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Vacuum Gage Monitoring

Why is monitoring the vaccum gage the best way to monitor fuel efficiency? How does one do it?

High manifold vacuum = small throttle opening, which means a light foot on the gas. This generally results in the highest fuel economy, pumping losses notwithstanding.

Is this the best way? The teachers from the local Communinty College recommended attaching a gage somehow and then driving to keep that small. So you are saying that they just want to minimize the quantity of fuel being burned and that should translate to maximizing your distance v. fuel burn?

In the old days of carburetor-equipped vehicles, the objective was to get the greatest vacuum reading at a particular throttle position. As the pistons move up and down, a partial vacuum is created. The faster the pistons move in the cylinders, the greater the vacuum. However, you decrease the vacuum as you open the throttle plate. To increase the speed requires a greater fuel/air mixture. The higher the rpm that can be obtained at a given throttle setting, the higher the efficiency.

In the old days of vacuum operated windshield wipers, you could observe this phenomenon. When the engine was idling, the throttle plate would be closed and the vacuum would be high. The wipers would be operating a a good clip. As one accelerated from a stop, the throttle plate would be open and the vacuum would fall off. The wipers would practically stop on the windshield. As the vehicle came up to speed and the driver eased off the accelerator, thus closing the throttle plate, the vacuum would be somewhat restored and the wipers would move slowly back and forth across the windshield.

I don’t know how effectively the vacuum gauge would perform on a fuel injected or a diesel engine. The trucks I have driven that had a vacuum gauge also had a vacuum assist on the brakes. I think the purpose of the gauge was to warn the driver if the vacuum to the brake booster was low. WIth today’s engines, there are better ways to boost fuel efficiency–specifically by using sensible driving habits.

It wouldn’t work at all on a diesel: there is no throttle plate and therefore no appreciable manifold vacuum. The strategy would work fine on all fuel-injected gas engines, except for some throttle-less ones that I think BMW was tinkering with.

Use of a vacuum gauge might have been useful a generation ago but even then it was a particularly crude method. It could only verify what every good driver knew by instinct – if you accelerate your fuel economy goes down.

Nowadays, with electronics sensors throughout and several computers per car, a modern auto has far better ways of monitoring fuel efficiency.

I use the fuel economy readout in my Pacifica. But many cars don’t have this gizmo, and a vacuum gauge is fairly easy to install, and while it won’t give actual fuel consumption, it’s a lot better than nothing.

Of course all fuel savings and more will be eaten up if you crash because you were focusing on the gauge and not on the road.

Nowadays, with electronics sensors throughout and several computers per car, a modern auto has far better ways of monitoring fuel efficiency.

You are right! However, I would bet a higher percentage of younger drivers don’t understand what is happening under the hood. We old geezers understood what was happening and instinctively knew how to drive for economy.