Scan Guage/Shifting for economy
 

I'm running a 4.3L engine with a 5 spd. MT and using a Scan Guage (along with other mods) in my full size truck. Somewhere I read that by accelerating briskly and up shifting when the SG shows the best mileage for that gear one's overall fuel economy will improve. I've tried searching for that type of information and have had minimal success. I'd like to hear from other members about this technique, especially if your running a vehicle similar to mine

Accelerating briskly would work with an automatic, but with a manual you're better off keeping your RPM low and opening the throttle more (and probably accelerating somewhat slowly).

NOPE - get out of the low gears quickly but don't give so much gas that the engine bogs down. In my xB there is a sweet spot where the gas pedal is very responsive - a combination of slow gas pedal and waiting for the fuel injection to keep up that seems to work well. This seems to result in shift points around 1900 rpm then dropping back to 1200 rpm in the next gear. The only other time I notice minimal impact in trip MPG average is sub 1000rpm shifting to take off from a sotp sign but that is aided by my use of Synlube which provides really low friction at those RPMs whereas your oils will not work as well.

THC's advice doesn't match my experience. You want to use your engine's brake specific fuel consumption graph to best advantage. With most 2 liter class engines coupled to manual transmissions, that's moderate acceleration between 1500-2200 rpm. Smaller/weaker engines get their best fuel economy with lighter acceleration, and larger/stronger engines seem to do better with more spirited acceleration. I don't know where your BSFC spots lie with your 4.3 V-6, but it's probably at a lower rpm range, and fairly strong acceleration. A Scangauge with its LOD readout will help you immensely in standardizing your acceleration.

You can't do this with most automatic transmissions, because their torque converters slip with moderate to spirited acceleration and you end up wasting gas at high rpms, with transmission slippage (going nowhere). So the best technique with most ATs is to accelerate very slowly to keep the TC from slipping.

So, in my experience, ATs get better fuel economy with very light acceleration, and MTs do better with relatively stronger acceleration.

my experience has been shift early and get to top gear as soon as the engine can handle it. A 4.3 is a torquey engine so it can probably handle 1500-2000 rpm in 5th. in my subaru's case I found shifting at about 2500 worked best for mpg but each car/engine is different.


in my auto brisk acceleration brought the car to torque converter lockup sooner and better mpg. slow acceleration took for ever to get to lock up and wasted fuel.

It definitely differs from one car to the next, and from one driver to the next. The only way to know for sure what will work for you in that vehicle is to try a variety of techniques until you get the results you want.

The 4.3 gets mixed reviews on whether to define it as torquey or not in the full size pickup. I suspect that it is plenty torquey for low-RPM heavy throttle fuel economy.

Is there a BSFC graph available for that engine? If so, that would probably save some trial-and-error effort.

That's what I find in my truck.

Thanks of the information. I haven't been able to find a BSFC for my truck, but will keep looking. The truck will handle 5th gear very nicely at 1500 rpm's. Watching the SG to show high mileage in each gear, my shift point seem fall between 1500-1700 rpm's. As an FYI, when the SG shows the highest mileage for each gear and if I stay in that gear the MPG's fall off.

I think the only way to tell how efficient your engine is at various RPM and gear is to climb a hill at constant speed and then you will see which RPM and LOAD on the engine yields the best efficiency / highest mileage for acceleration at that same load. I went up a steep hill in 5th 4th and 3rd and 5th was giving the best mpg but at a lower speed when the RPM drops too low it may not - then a lower gear would work better. But if you try top gear at 20 - 25 - 30 mph and see a big difference in MPG then that says something about the RPM since going up the hill at these low speeds should not be affected much by the speed but more by the engine RPM efficiency.