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Unklecueball 06-13-2008 12:16 PM

Alternator Mods
Hey there,

I am really loving the site, btw! Some really great info on here and everyone seems to be pretty cool.

My question is on alternators. I am reading about the drag it puts on the engine, people wating to upgrade to LED lighting, installing cut off switches and deep cell batteries, etc. Now I am a mechanical novice when it comes to autos. I understand the basics of how things work but I am by no means an expert. I know that the alt pulls power from the engine just like the AC does when it kicks on. Does the alt drag more on the engine with each extra amp it produces? If so, if you could turn it off and let it "free wheel" any guess as to what the increase in FE would be?

BITBY 06-15-2008 08:24 PM

I've been lurking here for a little while, but just registered and this is my first post.

Alternators don't operate like A/C compressors do. An A/C compressor has a clutch bearing, which when disengaged (no A/C) spins freely without resistance...that being said, removing the accessory belt on engines where the water pump is not a crank-driven accessory is somewhat common in drag racing for that very reason; less resistance on the engine.

If you so desire to bypass the alternator for brief periods, keep in mind its the alternators job to maintain current charge in the battery while powering the vehicle's electrical system; taking it out of the equation leaves the battery, and any fuel savings you may experience may be trumped by a much shorter battery life.

SVOboy 06-15-2008 08:35 PM

There is more drag based on how much it's putting out, but the main thing, I think, is just that it's always there, which is why a lot of people disconnect it.

Shawn D. 06-16-2008 07:59 AM


Originally Posted by BITBY (Post 35073)
... any fuel savings you may experience may be trumped by a much shorter battery life.

... and reduced FE. It used to be common to run without an alternator in certain racing circles, but it was found that the reduction in ignition efficiency (~14V from the alternator vs. 12.6 from a fully-charged battery) actually hurt power. Thus, I'm dubious that disconnecting the alternator is a good idea.

elhigh 06-16-2008 08:55 AM

I'm getting ready to pull my engine fan to switch to an electric fan; it'll drag more on the alternator but I'm certain that I'll land on the positive side of that equation. Switching out lights, when and if that ever happens, will make up some of the deficit - not much, rarely driving in the dark.

Hear's a thought, though: I've considered switching my alternator out to a high-output version, which I know would probably add drag to the engine regardless of whether I add loads, but then switch to underdrive pulleys to slow it down. My main goal here isn't reducing drag as much as reducing noise. There isn't a lot of noise insulation in my old stripper truck, and that little fan doing its thing has to be making a fair amount of racket. Slow it down and the noise gets lower. Any opinions?

This is all blue-sky of course, those alternators are couple hundred bucks and I'm not Daddy Warbucks.

ebacherville 06-16-2008 09:27 AM

the energy efficiency from a electric fan is its only doing any work when its needed.. even a good clutch fan is always adding some drag to the system..

truly if you can drive with no alternator or mod it to run only in situations where power is truly wasted, i think you'll get gains from it.

AnDoireman 06-17-2008 07:27 AM

Are there low resistance alternators on the market at all? Although I'm sure if there are, they'd be rather mainstream in the eco modding communities.

jamesqf 06-17-2008 02:56 PM


Originally Posted by elhigh (Post 35180)
Hear's a thought, though: I've considered switching my alternator out to a high-output version, which I know would probably add drag to the engine regardless of whether I add loads...

Actually, it shouldn't add much load, if any. When the alternator isn't producing energy, you just have friction losses from the belt & bearings. That's going to be pretty much the same with a high-output version.

As for finding one, scan the junkyards for old cop cars.

1GCRXHF 06-17-2008 04:43 PM

The "friction" in the alternator actually charging is the magnetic resistance the active circuit is supplying. The resistance is the current voltage the battery sitting at in the circuit. Generators (like your alternator) and electric motors are the same thing, the difference is the polarity of the circuit. If you hook up your alternator straight to your battery it will run; The engine has to overcome this force in order for the alternator to charge the battery, because it actually has to spin the alternator the OTHER way, in order put the battery in a charging state. I think the only way you could get less resistance from the alternator running would be to go with a low amperage unit. Adding a larger output alternator would be the exact same as installing a larger electric motor for your engine to fight. If you did your wiring right and went minimalist on your electrical goodies, you could easily get away doing this. Take a good look at anything that uses electricity in your car, and remember that every single milliwatt you is energy derived from your gasoline. Use less power, use less gasoline.

getnpsi 06-17-2008 06:41 PM

cop cars actually have smaller pullies that increase the output at idle. the upgraded cop package of the 80s on fords became the mainstram parts in the 90s as ecus, nice radios, power windows etc used more juice and became more "standard equipment."

to see a real gain you either need to use the smallest alternator you can get away with, OR use a high power alternator that only runs when you are decellerating. i dont know how to take one apart (and get it back together) to put a switch to turn off the field. there is a thread about it and from what i see is they just remove the darn thing and run a bunch of deep cell batteries. If someone is a rebuilder they can probably do it. id pay a guy to put some sort of switch on there to operate it by choice.

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