficient (accelerator almost all the way pressed). The 2 biggest reasons why we don't run an engine at the most efficient throttle opening at all times is that:

1. It's not safe to drive that fast
2. Efficiency drops as speed increases due to exponentially increasing wind resistance

[ST2008]

OK, but why some state that we should keep vacuum as higher as much possible in order to have higher mpg.

For myself, the vacuum shows in Torque pro are 21psi for idling and 8 to 12psi for driving in a freeway (60mph). And I have no clue how to control the vacuum.

[nemo]

Drive for the highest indicated instant mileage. Torque Pro has that feature correct.

[redpoint5]

Originally Posted by ST2008

OK, but why some state that we should keep vacuum as higher as much possible in order to have higher mpg.

For myself, the vacuum shows in Torque pro are 21psi for idling and 8 to 12psi for driving in a freeway (60mph). And I have no clue how to control the vacuum.

Are you referring to the MAP gauge? It's impossible to have more than about 14.7 PSI of vacuum because that is the density of the air. The only way to have more vacuum is to have denser air.

Vacuum is simply the difference in pressure between the atmosphere and the intake manifold.

Engine load is inversely related to vacuum, meaning the more of one you have, the less you have of the other. This generally means that a higher engine load is more efficient than running a lower load. Could engine load be what people are referring to when they say you want to keep it high for fuel efficiency?

So far we have been completely ignoring another very important factor in fuel efficiency, which is engine speed (RPM).

You need a BSFC map for your particular engine to know exactly what RPM and engine load is most efficient.

All of this is mostly irrelevant if you aren't using the pulse and glide method, as your engine speed is mostly dependent on how fast you need to travel, or how quickly you need to accelerate.

[ST2008]

I have both, the MAP sensor and the vacuum boost, in my Torque Pro. If I turn the key at On, but not starting the engine, the MAP sensor reading is 14.5 psi (atmospheric pressure) and the Vacuum indicates 0.4 psi. Not sure why then vacuum is not zero.

As for the engine load, I'm not sure if I understand what do you mean. For myself, I always think that the car is more efficient if I can drive it with less engine load for the same speed. Am I wrong ?

[elhigh]

Originally Posted by ST2008

So, can someone explain to me what is the deal about the Vacuum and the mpg ? and what is their relationship ?

Thx

Engine vacuum is generated by the pistons descending in the cylinders, pulling in air. But at less than wide open throttle, there is resistance and the cylinder is pulling a vacuum behind the throttle plate. Generally speaking, there is a direct relationship between how deep a vacuum vs. how much load there is on the engine: more vacuum means a smaller throttle opening, means you are asking less of the engine, means less fuel burned.

NOTE: not being controlled by a throttle, this relationship does not exist in vehicles with diesel engines.

Vacuum readings are only relevant to the car in which they are taken. 15" vacuum in one car doesn't mean the same thing in someone else's car also showing 15" vacuum.

The vacuum gauge's main advantage to a hypermiler is showing, moment to moment, approximately how much demand he is placing on the engine and allowing him to alter his behavior in that moment to maximize his fuel economy.

[Ecky]

I feel redpoint5 and elhigh have very thoroughly explained how vacuum works. For practical purposes though, all else being equal, more vacuum means you're demanding less power from the engine, and thereby using less fuel. Although your engine is less efficient at lower load (higher vacuum), you're still burning less fuel.

Ideally, you could shift to a taller gear, thereby increasing load and decreasing vacuum, but transmissions have limits in their maximum gearing. In the same gear, higher vacuum means you're burning less fuel, even if efficiency is lower.

[Gasoline Fumes]

I don't use a vacuum gauge, but I think you want to cruise at low load/high vacuum, and accelerate at the highest load/lowest vacuum you can get without going into open loop. Keeping RPM low while accelerating will reduce vacuum too.