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Question about fuel consumption

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  • Question about fuel consumption

    Which of the two scenerio consumes more fuel. This is for a carburated engine with no computer and nothing fancy.

    1. The vehicle is stopped. The engine is idling at 600 rpm and the accelerator is not depressed. Transmission is in neutral.

    2. The vehicle is going down a hill in 5th gear. The engine's rpm is 2500. The accelerator is not depressed.

    In both cases, because the accelerator is not being depressed, the butterfly valve in the carburator is fully closed. For sake of the argument, the fuel pressure and the atmospheric pressure can be assume to be the same in both cases.

    I need your help to resolve this debate.


  • #2
    I would say the car going downhill would use more fuel since to keep the engine running, a specific minimum fuel-air ratio is needed and if the same amount of fuel was being burned at 600 and 2500 rpm, the fuel-air ratio would be 1/4 at the higher speed. I don't think the engine would stay running under those conditions.
    Also, the increased airflow through the carborator will draw more fuel whether the butterfly valve is open or not. There is an idle port that adjusts the air-fuel ratio, so if the airflow increases, it will draw more fuel.


    • #3
      On mb's comment, if the engine is to be kept running, that implies that something ( a computer) is sensing whether or not it is running, and making adjustments to the fuel. Since there is no computer in your scenario, and the engine is running, idling in case # 1, and overspeeding as in case #2 (still running, but faster) I'm going to nix that explanation, and go on. IMHO. In case #2, the engine is being driven by gravity, and not combustion. I'd say it's fair to say that combustion does not enter into the equation here. My thinking is that the ammount of fuel drawn by the engine is directly related to the ammount of air suctioned through the carburetor. At idle, the intake vacuum sucks a certain ammount of air, and fuel. At higher rpm, the intake vacuum increases, so does the ammount of air sucked past the small but fixed passageways, so therefore, the fuel intake increases. Another factor is the small, but measureable rise in fuel level in the carb under full vacuum. Higher fuel level in the venturi, higher intake of fuel. The effect is small, as the vacuum at idle is high, and with the engine overspun the vacuum is not much higher, but it is higher. Injected engines theoretically would shut the fuel off entirely if the computer's goal was to maintain idle rpm, and it found the engine turning too fast. Now if fuel comsumption per mile is what it's all about, the downhill coast supplies the power to go the miles, and to overspeed the engine, so gravity is the power source, and the small, but measureable fuel increase is nothing compared to the energy being supplied to the vehicle from coasting downhill. This then is the most mpg. This MHO

      [This message has been edited by darryl (edited 04-01-2003).]

      [This message has been edited by darryl (edited 04-01-2003).]
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #4
        with the carb you will be useing more fuel at 2500 even if the butterfly is closed because of the way the fuel circuts are made. now if fuel injected not as much because the computer cuts the fuel completely when it is not needed.


        • #5
          Ther also is the tendency for idle to be set a bit rich. That means more fuel than it looks like it should use.

          But manifold pressure will be lower with the engine pumping (on the downgrade), sucking more fuel than idle does since the throttle is closed eother way.

          And then some have a throttle bypass to keep the vacuum from going way up on deceleration.

          Heck, I dunno.


          • #6
            The same in both cases as the flow past the closed butterfly plate is choked flow, i.e. it is critical flow or supersonic,thus the pressure at the idle port is the same in both cases. Same pressure means same fuel flow rate.

            Neil Peters
            Neil Peters

            When on the hunt, a broken part is better than no part at all.


            • #7
              Buzzzzzzzz.... All wrong.
              As manifold vacuum increases in the down hill scenerio, more fuel is drawn in through the idle circuit and causes the engine to run rich. .(Period)

              This is the reason Vacuum Breaks (or pull-offs) are used on carbs. These OPEN the butterfly slightly to allow air enrichment of the air/fuel mixture. In any case, MORE fuel is used than in a straight idle scenerio.


              • #8

                This is a moot exercise. Companies like Chrysler had 2.4l Turbos with tons of snort (even more when the MAP sensor fails and you get a BIG BOOST - trust me, my head blew off) but the dumb ****s cannot make the engine run lean even with MPFI! They are affraid that a sensor will fail and the system will lean out after hard acceleration and explode. That is what happens when you use cheap **** in your electronics instead of doing it right for two dollars more...

                But I am not bitter. So what do you think the Chrysler pukes said when I stuck my finger in the exauhst and pulled out a blob of black soot? "OH, they are supposed to run that way."

                I had 450HP small blocks that turned 7500rpm that just dripped water out the tailpipe. And got 25 MPG driving like a mad whore. But then, I am not an idiot - sorry, I did buy the car and two fords - I am an idiot. (but they were shiney!)


                • #9
                  Ops, mostly right on the first paragraph. The lower pressure on the manifold side will actually draw fuel from the run circuit, not the idle circuit.
                  The rest is correct.


                  • #10
                    I agree with CCWken... More manifold vacuum more air velocity through carb (through the lil bitty holes in butterflys), more draw on idle circuit going downhill under no load with higher engine speeds. ON.... a lot of race engines the rings will seat, then on decell the pipes will smoke, closed throttle, high vacuum, pulls oil past the rings. But, don't oil lubricate the top end too? better that then lean and burned.
                    To break in a harley engine I recomend you accell and decell rapidly several times each 100 miles to shock load the rings. Hell, I ride like that anyways, except a lot more than once a hundred miles. if it ain't wide open, I must be afraid something will break, you don't want that bike.


                    • #11
                      I had a double post, sorry guys... I hate bill gates...

                      THRUD>>> 450 hp small block? must have been a 302 or a 327 cause I ain't never built a 350 that got that good, most they do is 16-19.. I have one that has been in 7 vehicles, did wheel stands in two of them. Something about stroke-bore-friction efficiency?
                      I took a perfectly good 283 out of a 64 el camino, it got 25mpg and was smooth, put my hoss motor in and it got 16mpg, same tranny, same axle. but a coin throwed on the metal dash would dance from side to side of car during idle. It would set burgalar alarms off at a 1/8 mile away. Would do 160, (float all over the road) and sling belts outa BF goodrich tires on both ends.

                      [This message has been edited by ibewgypsie (edited 04-02-2003).]


                      • #12
                        "then on decell the pipes will smoke, closed throttle, high vacuum, pulls oil past the rings."

                        Or the valve guides if they are worn.



                        • #13
                          Slightly ot for Paul....
                          I have always noticed and been told that worn guides give a puff of smoke next time you accelerate, but not much otherwise.

                          And that bad rings/cylinder wear give you smoke when you are under acceleration or pulling a load.


                          • #14
                            Smoking (Blue/white) on decel can be either valve guides or valve stem seals. Smoking on accel or load is usually rings.


                            • #15
                              My friend who read this indicated that the answer is obvious. It's #2 because the ignition is off.

                              I didn't intend for this to be trick question, but then I wasn't expecting 12 different answers. Oh well, I guess I'll just have to try it...if I can only get a car with no computer.