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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...)

Reply to
njot
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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

Reply to
sdlomi2

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:

Reply to
njot

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