Originally Posted by 1cleanmav!
this coming spring i plan to rebuilt an old school stock small block 302 and im going to put EFI on it and put it in my ford maverick(Daily driver). i already have most of the parts that i need but my main concern is gas mileage. i read an article on the mustang and ford website that talks about what i plan to do, the car that was uesd was a full size car i think it was a fairlane or something. with the EFI that they put on the V8 it boosted the mileage up by five miles. since my maverick is pretty light i figured i could do the same and get pretty descent gas mileage.
what do you all think?
By itself EFI has a small impact on mileage generally through being able to hold the mixture tightly around what's needed at any point in time along with the fact that it doesn't underun or overun fuel requirements when throttle position is changed and it shuts the fuel off in coasting situations. But its big impact comes with it making the engine compatible with very high ratio final gearing.
Taking this apart:
- At cruise with an O2 sensor, EFI holds the mixture right at the chemical optimum of 14.7 pounds of air for each pound of fuel. It does this over a broad range of RPMs. This compares to a carburetor where it has a "sweet spot" but gets wiggy in other RPM settings. In this regard, of domestic carburetors, the Q-Jet and the Carter AFB/AVS models get more mileage from a gallon of fuel than a Holley. Not that you can't dial a Holley in, but comparatively you'll work your tail off getting it there where the Q-jet and Carters will usually drop in with a couple part changes.
- Injectors being just behind the intake valve, an accelerating pump shot to get some fuel into the cylinders to prevent them falling lean when the throttle is opened isn't necessary, a little fuel saved.
- Unlike a carb, injectors don't continue to dribble fuel when the throttle is suddenly reduced, so there's no momentary rich over-run, a little fuel saved.
- When the throttle is closed and the vehicle is pushing against the engine, the computer shuts the fuel off, so the typical over-rich coasting mixture of a carb is eliminated, actually a fair amount of fuel saved.
- With unusually high cruise ratios found with high ratio final drives combined with overdrive transmissions, EFI is not effected by the reduced signal received by the carb at low RPMs and fuel fall out through the intake track caused by these very low intake velocities is no longer a problem. With a carbed engine low RPMs and moderate to high loading on the crank lead to unstable combustion with some cylinders running very lean in some and rich in others. This shakes the daylights out of the crank which is hard on transmission components since these vibrations are transmitted through to the tranny, shaft and rear end; not to mention hard on the cam drive as it rattles the chain and driven cam gear, often you can feel the car buck and surge with these forces. With EFI, each cylinder is doing its part with consistency to those around it, so surging is eliminated making for a comfortable ride and longer lived components. This is where EFI really provides a mileage benefit, probably half, or better, of the improved fuel mileage comes from this aspect. You will quickly note that as road speed and RPMs of a typical EFI V8 increase, that the mileage will quickly drop to something close to a carbed model.
But I rather doubt you can recover the cost of the EFI system plus an AOD transmission from fuel savings in anything like a short time.