plenum vacuum and efficency
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From: mandan nd
plenum vacuum and efficency
Ive got a 75 f250 supercab with a mostly bone stock 360 topped with a 2V. It has a funny idiot light that reads "low fuel economy" when the vacuum drops below approximately 6 inches. I tested this with one of those hand operated vacuum pullers. How low is 6 inches really? I dont have a vacuum gauge on the truck (yet) so i am not sure what it really has running down the road. I know i can make the light come on clear through 3000 rpm. Ultimately i am wondering whether or not it pays to go to a larger 2V or switch to a 4V set up.
FTE Leadership Emeritus
Joined: Jul 2002
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From: Long Island USA
Is this an add-on light?
My father used to run a vacuum gauge mounted on the steering column of all his cars. He liked to keep an eye on it and keep it as high as he could while driving to keep the fuel economy as high as possible. I think I still have that gauge around somewhere
Anyway, it was an old way to try to drive "economically" and doesn't really mean a hill of beans on a truck that size.
What happens is the more you open the carb plates, the lower the vacuum. The faster you accelerate (open the carb plates) the more gas you use.
Lots of things affect the vacuum at any given RPM and carb opening. Timing advance, cam timing, compression (or lack thereof), etc.
Does it run well? And move?
My father used to run a vacuum gauge mounted on the steering column of all his cars. He liked to keep an eye on it and keep it as high as he could while driving to keep the fuel economy as high as possible. I think I still have that gauge around somewhere
Anyway, it was an old way to try to drive "economically" and doesn't really mean a hill of beans on a truck that size.
What happens is the more you open the carb plates, the lower the vacuum. The faster you accelerate (open the carb plates) the more gas you use.
Lots of things affect the vacuum at any given RPM and carb opening. Timing advance, cam timing, compression (or lack thereof), etc.
Does it run well? And move?
Cargo Master
Joined: Apr 2004
Posts: 2,574
Likes: 4
From: Golden, CO
My '74 had that light, it was somewhere in the instrument cluster. It was disabled by the time I got it. Sadly, I had the cluster out of the dash to replace some lights and the plastic shell crumbled in my hands.
People used to give me astonished looks when I told them my 390 got 10 to 12 MPG. After all, that was a 25% increase in fuel economy!
People used to give me astonished looks when I told them my 390 got 10 to 12 MPG. After all, that was a 25% increase in fuel economy!
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
There are other factors too, the motorcraft carburetors had different power valves depending on the general altitude, in the same way they had different jetting for altitude. If a flatlander drives his carbureted truck to say, Colorado, the overall manifold vacuum will drop quite a bit on the high passes. You'll see raw fuel drizzling out of the tailpipe. The power valve will never hardly close. This won't do much for economy.
Cargo Master
Joined: Apr 2004
Posts: 2,574
Likes: 4
From: Golden, CO
Ive got a 75 f250 supercab with a mostly bone stock 360 topped with a 2V. It has a funny idiot light that reads "low fuel economy" when the vacuum drops below approximately 6 inches. I tested this with one of those hand operated vacuum pullers. How low is 6 inches really? I dont have a vacuum gauge on the truck (yet) so i am not sure what it really has running down the road. I know i can make the light come on clear through 3000 rpm. Ultimately i am wondering whether or not it pays to go to a larger 2V or switch to a 4V set up.
Cargo Master
Joined: Apr 2004
Posts: 2,574
Likes: 4
From: Golden, CO
There are other factors too, the motorcraft carburetors had different power valves depending on the general altitude, in the same way they had different jetting for altitude. If a flatlander drives his carbureted truck to say, Colorado, the overall manifold vacuum will drop quite a bit on the high passes. You'll see raw fuel drizzling out of the tailpipe. The power valve will never hardly close. This won't do much for economy.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Nope, it was a factory option in '75
More than likely it wasn't disabled, it was never hooked up in your truck. This was a '75 only option. Your truck probably had a swapped in instrument cluster.
The light was operated by a vacuum switch (D5TZ-10A922-D) that was mounted on the firewall.
The "low fuel economy" instrument cluster jewel (D5TZ-10A936-D) is listed in the parts catalog as '75 only, same as the vacuum switch.
Just out of curiosity I checked, Green Sales has 146 (!) of the vacuum switches and 1 of the instrument cluster jewels.
The light was operated by a vacuum switch (D5TZ-10A922-D) that was mounted on the firewall.
The "low fuel economy" instrument cluster jewel (D5TZ-10A936-D) is listed in the parts catalog as '75 only, same as the vacuum switch.
Just out of curiosity I checked, Green Sales has 146 (!) of the vacuum switches and 1 of the instrument cluster jewels.
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