Alternator disable switch -- how much MPG improvement?

[sl2eggplant]

I know a few of you guys have installed a alternator disable switch. Just trying to get an idea of how much that will improve MPG (actual number or %). A "guesstimate" is fine since I'm still months away from completing all my mods.

My daily round trip commute with my 99 Saturn SL2 automatic is 64 miles (56 highway, 8 city) with some construction and traffic.

[sl2eggplant]

Forgot to mention that I did read the second half of DOAX's Paseo thread which includes the alternator switch.

[Daox]

Darin (metrompg) tested this a while back. He saw a 10% improvement at steady state cruising.

Test: Alternator vs. no alternator = 10% MPG gain @ 70 km/h

[sl2eggplant]

Thanks Daox

[dcb]

technically, Darin had removed the belt from the alternator so that it wasn't even turning. There would be less gains from just turning off the field. And it was a metro.

[busypaws]

I did it on my 94 Toyota PickUp. I was not able to measure any change, I don't have a computer and tank to tank fillups with variations in temp/load didn't see any changes.
I do know I can feel a change in acceleration up a hill or a quick left turn in traffic with it off. Maybe I'm just using the switch in improve performance not improving gas mileage.

Don't get discouraged. I think if I had a scanguage or similiar and did A-B-A testing I would see a difference. Removing belt gets 10% so I would expect field switch at 5% or so.

[thecheese429]

I have a field switch, so when I get my MPGuino, I will be able to tell you a % gain. I won't be able to say MPG improvement, because I won't have it calibrated by then.

Sometimes, however, when I have the alternator off, the engine feels funny. Like the throttle is closing partway. It goes away when I turn the alternator back on. I suspect that it is the ignition getting less powerful with the low battery volts.

[sl2eggplant]

This alternator disable switch is one of the more extreme mods/tactics that I may end up not doing ... in the same boat as power steering pump delete and turning off the engine (during P&G). Maybe after exhausting the aero mods, I might reconsider.

Thanks for the input, though.

[MetroMPG]

Originally Posted by busypaws

Maybe I'm just using the switch in improve performance not improving gas mileage.

Were you also recharging the battery from an external power source?

[MetroMPG]

busypaws: saw you answered my question in another thread:

Originally Posted by busypaws

I get about 20-30 miles before the radio starts lossing power then I know to turn alternator back on.

If you're not using an external power source to re-charge the battery, that explains why you're not seeing any fuel economy benefit.

Some vehicles use smart alternator control to eke out a few percent savings, but mostly by maintaining a minimum voltage under accel/cruise, and boosting alternator output under engine over-run and braking conditions.