how to read/drive by a vacuum gauge

[lifted4.0]

whats the trick to driving by a vacuum gauge, i hear that you wanna keep the vacuum as high as possible..but im wondering if having the vacuum at 20ish with rpms at 3k in 3rd gear CRUISING is bad?.....when your driving by a vacuum gauge, do the rpms matter? do you try to get into the highest gear possible while still maintaining the highest vacuum you can?? help me out! thanks

 

[Wicked_Sludge]

highest vacuum possible no matter what rpm or what gear it takes. depending on what motor you have, this likely means you'll be turning 3k rpm at highway speeds.

 

[Ranger5.0]

so whats this driving by a vacume gauge mean? and why would one want the highest vacume?

 

[Will]

so whats this driving by a vacume gauge mean? and why would one want the highest vacume?

It's a way to moniter your throttle foot. Every movement you make instantly flaps the gauge needle. It will help you improve your driving style.

I wouldn't necessarily say that always keeping the highest reading is the most efficient, but trying to follow that rule is a good place to start. With a carb, every movement of your foot is going to either dribble raw gas or activate the power circuit so it's best there to always keep the vacuum high. With EFI I think they say to get up to speed more quickly and get the system into feedback mode. At that point, I think they could tune out small, incidental throttle movements if they wanted and make them not matter, but I don't know if they do.

 

[lifted4.0]

soo if im trying to maintain 40mph on a flat surface, 4th gear can maintain it at 20inhg, and 5th gear can do it at like 18ish..maybe 17.....so going 40 mph, i stay in 4th gear?

 

[Will]

unless going in 4th gives you better mileage. I used it for more drastic drops. More like, you are going along at 15" and start up a hill and suddenly you are down to 8". Then you should downshift instead of making heat. But steady state and trying to read a 10% difference isn't worth the effort. The best value is in watching how steady you can keep your foot on the pedal--and how gradually you can accelerate when it isn't urgent that you go up 10mph. That's where the benefit is.

 

[Hahnsb2]

highest vacuum possible no matter what rpm or what gear it takes. depending on what motor you have, this likely means you'll be turning 3k rpm at highway speeds.

Not sure I agree. At 50 MPH for example in 2nd gear (5K rpms) with my 2.9 I could have 15HG/in of vacuum, same speed in 5th (1800RPMs) I had 10HG/in of vacuum, I'm sure I was getting better millage in 5th. I agree with Will.

 

[Wicked_Sludge]

you know what they say about assuming...

internal combustion engines achieve their best fuel economy at their peak torque RPM for a given throttle setting. most RBV engines make peak torque well over 3,000 RPM...so this is where you will have the best economy. this may mean going down the highway in 4th or even 3rd gear.

my 3.0 goes from 20mpg to 24mpg just by going from 5th gear to 4th gear at 60mph.

 

[rickcdewitt]

you know what they say about assuming...

internal combustion engines achieve their best fuel economy at their peak torque RPM for a given throttle setting. most RBV engines make peak torque well over 3,000 RPM...so this is where you will have the best economy. this may mean going down the highway in 4th or even 3rd gear.

my 3.0 goes from 20mpg to 24mpg just by going from 5th gear to 4th gear at 60mph.

look at the guys name,peak power is not over 3000

 

[MAKG]

look at the guys name,peak power is not over 3000

Huh?

Peak power is over 3000 RPM even on the slow clunker on my Chevy, with its 1 7/16 single barrel carb. Peak torque is more like 1200 RPM, though.

 

[rickcdewitt]

Huh?

Peak power is over 3000 RPM even on the slow clunker on my Chevy, with its 1 7/16 single barrel carb. Peak torque is more like 1200 RPM, though.

i wondered later if someone would give me a hard time for typing power instead of torque.