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I know most people swap for more power but I'm looking for gas mileage. I have a 1999 Chevy 2500 crewcab with the 7.4 454 Vortec engine - 10 mpg - and I'm wondering if anyone has ever swapped down to a Vortec 350. The bolt in stuff doesn't bother me but I'd like to know if the wiring would be a challenge. I've heard that the fuel pumps are different and, of coarse the computers. Anybody ever done this?
 

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I was discussing the same swap with a friend in the automotive repair buisiness (I have a 7.4 powered 96 4x4 Suburban). The swap is relatively easy if your using the same engine year groupings. Wiring is a close match, transmissions can be adapted, cooling systems will work, fuel systems can be adapted with a fuel regulator, and all gauges and gauge functions should be the same.
The problem is engine size, the 350 is a great engine, but the mileage increase wont be as much as you expect in a truck that heavy. The smaller engine works harder doing the job and consumes almost as much fuel as the big block. You could add some performance enhancements, then the mileage goes back down.

He recomended the following:
Change gearing to a friendlier ratio, 3.07 if possible.
A good tune-up, injector, intake, and top end cleaning included.
Run Synthetic oils and fluids if possible (not always recommended on an older engine due to the possibility of developing leaks).
Performance enhancements that maximize fuel mileage, exhaust, water injectors, and better air filters.
Remove unnecessary weight from the vehicle, grill guards, roof racks, tool boxes.
Change driving style.

A well tuned big block truck is capable of 16 to 18 mpg highway unloaded, 14 to 16 loaded, (I can get these numbers from my Suburban when I watch my driving style). These arent far from the numbers a 350 powered truck of the same size gets, now factor in the cost of the engine conversion. How much will you save?
 
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