Thread: MPG aids
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Old 11-08-2020, 10:11 AM   #13 (permalink)
Ecky
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ND Miata - '15 Mazda MX-5 Special Package
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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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