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Old 11-08-2020, 09:41 AM   #11 (permalink)
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Originally Posted by jakobnev View Post
How/when is the vacuum gauge used really?

I'm thinking on highway cruise for example, without a gauge you'd give it enough gas to not lose speed, what would you do differently if you had a gauge to look at?
jakobnev, at highway speeds one can't always tell the road has started to climb, and one's foot is applying more gas, but the gauge will automatically remind you. In much the same way a cruise-control can't foresee the top of a hill, but the driver can let-off before reaching the top to save gas. It became a habit to watch the light (gauge) in our Pinto and MPG did improve. Understand, I am looking at ways to improve MPG in older cars with more modern engines but not necessarily Fuel Injection. My Sunbeam has a 1985 Ford 2.3 with overdrive automatic. The Autolite 2100 carb invented annular atomization which did the best job breaking up fuel droplets into vapor, which increased MPG, HP, and Torque...and with just 5 lbs. fuel pressure.

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Old 11-08-2020, 11:10 AM   #12 (permalink)
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When I drove the '88 Escort I had a vacuum gauge mounted on the steering column. All the cars I drive now are OBDII so use either a Scan Gauge or Ultra Gauge in them.
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Old 11-08-2020, 11:11 AM   #13 (permalink)
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Even with fuel injection, vacuum can have some use. My car uses MAP based fueling and timing tables. By looking at vacuum, I can infer load. Above a certain pressure the computer starts fuel enrichment and pulls timing advance, so when I'm driving for economy, I stay below that manifold pressure.

It's also useful when tuning. Although in an "all else equal" scenario higher vacuum is a direct indicator that less fuel is being used, vacuum itself is an inefficiency. Take for example if I advance (or retard) the intake cam's phasing a few degrees and vacuum decreases at the same road speed, I don't need to look at the fuel economy gauge to know that fuel economy is (marginally) improved. Less fuel is being wasted creating vacuum.

Honda's R series engines use exactly that. At highway speeds, where throttle changes aren't rapid and frequent, the engines open their throttle butterflies completely and start phasing the intake cam to control engine power, closing the valves when just enough air has entered to produce the power needed. Manifold vacuum is basically zero. I can't recall offhand how much fuel this saves but it's significant.
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Old 11-08-2020, 04:13 PM   #14 (permalink)
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Originally Posted by Ecky View Post
Even with fuel injection, vacuum can have some use.
Most of the Japanese vehicles in the '90s had a lot of vacuum-based input for the EFI. I remember being called by a client of my father to take a look at his nephew's '98 Mitsubishi Pajero which had some random failure no-one was able to troubleshoot. I didn't find out what went wrong, but a few days later I've been told it was a replacement hose which couldn't handle the vacuum it was supposed to route.
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Old 11-09-2020, 04:37 AM   #15 (permalink)
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MAP sensors are still there...as back-up and/or verification of MAF input. MAFs are more accurate than, so better for emissions and for running an engine at peak performance safely. AFAIK anyway.
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Old 11-17-2020, 05:53 PM   #16 (permalink)
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vacuum

Manifold Absolute Pressure / Mass Air Flow / and Knock sensors can control a lot but not necessary with a simple points ignition and carb fuel system. No doubt, with drastic differences in fuel grade from country-to-country, a good computer with many references, is great. I remember drivers talking about such things years ago, but not sure if they still exist. In Texas I am fortunate. My only problem is staying away from too much timing advance, so I limit it to 36 degrees maximum, all in by 2900 rpm. These are the numbers I used with my 6-cylinder 1965 Mustang that worked great...with a carb. I do have a vacuum gauge to put in the dash and will probably mount it right in front of the driver between the Tach and Speedometer. Building the suspension now. The Bearings, bushings, shocks, springs, steering, etc. are mounted to the front crossmember...which I have removed.

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