I would imagine that being completely mechanical, mechanical fuel injection would only be as good as a carburetor in terms of metering the volume of air being delivered.

It will have better fuel atomization.

FYI, comparing like for like:
85 F-150, 2WD, 3-speed auto, feedback carb: 13mpg city, 14mpg freeway, 15.5 tons CO2
85 F-150, 2WD, 3-speed auto, EFI: 14mpg city, 16 mpg freeway, 13.3 tons CO2.

Source: fueleconomy.gov.

 

A bit off topic here I know but I am thinking about fuel economy. I was driving my 95 Dodge with Fi home tonight. I was climbing hills and mirroring when I thought the vac secondaries when my Ford would kick in. Lol no real reason just jacking around. I am hoping I'll notice some big rush of power but we shall see.

 

Only if they're extremely out of adjustment.
The whole point of vacuum secondaries are to provide a linear throttle response.

 

Lol oh dang. I was all fired up thinking I could jam the pedal and it would kick in and off I go hahah. Oh well I guess.

That would be throttle response/accel lag wouldn't it anyway?

 

That would be too stiff a spring causing a flat spot before the secondaries have a chance to open.

 

I think it's the other way around - too light a spring will cause the car to bog down as the secondaries open up too quickly and kill the vacuum signal. A heavy spring will just make the car feel down on power overall.

 

I always start with a stiff spring, go too far and then switch back one step when I notice the engine falling on its face. (much easier when you have a quick change cover)
So I guess you could have either a big flat spot on top where you don't have enough venturi or you could have a bog where the secondaries flop open.

 

Gotcha, I was reading "flat spot" as bog.

What's a quick change cover? The P/O of my 460 put in one or two steps from the stiffest spring Holley sells. I went down one step and was tired of finagling with all the crap on my CA 460 that I decided to stop.

 

The 20-59 diaphram cover allows you to change springs by only removing two screws on the top, instead of having to access the three screws on the side behind the choke linkage.



Holley Performance Products Quickchange Vacuum Secondary Housing Cover 20-59

 

That is for the 302 right?



The 300-I6 had different numbers...



Also comparing like for like...



1986 F-150, 2wd, 4 speed manual:OD, Feedback Carb, 23 mpg highway, 20 mpg combined, CO2: 9.3



1987 F-150, 2wd, 4 speed manual:OD, EFI, 20 mpg highway, 17 mpg combined, CO2: 11.0



Source: fueleconomy.gov



1986 was last year for Feedback carbs, and 1987 was first year of EFI for the 300-6. And according to the website, the EFI was dirtier with 1.7 more Co2 output than the Feedback carb just a year earlier.



If you keep going forward, the 1988 is even worse.



1988 F-150, 2wd, 5 speed manual (the four speed OD was dropped), EFI, 18 mpg highway, 16 mpg combined, Co2: 11.6

 

Yes.

I guess from the statistics we can draw that I6 motors should be using feedback carburetors and 5.0L V8s respond better to EFI.

 

Actually, that might be just the reason why it took two years longer for the EFI to be introduced to the 300-6. No doubt they had prototypes in 1985. I bet they were really bad if I was to speculate from the numbers given at the EPA website.

 

Could be the case. I think we could pull numbers that supported either cause, when comparing Chevy V8s of similar displacement and drivetrain, the EFI wins out on HP, TQ, emissions and MPG. Of course, the typical EFI motor came with higher compression engines, aluminum heads, etc vs iron heads and other junk.

I also wonder what kind of gearing an EFI truck would come with vs a feedback carb. Not enough of the picture is really painted from the site.

Taller gears will produce lower CO2 and better fuel economy, to a point.