Re: Improve Gas Mileage Without Damaging Your Car

I had got the impression that open throttle (and low rpm) yielded best efficiency. Would could care to explain/reference to the half/ 3/4 throttle reasoning for best efficiency? (or are you including auto trans effects with respect to a torque converter and gear change timing?)

Martijn

With due respect to the original poster, there is a fairly narrow band of conditions which yields best economy. The manifold vacuum gauge helps the driver stay within that band....but is is not,of itself, a device which improves gas mileage.

For each car, there will be an optimum operating range. It is hard to say that it will come at any particular throttle opening, especially since there is considerable difference in engines, injected or carbureted.

NONE of the devices tested (and sometimes quoted as having improved mileage) have ever shown statistically significant improvement in fuel economy, over the standard engine operating within specifications.

The freaking magnet fraud is one of the more prevalent scams. Worthless as teats on a terrapin.

Sure. low manifold-absolute-pressure (high vacuum) helps vaporize the fuel. Under true full throttle the MAP is nearly atmospheric and the fuel does not vaporize in the manifold as well. Thus carburetors had "power enrichening" circuits that richened the mixture at above about

70-80% throttle opening. I know TBI does the same thing. I don't know about PI but suspect that there is still some enrichening. So the best efficiency comes at the widest throttle before power enrichening.

The power enrichening is a drivability thing. The engine would still run, but can miss, and throttle response is very bad with the lean mixtures that result from wide open throttle. Also, the mixture can go so lean that efficiency would actually suffer.

A lock on the gas cap to keep your neighbor's kid from siphoning your gas at night is a remarkably effective one. I had a car go from 15 mpg to nearly 25 mpg with this effective expedient.

--scott

Actually one device did: Pass Master, an A/C compressor cut-off, improved city mileage by 4% in EPA testing done several decades ago. But since it was just a simple vacuum switch, the A/C clutch would chatter at every transmission shift, which was probably why the contacts in my Pass Master burned out in a month. Sears gave me a refund, and I built my own cut-off with a 2-second turn-on delay that eliminated the chatter.

There's no benefit in adding a similar device to any modern car since it has it built into the computer programming.

This discussion is nuts. Engine efficiency doesn't mean good gas mileage. Accelerating a vehicle is like driving up hill. The faster you accelerate the steeper the hill. Now it may be true that your engine is running more efficiently when you go up a steep hill but it doesn't mean you are getting good gas mileage. Accelerating slowly requires much less energy than accelerating quickly. As a result accelerating slowly will give you better gas mileage even if your engine isn't running as efficiently. Same thing with driving fast - your engine may run more efficiently at 70 mph, but you will still get better gas mileage at 50 mph simply because it takes a lot more energy to drive at 70 mph.

The only way to get excellent gas mileage is to buy a car with a very small engine. That way you can drive with the gas pedal to the floor and accelerate slowly at the same time.

-jim

-jim

No, you are confusing energy and power. The energy is the same if you use little power for a longer time, or a lot of power for a short time. It requires a certain amount of energy to accelerate a car to cruising speed, 1/2 M V^2. If the efficiency of the engine were the same at all operating conditions it would take the same exact amount of energy to reach, say, 60mph regardless of how rapidly you accerate.

Yes, a smaller engine gives better gas milage because you normally operate it at a higher throttle setting than with a larger engine. However, given a certain size engine, you get better economy/ efficiency by accelerating at a higher throttle setting as long as you do not invoke power enrichening.

The problem is it just isn't that simple. If your theory was correct you would get better gas mileage driving on hilly roads than flat roads. After all you have your equations that prove you get back all the energy going downhill that you used going up hill. But you don't get better mileage driving up and down hills that you do driving on the flat. And you don't get better mileage driving the way you advocate - at least most cars on the road don't. It's just wishful thinking.

-jim

True, I oversimplify a bit. I did not include losses in air resistance. These are actually a big factor in hilly terrain. But on level ground if you are going to be cruising for a long time compared to the time accelerating and decelerating to cruising speed the differences in aero and friction losses are inconsequential.

That is NOT the case in hilly terrain. Friction (either air resistance or rolling friction) is a so-called dissipative loss- not recoverable)_. So speeding downhill to gain KE from PE doesn't work. You loose some of the energy you gain from the conversion, so driving in hilly terrain takes more energy than driving on flat terrain.