Relationship between vacuum and FE? New questions
 

Can someone please help me out with the relationship between vacuum and FE? I'm not really sure exactly what vacuum's function is in an engine either. IIRC low vacuum is the engine running more efficiently right? I have an idea for a mod/improvement but I'm not quite sure how to implement it. I wanted to ask about it, but need to understand how vacuum works and the terminology first.

Basically, I want to temporarily hook up a vacuum gauge and see what RPM range my engine likes. The "sweet spot" so to speak. Then some fancy ciphering using my tire size and transmission gear ratios and various rear end gears to try and get as close to that "sweet spot" at 55-60 mph in 4th gear and 65-70 in 5th gear. Maybe Ford has already done that, I don't know.

I guess it all starts with finding out where my engine is happiest. That's where learning about vacuum comes in.

Thanks

Apart from some older ignition systems that relied on vacuum to variate the timing, I don't remind any other utility for it in the engine. Well, on the other hand it does work as a power source for brake servos and other auxiliary systems...


Low vacuum is a result of fewer restrictions at the intake, such as a wide-open throttle valve, thus the engine in fact runs more efficient because it has to do fewer work to intake a given amount of air (leading to smaller "pumping losses").

Your engine sucks in air as the piston moves down. Same way as you suck in air as your diaphragm contracts to make your lungs more voluminous.

Gasoline engines run at a fairly fixed air fuel ratio. So every bit of air has a bit of gasoline injected to it and the ratio is constant.

So the wat the engine power/speed is controlled is by a valve to limit the incoming air. (Valve is connected to the gas pedal.
You can imagine this as you wanting to run like Forest Gump but some one controls your speed by closing your mouth.


Obviously trying to breathe with a semi closed mouth is not optimal because you have to suck in air through a restriction which wastes energy. Same applies to an engine. The vacuum gauge is measuring the air pressure in your mouth. The less it is, the more efficiently the engine runs.

Thank you Cripple Rooster.

My thoughts were this.....

Let's assume that my engine really likes the 2000-2200 rpm range verified by my temporary vacuum gauge hook up. 55 mph in 5th gear gives me this range with my current tire size (already 5% larger than stock) and rear gear (3:73). 15 miles of my drive every day is at 65-70 mph. Even in 5th gear it's nearly 3000 rpm and I'm sure not very fuel efficient. If I do some math I might find that swapping to say a 3:55 or 3:45 rear gear may give me the magical 2100 rpm range at 55 mph but in 4th gear rather than 5th, and the same 2100 rpm range at 65 mph in 5th gear. This would give me optimal FE for the whole drive to work except for the short in town bits. Maybe I'd get lucky and 3rd gear would be my <40 mph gear. I just need to find the optimal rpm range for each gear and speed and then get out the pencil and paper.

Thanks teoman.

Too bad it's not easy to find a dual-speed rear axle for a Ranger, isn't it? BTW doesn't your '03 have an electronically-controlled throttle plate instead of a cable-operated one? Maybe that could allow you to get it tuned to remain wide-open at a broader RPM band.

Please review:

- Classic-Car-Magazine - Tuning with a Vacuum Gauge

Great description, teoman.

On the point of vacuum control in general... My understanding is that the Chevy ecotec turbo engines are vacuum controlled somehow. I learned this at a Chevy dealership, looking over a running and partially disassembled 2012 Chevy Sonic 1.4L turbo that I was considering buying and that had just thrown a code for the vacuum system.

Apart from power brakes, I can't remember any other vacuum-dependent feature on the new cars in general.

The lowest speed in the highest gear generally nets the highest FE....what's with trying to complicate things with a vacuum gauge?