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Old 11-07-2020, 03:53 PM   #1 (permalink)
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carb engines and vacuum gauges

I posted a similar thread somewhere but it probably was in the wrong place. We had a 1975 Pinto V6 that had an economy light on the dash. I am sure it was a simple vacuum gauge that was adapted to the light for ease of reading. As the vacuum dropped the light got brighter or dimmer...I forget which, but it told the driver if the throttle was being pushed too far for best MPG. It was a great idea one could adapt to any carburetor fed engine. The gauge works but one must watch the needle while with a light, IT GETS YOUR attention.

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Old 11-07-2020, 04:01 PM   #2 (permalink)
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What's a carb?

Joking. It's all those potato chips I've been eating!

Light is a great idea for a carborated vehicle. These new-fangled cars have numbers, usually accompanied with a bar...when you light the whole bar up, you win!

An extra trip to the gas station, at least.

Carry on, nothing to see in my reply.
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Old 11-08-2020, 03:30 PM   #3 (permalink)
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Even though a vacuum gauge is often reporter to be more useful in a carburettor-fed engine, while it doesn't seem to make so much sense on newer EFI systems due to their electronic throttle bodies which position doesn't have a fixed ratio for each RPM or throttle pedal position, I remember having seen some fitted to vehicles which already had an EFI even though the throttle body was linked to the pedal through a cable.
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Old 11-09-2020, 09:02 AM   #4 (permalink)
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vacuum gauge

If memory serves, the old corvette Rochester mechanical FI worked like a carb as far as the throttle affecting vacuum.
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Old 11-09-2020, 01:20 PM   #5 (permalink)
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The vacuum guage serves mostly as a load indicator.

I always installed a vacuum gauge in all my carburetor equipped vehicles. It was most effective in our big 78' Dodge van with a 360 cubic inch V8 and a large spread bore ThermoQuad carburetor.

By fooling with various springs, we could delay the opening of the huge vacuum secondaries and drive around mostly with the tiny primaries. Only when the vacuum gauge hit a set reading would the secondaries open. A piece of tape indicated this point on the dial. It was great fun to putter around trying to keep out of the secondaries. The 8000 pound van was capable of 8-10 MPG factory. Just putting in the vacuum gauge and carburetor springs jumped us up to 12-14 MPG.
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Old 11-10-2020, 12:18 AM   #6 (permalink)
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vacuum gauge/ fuel consumption

I not sure what the question is ,but you can buy a modern day vacuum gauge (stepper motor drive)which has a warning Led , that you can set the vacuum level at which it illuminates ,same as a shift light on a High performance Tachometer.

Real world long term road testing of various carburetor vehicles( I travelled 100,000 kms /year,during the late 90's through 2009) ,I found for best economy , avoid opening the power valve ,although you need to know the opening point(inches Hg)
If you open the throttle just above this opening point ,engine pumping losses are reduced as well as fuel consumption.

Best power is generally thought to be about 8%(12.5:1 afr) gasoline to air(power valve open) & best economy is about 6.25%(16:1 afr) gasoline to air(power valve closed ,although these figures are from non emission engines.

The power valve can be open without the secondary barrel(s)opening so careful use of a vacuum gauge ,while knowing the power valve opening point can reduce fuel consumption.

hope this helps
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Old 11-16-2020, 08:20 PM   #7 (permalink)
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I have a vac gauge - right next to the A-pillar as it happens, giving my geriatric old truck a grossly misplaced bit of boy racer vibe - that got wildly out of calibration shortly after installation. I re-timed it so it was correct and it went wildly out again. So my truck is evidently pulling 10" vacuum sitting still, with the engine off.

In terms of concrete numbers therefore it's useless, but still it gives instant feedback on minute changes of throttle position. So as far as that goes it gives a visual feedback on inputs that the pantsometer can't detect.


Lead or follow. Either is fine.
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