Gas Mileage Improvements

[piecesparts]

Originally Posted by jimfulco

My '79 Sub has the stock 350 w/dual exhaust and with a 3.08 gear it got 10mpg, highway, city, sitting in the driveway, whatever, 10mpg. I changed it to 2.73, and it now gets barely over 11 on the highway, still 10 in the city, but I drive around town in D2 quite a bit now.

I'd say get your Q-jet rebuilt if there's a decent carb place near you. One of the local auto electric shops here has a good carb guy, so that's where mine get fixed. A Q-jet rebuild is ~$125 plus any extra parts needed (kit is included in the $125). I'd expect the Q-jet to get better mileage than any other carb, plus yours is probably already calibrated for your vehicle, and all the linkages, etc., already fit.

I'd keep the stock manifold, because it's designed for lower-rpm operation than the aftermarket ones, and as such, I'd expect better mileage from it.

I am guessing that after you changed the gears in your Sub, the RPMs are really low when you are driving around on the streets. You stated that you are driving around in D-2, that might be the clue that you need to get the RPMs up a little to make the motor more efficient. Idling is not ALWAYS the answer. It is how hard your motor has to work to keep up speed and accelerate. The taller tires will hurt performance also.

The factory intake for any vehicle will work, however the "Volumetric Efficiency" of those intakes are questionable at the most. The ones that the car manufactureres build for performance cars (specialty vehicles) are designed to flow very much better than your "Run of the Mill" intakes. Keep in mind that it is all what can be made cheaply, so that they can make a good profit. Look at each aftermarket manufacturer's info and you will see that they HAVE designed flow patterns to improve across the power band. Different manifolds are designed for higher or lower profiles and you can decide where you want to drive the most.