This seems to be a very common thing for tuners/ race oriented people to do, but I really haven't heard of anyone metioning it for fuel economy. Of couse they cost $100 or more and gains must be minimal. You would have to drive 1,000,000 miles to get your money back.
2008 EPA adjusted:
Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)
(hey it's the little things that count sometimes Smiling)
Yeah ****, people who are in it for gas mileage complain a lot about it, but I don't care so much. Something like this I might do just for the power, people drop a grand on some fancy custom exhaust to get ten hp, hell, it's all in the sport, :-)
i still think it's going to be much lower than 8%.
ooo! surfing around, just found this... some serious underdrive debunking:
What many people fail to comprehend with the alternator is that if you slow it down, the voltage regulator will simply crank up the field excitation to whatever value it takes to maintain voltage. The mechanical effect is the torque requirement rises as the speed decreases IAW torque*RPM/5252 = hp. Since watts are the same as hp, the alternator will draw a constant hp to supply a given watt load. The only difference will be the very minor air drag difference. On the Cordless Battery Charger, I can't see the govenor lift at all when spinning an un-excited alternator so the drag is trivial.
There was an article in Circle Track mag awhile back where Steward Pumps build a water pump dyno to see what the story was on water pumps. The worst pump consumed about 20 hp. His racing pumps were down around 8 hp. (anyone see a big win?) Underdriving MIGHT help IFF you have extra cooling capacity AND can cool adequately at low speed (if you even need to, of course. (source)
so the argument there is the only gains to be had in underdriving are the water pump and the reduced air drag from the alt pulley "fan".
water pumps... the quote above is talking about racing engines (v8's i gather). obviously the size of the gains to be had depend on engine size.
let's do some rough math for my car:
on the teamswift site, one guy replaced his water pump with an electric one that is rated at 1/2 hp, and is stronger than stock. so let's say the stock one costs 1/3 hp. underdriving it by 25% would save me 1/12th of one hp. let's be generous and round it to 1/10th to accound for reduced aero drag on the alt.
1/10th of 1 hp = 75 watts. that won't even power my headlights. yes, it will save fuel, but looks like the savings will be negligible.
continuing further down my very rough calculation road, another figure you see quoted often is road horsepower at highway speed. i.e. i've read several estimates that it takes a modern car between 10-15 hp to cruise at highway speeds on level ground.
assume my car gets 50 mpg (US) at 60 mph and needs 12 hp. if i save 0.1 hp with the underdrive calculations, my highway load drops by 0.83% (0.1 / 12 hp). if the fuel consumption drop is linear, i have just gained 0.42 MPG (50 mpg * 0.83).
suddenly the investment "risk" doesn't look so good (from a strictly fuel saving perspective)...