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
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
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.
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.
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