Quote:
Originally Posted by Hersbird
Ironically I have an old "fuel economy" gauge that runs on vacuum. I think it's for a 64 Pontiac Bonneville console, it's really a pretty thing. Anyway it goes from 24 to zero with a secondary color bar code under that goes from economy to power, economy being the 24 side and power being zero. So on a 64 big block Pontiac they figure the more vacuum you have the more economy you are running, based on the throttle being closed, and the less vacuum the more power you are making. It doesn't really consider that the motor is operating with better thermal efficiency with the throttle open because with the throttle open on a big block carbed Pontiac you will be going 100mph in a 1/4 of a minute or so and you can probably watch the gas gauge go down. In that case high vacuum, throttle closed as much as possible is where you get the best mpgs.
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What a beautiful gauge! Almost the first sight one encountered on opening the door to an early Grand Prix (its late enough at night I can recall the scent of new).
Vacuum inches (beyond diagnostic issues from article linked by freebeard) was how we’d travel the highway and determine downshift points on grades substantial enough to warrant a downshift. Especially, when towing.
One watched this gauge versus the tachometer: as vacuum declined, downshift early to keep HP high. But not so early as to burn fuel unnecessarily. (Too late and one was relegated to a significantly slower uphill pace; and at lower rpms where heat control was more difficult) It wasn’t (and still isn’t) about speed to the top of the grade. It’s about using power (fuel) to keep engine strain (load) at a peak of no more than 80%.
Long life trumps any momentary FE concern.
A Pontiac 389 or 428 had to work pretty hard. Just didn’t breathe so well up top and under a load. Except Buick, what it shared with other GM. Great off the line, though.
You sure could surprise a guy back then when you could out-drive him this way. Understanding engine (drivetrain) architecture predicted performance pretty well.
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