MPG aids

[Charlie Cheap]

Originally Posted by jakobnev

How/when is the vacuum gauge used really?

I'm thinking on highway cruise for example, without a gauge you'd give it enough gas to not lose speed, what would you do differently if you had a gauge to look at?

jakobnev, at highway speeds one can't always tell the road has started to climb, and one's foot is applying more gas, but the gauge will automatically remind you. In much the same way a cruise-control can't foresee the top of a hill, but the driver can let-off before reaching the top to save gas. It became a habit to watch the light (gauge) in our Pinto and MPG did improve. Understand, I am looking at ways to improve MPG in older cars with more modern engines but not necessarily Fuel Injection. My Sunbeam has a 1985 Ford 2.3 with overdrive automatic. The Autolite 2100 carb invented annular atomization which did the best job breaking up fuel droplets into vapor, which increased MPG, HP, and Torque...and with just 5 lbs. fuel pressure.

[2016 Versa]

When I drove the '88 Escort I had a vacuum gauge mounted on the steering column. All the cars I drive now are OBDII so use either a Scan Gauge or Ultra Gauge in them.

[Ecky]

Even with fuel injection, vacuum can have some use. My car uses MAP based fueling and timing tables. By looking at vacuum, I can infer load. Above a certain pressure the computer starts fuel enrichment and pulls timing advance, so when I'm driving for economy, I stay below that manifold pressure.

It's also useful when tuning. Although in an "all else equal" scenario higher vacuum is a direct indicator that less fuel is being used, vacuum itself is an inefficiency. Take for example if I advance (or retard) the intake cam's phasing a few degrees and vacuum decreases at the same road speed, I don't need to look at the fuel economy gauge to know that fuel economy is (marginally) improved. Less fuel is being wasted creating vacuum.

Honda's R series engines use exactly that. At highway speeds, where throttle changes aren't rapid and frequent, the engines open their throttle butterflies completely and start phasing the intake cam to control engine power, closing the valves when just enough air has entered to produce the power needed. Manifold vacuum is basically zero. I can't recall offhand how much fuel this saves but it's significant.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

Even with fuel injection, vacuum can have some use.

Most of the Japanese vehicles in the '90s had a lot of vacuum-based input for the EFI. I remember being called by a client of my father to take a look at his nephew's '98 Mitsubishi Pajero which had some random failure no-one was able to troubleshoot. I didn't find out what went wrong, but a few days later I've been told it was a replacement hose which couldn't handle the vacuum it was supposed to route.

[Stubby79]

MAP sensors are still there...as back-up and/or verification of MAF input. MAFs are more accurate than, so better for emissions and for running an engine at peak performance safely. AFAIK anyway.

[Charlie Cheap]

Manifold Absolute Pressure / Mass Air Flow / and Knock sensors can control a lot but not necessary with a simple points ignition and carb fuel system. No doubt, with drastic differences in fuel grade from country-to-country, a good computer with many references, is great. I remember drivers talking about such things years ago, but not sure if they still exist. In Texas I am fortunate. My only problem is staying away from too much timing advance, so I limit it to 36 degrees maximum, all in by 2900 rpm. These are the numbers I used with my 6-cylinder 1965 Mustang that worked great...with a carb. I do have a vacuum gauge to put in the dash and will probably mount it right in front of the driver between the Tach and Speedometer. Building the suspension now. The Bearings, bushings, shocks, springs, steering, etc. are mounted to the front crossmember...which I have removed.

[Isaac Zachary]

When I had the VW Type 1 I installed a CB MagnaSpark electronic distributor. It had a built in MAP sensor, and I could hook up a laptop and see it work from inside the cab. I could also change both vacuum advance and RPM advance curves on the fly.

If I ever got a car with a carburetor and distributor again, I'd get a CB MagnaSpark box.

With the Bug I was able to advance clear up to a whopping 45° without any pinging that I could tell. Engine was peppy and I never got under 30mpg on trips. And all trips were over mountain passes.

One thing to note is that vacuum gauges and MAP sensor readouts aren't always what you think. Best fuel mileage isn't highest vacuum like some gauges tell you. It's not even highest spark advance. Best fuel mileage is usually the vacuum level right before secondaries open up and the carb goes rich. (I had a stock carb on the Bug that didn't have mechanical or vacuum secondaries, but I did also have an O2 sensor and AFR gauge to see when it would enrich for high load).

If you want the best fuel mileage with a vacuum gauge, first figure out how much vacuum it takes to open up your secondaries or power enrichment circuit. (If it's mechanical maybe you need a gauge that shows throttle position.) Mark a line there when the secondaries open and while driving open up the throttle to just before that line and get up to speed. Then let off the throttle, throw it into neutral, and shut off the engine and glide. Getting down to a slow speed, hit the clutch, put the stick back in gear and pop it and start all over again.

[Charlie Cheap]

Isaac Zachary, my 2-barrel does not have a secondary barrel but is a basic 2-barrel Autolite 2150 on the 4-cylinder I am building and a 2100 on my Mustang 6-cylinder. Vacuum is a very good indication of load with such a carb and with my cam making maximum Torque in the 3000 rpm range. No doubt I could improve MPG with a computer reading the sensors new motors use, but THAT is what I am trying to avoid. My purpose is to find maximum economy WITHOUT any electronics or computer controls. I think today's electronic ignitions main reason for existing is because they can be matched to to those sensors for emissions purposes. The dual-vacuum canister on older ignitions was NOT to improve economy but to make idling engines run hotter...for emissions purposes. Way back in the mid 70's my 32 Ford coupe was tested by Baylor University with a modified 289 V8 Ford engine and NO smog devices except a PCV valve. It met emissions standards for 1978 in 1974. A well tuned engine with basic improvements (Hot Ignition 'yes points', Proper Timing, Mild Cam, Carb matched to engine size, Free Flowing Exhaust, Cool-Air intake) can make more horsepower and run cleaner than when factory new, with these upgrades. Putting that engine in a car 1000 pounds lighter and using a .75 overdrive with lockup torque converter, I hope for well into 30's MPG...then I'll try for low 40's. THAT is my objective with ZERO computers.

[Isaac Zachary]

Originally Posted by Charlie Cheap

Isaac Zachary, my 2-barrel does not have a secondary barrel but is a basic 2-barrel Autolite 2150 on the 4-cylinder

Nor did my stock Solex carb on my VW Type 1. It didn't even have a mechanical or vacuum operated power jet. The power jets were just located higher in the choke area and activated when there was enough air flow to pull fuel up that high. But regardless, the idea is the same. The difference is that if your power jet doesn't work with a mechanical or vacuum actuator, whether it has it's own barrel or not, then it's harder to determine when high load enrichment comes in without an O2 sensor. But if it does have a vacuum or mechanically actuated power jet you can then figure out what will cause it to open without any need of an O2 sensor. Again, this is whether or not the power jet has it's own barrel. All carbs have a power enrichment circuit of some sort, the question we need to answer is what activates it and when.

Originally Posted by Charlie Cheap

Vacuum is a very good indication of load with such a carb and with my cam making maximum Torque in the 3000 rpm range.

Of course vacuum is a very good indication of load. But what I was saying is that a lot of people, as well as what's written on cockpit installed vacuum gauges, assume that less load equals better efficiency. But the opposite is true. You want to be under high load (up to a point) to get better fuel mileage.

This is why smaller engines and taller gears tend to get better fuel mileage. It's because they are under a higher load most the time. Higher load means the throttle is open more which means there is less vacuum which means there are less pumping losses and also means the fuel/air charge is more dense, all leading to better efficiency.

This is why knowing when your power fuel circuit activates is so important. For best fuel mileage you want to use as high of a load as possible without activating it.

Originally Posted by Charlie Cheap

No doubt I could improve MPG with a computer reading the sensors new motors use, but THAT is what I am trying to avoid. My purpose is to find maximum economy WITHOUT any electronics or computer controls. I think today's electronic ignitions main reason for existing is because they can be matched to to those sensors for emissions purposes.

I didn't put those electronic sensors and gauges on my 1971 VW for emissions purposes. The better your instrumentation the more accurate your numbers are and the better you can fine tune things to get better fuel mileage. Or emissions. Or power. Or some happy medium between the three. But it doesn't have to be one or the other.

I was bummed it's so expensive to put the engine on a dyno. I wanted to set the engine to every load and RPM combination possible and find the best carb jetting and vacuum/RPM advance curves to get the best fuel mileage. Just using the seat of your pants and adjusting from time to time will not get you as fine of a tune-for-efficiency as you can with better instrumentation.

But that's not to say you have to have such fancy instrumentation. If you want to go old school, go right ahead! That's perfectly fine too. It's just going to be more difficult to get done what you can do with finer measuring tools.

Originally Posted by Charlie Cheap

The dual-vacuum canister on older ignitions was NOT to improve economy but to make idling engines run hotter...for emissions purposes.

I know that, but wasn't talking about that. VW had dual vacuum distributors that ran as much as 5° ATDC. My 2013 Avalon will run as retarded as 25° ATDC when warming up. Sure, it warms up the catalytic converter much quicker, but it sure takes a big gulp of fuel doing it.

I was talking about vacuum actuated power fuel circuits. All carbs have a power fuel circuit, and a lot have one that's actuated by vacuum since it's a good indicator of load (as you already said). Basically you floor the gas pedal, the intake manifold loses vacuum and the vacuum canister on the carb opens up and lets the power fuel circuit dump more fuel in to enrich the AFR way rich. (It may or may not have it's own secondary barrels.) For best efficiency and fuel mileage you want to avoid that activating. So, hence my suggestion of finding out at what vacuum it activates and marking that on your vacuum gauge. That way on one side is leaner, more efficient running, and the other side richer, less efficient running. Again idealy you want as much load (least amount of vacuum) you can get without activating the power fuel ciruit.

Originally Posted by Charlie Cheap

Way back in the mid 70's my 32 Ford coupe was tested by Baylor University with a modified 289 V8 Ford engine and NO smog devices except a PCV valve. It met emissions standards for 1978 in 1974. A well tuned engine with basic improvements (Hot Ignition 'yes points', Proper Timing, Mild Cam, Carb matched to engine size, Free Flowing Exhaust, Cool-Air intake) can make more horsepower and run cleaner than when factory new, with these upgrades. Putting that engine in a car 1000 pounds lighter and using a .75 overdrive with lockup torque converter, I hope for well into 30's MPG...then I'll try for low 40's. THAT is my objective with ZERO computers.

Sounds like a great project! And perfectly fine to do without computers.

To sum up all that I said, the average vacuum gauge is falsely designed. It should start out as 0% efficient at very high vacuum, move to higher efficiency (5%-10%-15%-20%-25%-30%) as you get less vacuum, then at the point the power fuel circuit opens it goes back the other way (25%-20%-15%). That's what should be printed on your vacuum gauge.

[Isaac Zachary]

After doing a little research, your 2 barrel Autolite 2150 carb does indead have a vacuum actuated power valve.

Again, the question isn't if more or less vacuum is better or worse for efficiency?, but rather: At what vacuum level does the power valve go from closed to open. Figuring that out and marking that line on your vacuum gauge will be of better help than just going by your gauge's generic "poor/good" fuel mileage drawings.