Thread: MPG aids
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Old 12-13-2020, 02:52 PM   #19 (permalink)
Isaac Zachary
High Altitude Hybrid
Join Date: Dec 2020
Location: Gunnison, CO
Posts: 1,320

Avalon - '13 Toyota Avalon HV
90 day: 40.45 mpg (US)

Prius - '06 Toyota Prius
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Originally Posted by Charlie Cheap View Post
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 View Post
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 View Post
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 View Post
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 View Post
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.
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