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Brisk acceleration

One of the characteristics of carburetor aspirated engine cars is brisk acceleration resulted in better fuel mileage than slow acceleration.

Is this true for fuel injection?

I don’t consider that to be true for carbureted cars.

I always found rapid acceleration resulted in poor fuel mileage. Both with the carburated vehicles I use to drive and with the fuel injected vehicle I now drive.


It seems to me that brisk acceleration with a carbureted engine would result in worse fuel mileage than slow acceleration. Brisk acceleration would cause the accelerator pump on the carburetor to have a longer stroke as you pushed the pedal down and hence pump in more fuel. On some of the 4 barrel carburetors, the secondary barrels only open on hard acceleration thus using more fuel.

I owned a late 60s Roadrunner and Superbee; both with the 383 Magnum and AFB carburetors.

If driven around town with an egg on the accelerator pedal I could squeeze out 8 MPG; maybe.
Driven more aggressively (as in normal acceleration) it was more like 5 along with watching the gas gauge drop very quickly.

Yep, the accelerator pump on a carb is crude compared to EFI, dumping gas in with every push of the pedal. Much worse mpgs when driven ‘briskly’.

I think what is being asked is whether carbureted cars ran more efficiently with the throttles open further than at lower speeds. The answer is probably yes, since any engine with a throttle is more efficient with the throttle plate open further. But in every range and whether slowly accelerating, quickly accelerating, or driving a steady speed, a fuel-injected car is going to be more efficient, even one with a comparatively simple throttle body injection system.

I don’t believe “brisk acceleration” will give better mpg in a car and that whether the car has a carb or EFI does not matter at all. EFI is more precise in modern cars because of all the sensors of motor function and computer controls based on what the sensors report. Most carb motors have virtually no computer controls and if they do it is very basic electronic ignition only.

It could be all about what you consider “brisk”? In general, a gentle touch on the accelerator with enough acceleration to move you into the next higher gear is the best approach. Shifting into the higher gear as soon as you have enough rpm’s to sustain momentum without bogging down the motor is key to good mpg. The optimum rpm varies based upon whether you are starting on level ground or an uphill grade.

All the cars I have had (got my first car in the early 60’s.) gave me better mileage driving conservatively.

There are some good theories that SOME cars will get better mileage running a little more aggressively. There are limits and a lot depends on your day to day driving style.

I remember reading a study done by BMW a while back, where they concluded that for their cars accelerating using roughly 70% throttle yielded the best fuel economy.

I remember that BMW study. One key was to shift ‘short’, at about 3000 rpm. The idea was to have the open throttle reduce pumping losses.

EDIT - note, this was for EFI cars, not carbs.

That is a completely bogus theory. If you stomp on the accelerator with a carburetor…you are expelling most of the fuel out through the exhaust system. I don’t know where you heard this old wives tale but you need to disregard it.

Missileman is correct. However I wanted to dispel some other misunderstandings.

“Carburator aspirated” is a misnomer. Aspiration is a term to describe vis what method the air enters the engine. The two options are “naturally aspirated”, which means that there is no additional pump pushing air into the engine and the only air entering the cylinders is what the pistons draw in in th eintake strokes, and “boosted” means that air is pushed into the cylinders by way of a turbocharger or supercherger. Carburated engines and fuel injected engines can both be either naturally aspirated or boosted.

Carbuators work via a principle wherein airflow going through a restriction in the carburator (called a “venturi”) accelerates but also lowers pressure on the wall of the passage (called “lateral wall pressure”) That reduced pressure draws fuel through a tiny orafice in the wall that’s connected to a bowl of fuel in the carburator (called the “float bowl”). The difference in pressures between the fuel and the wall of the passage are relatively small, aond the fuel goes through in ralatively large droplets. Large droplets don’t burn quickly or well in the cylinders. And the only control on the fuel is the wall pressure.

I’m gonna skip throttle body injection for the illustration.
Modern electronic fuel injection works by maintaining high pressure vaporizers (injectors) either in the entry hole (port) right behind each intake valve, or directly into each cylinder. These high pressure injectors spray a fine mist of gasoline, which burns much more rapidly, much more effectively, and provides more power than large droplets. The amount of fuel dispersed is controlled by various sensor that provide signals to a computer telling it exactly how much fuel the engine “demands” and sprays exactly the amount needed. It all happens faster than you can blink.

Think of it as the difference between squeezing the Windex bottle slowly (large droplets) and fast (fine spray). In an engine that difference is huge.

Now, how fas an engine accelerates depends on the overall engine design, but if that’s the same between two engines the difference will be (1) how much fuel the fuel metering system allows the cylinders to pull in and (2) how finely misted the fuel is. If the amount of fuel is appropriate for the demand of the engine, iether will accelerate well, but the carburated engine will use more fuel to do so. It isn;t burning as efficiently. If the amount of fuel is too high for the engine demand, the engine will run rich and dirty…and may flood. If the amount is too small, ,the engine will run “lean”, with low power and possible damaging operating conditions.

There’s actually a lot more involved in the difference between mutiport (or direct) EFI and carburation, but in the interest of not writing a book I’ll leave it here. I’ll be happy to answer any questions.

Anything done briskly with a car tends to increase fuel consumption and wear.