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Discuss carb, fuel inj, and fuel economy in the alt.autos.gm forum at Car Dealer Forums; Are there any carb vs. fuel injection experts out there? I am curious why two ...
  1. #1
    njot
    Guest

    Default carb, fuel inj, and fuel economy

    Are there any carb vs. fuel injection experts out there?
    I am curious why two cars of same size and weight, with the same engine
    and transmission, except one is older and has a carburetor, the other
    has fuel injection...
    both cars run fine... why would the carbureted one get significantly
    worse gas mileage at interstate speeds (65-70 mph)?
    In this case we are talking about two front wheel drive GM A body
    vehicles (which are obviously not new cars at this point) with 2.5 4
    cyl engines and auto trans... Both are in a good state of tune... why
    would both cars get about 25 mpg city...
    but on the highway on a long trip, going 65-70, would the one with fuel
    injection, get 32 mpg, while the one with a carburetor get only 27 mpg?

    I don't think it is an ignition issue as both run great. It isn't a
    tire pressure issue since both have equal tire pressure. Any thoughts
    appreciated. I plan to keep both cars but think the older, carbureted
    one could do better on gas (who wants to spend extra money on gas these
    days...)




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  2. #2
    sdlomi2
    Guest

    Default Re: carb, fuel inj, and fuel economy


    "njot" <njot33@juno.com> wrote in message
    news:1154993161.658075.156290@75g2000cwc.googlegro ups.com...
    > Are there any carb vs. fuel injection experts out there?
    > I am curious why two cars of same size and weight, with the same engine
    > and transmission, except one is older and has a carburetor, the other
    > has fuel injection...
    > both cars run fine... why would the carbureted one get significantly
    > worse gas mileage at interstate speeds (65-70 mph)?
    > In this case we are talking about two front wheel drive GM A body
    > vehicles (which are obviously not new cars at this point) with 2.5 4
    > cyl engines and auto trans... Both are in a good state of tune... why
    > would both cars get about 25 mpg city...
    > but on the highway on a long trip, going 65-70, would the one with fuel
    > injection, get 32 mpg, while the one with a carburetor get only 27 mpg?
    >
    > I don't think it is an ignition issue as both run great. It isn't a
    > tire pressure issue since both have equal tire pressure. Any thoughts
    > appreciated. I plan to keep both cars but think the older, carbureted
    > one could do better on gas (who wants to spend extra money on gas these
    > days...)
    >

    There exists what is known as an ideal gasoline-to-air mixture. Any
    deviation richer or leaner has a negative effect on mileage/efficiency.
    Electronic fuel inj. continuously monitors this ratio and alters the
    mixture, many times per second, to keep it at this ideal. Carbs just cannot
    achieve this accuracy, even with computor-monitoring and altering. Also,
    carbs put the mixture into the general proximity of all the cylinders,
    meaning that with all 4 cylinders receiving fuel from the same source, some
    cylinders will get more, some less, depending on its distance from the carb.
    With elec. fuel. inj.--esp. multi-port inj.--each cyl is the same distance
    from its injector and can receive the same, theoretically exact, input of
    fuel on each pulse of the injector:hence a second important feature for
    economy.
    The throtle-body injection, esp. before the "Vortec" type that GM has,
    has the similar precise control of mixture, but lacks the
    equi-distant-to-cylinders characteristic of multi-port. Its efficiency
    probably lies somewhere between carbs and multi-port.
    HTH, s



  3. #3
    njot
    Guest

    Default Re: carb, fuel inj, and fuel economy

    I realized another difference between the two cars that probably makes
    a big difference. The older, carbureted car has 13 inch wheels, while
    the newer one has 14 inch wheels. The older car's engine would turn at
    higher rpm's at a given speed, and burn more fuel.

    For some reason, though, driving at say 60 mph instead of 67 or 68 mph
    makes a big difference on the carbureted car. I know I used to get
    more like 30 or 31 mpg with it, and I think driving slower was why.
    I'm just surprised what a huge difference it makes.


    sdlomi2 wrote:
    > There exists what is known as an ideal gasoline-to-air mixture. Any
    > deviation richer or leaner has a negative effect on mileage/efficiency.
    > Electronic fuel inj. continuously monitors this ratio and alters the
    > mixture, many times per second, to keep it at this ideal. Carbs just cannot
    > achieve this accuracy, even with computor-monitoring and altering. Also,
    > carbs put the mixture into the general proximity of all the cylinders,
    > meaning that with all 4 cylinders receiving fuel from the same source, some
    > cylinders will get more, some less, depending on its distance from the carb.
    > With elec. fuel. inj.--esp. multi-port inj.--each cyl is the same distance
    > from its injector and can receive the same, theoretically exact, input of
    > fuel on each pulse of the injector:hence a second important feature for
    > economy.
    > The throtle-body injection, esp. before the "Vortec" type that GM has,
    > has the similar precise control of mixture, but lacks the
    > equi-distant-to-cylinders characteristic of multi-port. Its efficiency
    > probably lies somewhere between carbs and multi-port.
    > HTH, s



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