As you can see from the chart, disconnecting your alternator will save you gas mileage. Source: http://metrompg.com/posts/alternator-optional.htm
Why would you want to add a HHO device to your car that increases the amount of power needed from your alternator?
This HHO webpage even encourages you to add a bigger alternator and even add a secondary alternator!
I learn a lot about electronics and engines from sailing boats. Read this quote about the power required to spin an alternator:
The genset starts and runs at a preset throttle level providing 8-12 horsepower at around 2800-2900 rpms and consumes fuel (with slightly fluctuating rpm) based on the alternator loading. A 200 amp alternator requires a minimum 8HP motor, so the Kubota 482 is a good match. Because it is direct drive and there is no 3:1 pully driving the alternator up to 4500-6000 RPM (at 1500-2000 engine rpm) as with most engine alternators, the sustained amp output sits comfortably around 180 amps (200 when cold, 180 when hot). Fuel consumption is approximately 1 liter per hour at the 2800 rpm setting. That's enough on this for now, back to the installation...
The man ordered an engine that would spin his 200Amp alternator on his sailboat.