Does a high output alternator put more load/drag on the engine only when it is being called upon to deliver extra current for accessories such as lights, sound systems, missile launchers, etc. ? If all of those are turned off, will something like a 250amp alternator still require more engine power to spin than the stock 120amp alternator assuming the vehicle is operating in stock form with something like 60amp's of current demand?
High Output Alternators and Engine Load question
- Thread starter Johnny248
- Start date
Yes and no.
You're pretty much spot on with more demand = more load so those things are a wash...but.
Larger units usually have smaller pulleys (to spin them faster) and heavier rotors, as well as bigger fans. Takes a smidge more power to spin those but shouldn't be much more than a standard unit. Probably not enough to notice.
You're pretty much spot on with more demand = more load so those things are a wash...but.
Larger units usually have smaller pulleys (to spin them faster) and heavier rotors, as well as bigger fans. Takes a smidge more power to spin those but shouldn't be much more than a standard unit. Probably not enough to notice.
Awesome. Those where my thoughts, but I've heard of people making claims of losing 3-4mph when installing a HO alternator. I guess I can see that if you are pulling 300amps all day long, but I wouldn't think there would be much difference with normal draw use.
MolaKule
Staff member
@Johnny248 1 Hp Mechanical Energy =746 Watts of Electrical Energy.
250A X 12V = 3000 Watts; 3000 Watts/ 746 = 4 Hp
120A X 12V = 1440 Watts; 1440 Watts/ 746 = 1.9 Hp
60A X 12V = 720 Watts; 720 Watts/ 746 = 1 Hp
250A X 12V = 3000 Watts; 3000 Watts/ 746 = 4 Hp
120A X 12V = 1440 Watts; 1440 Watts/ 746 = 1.9 Hp
60A X 12V = 720 Watts; 720 Watts/ 746 = 1 Hp
It depends. If nothing but the alt is changed, I say no. If you added more electrical demand as well, it might, but the battery will remain in a fully charged state more than it would with the smaller alt.
You can easily double those numbers since alternators are only around 50% efficient.
Also, a lot of high output alternators are actually worse at charging at idle/slow engine speeds. That's why they're usually equipped with a smaller pulley, to get them spinning faster. Just the nature of the beast.
MolaKule
Staff member
It is important to note that alternators typically consume 0.5–1% of the truck’s fuel to perform their function, so any fuel economy gains from new technology will be modest."
https://nacfe.org/research/technology/powertrain/high-efficiency-alternators/
These efficiency gains are mostly due to using conductors that allow greater fill of the windings in the alternator.
This means they use larger wire to reduce I^R losses.
https://nacfe.org/research/technology/powertrain/high-efficiency-alternators/
These efficiency gains are mostly due to using conductors that allow greater fill of the windings in the alternator.
This means they use larger wire to reduce I^R losses.
Ctechbob is right on. Electric power consumption vs engine load will be very similar. But usually the high output alternators have a smaler diameter pulley so they spin faster, and usually the cooling fan of the higher output alternator pushes more air to supply more cooling and that increases load on the belt and engine some, even when under light electric load.
Also, the manufactures of high output alternators with smaller dia bullies usually highly recommend using a slightly smaller length serpentine belt. And a new tensioner is a good idea if its old.
It can be difficult to find the correct new length serpentine belt. Along with the shoter pulley, usually there is one or more items that are driven by the back of the belt. If so, then the thickness of the belt plays a huge role in how much of the belt is used as it winds amount pulies. So while a specification may say a belt is shorter but yet long enough to still go on, a thick belt may not go on. It can get frustrating, before you find the correct belt.
By the time a thick serpentine winds around pullies using both sides, the belt thickness uses up enough length that it might not go on.
Also, the manufactures of high output alternators with smaller dia bullies usually highly recommend using a slightly smaller length serpentine belt. And a new tensioner is a good idea if its old.
It can be difficult to find the correct new length serpentine belt. Along with the shoter pulley, usually there is one or more items that are driven by the back of the belt. If so, then the thickness of the belt plays a huge role in how much of the belt is used as it winds amount pulies. So while a specification may say a belt is shorter but yet long enough to still go on, a thick belt may not go on. It can get frustrating, before you find the correct belt.
By the time a thick serpentine winds around pullies using both sides, the belt thickness uses up enough length that it might not go on.
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Somewhere is a formula to see how much hp is needed to go 3 mph faster at a car's top speed/ wind resistance. Even if they ignored drag from tires you'd get pretty good numbers. I bet its a bunch of hp, dozens if not 40-50. Alternators, even 300 amp ones, don't draw that much.
For fun, take off your serpentine belt. Give the alternator pulley a spin. It'll keep spinning from the inertia of the internal moving parts. Now turn the key on, which will activate the field current. It'll spin with much more resistance.
For fun, take off your serpentine belt. Give the alternator pulley a spin. It'll keep spinning from the inertia of the internal moving parts. Now turn the key on, which will activate the field current. It'll spin with much more resistance.
We do all realize what new cars are coming with, right? My JL came with 240A due to electric-over p/s pump and of course all the modules, heated seats, heated steering wheel etc.
I dunno if electric steering racks take even more?
ANYWAY the point is if it really affected MPG that much someone (like those weird people sitting in a CAFE making rules) would be up in arms. As it is we're running water for oil and ATF at ~240F claiming it gets some (miniscule) degree of improved MPG.
Now I realize this is far from a technical argument, I'm just saying step back and look at the view from 30,000 feet
I dunno if electric steering racks take even more?
ANYWAY the point is if it really affected MPG that much someone (like those weird people sitting in a CAFE making rules) would be up in arms. As it is we're running water for oil and ATF at ~240F claiming it gets some (miniscule) degree of improved MPG.
Now I realize this is far from a technical argument, I'm just saying step back and look at the view from 30,000 feet
I thought some of the ones for boat engines needed dual V belts.
Great point. Ideally, the work seen by the engine is the same, or less - and I’m betting Less, because it’s all being done for fuel economy. Instead of cranking on the hydraulic pump for steering, the engine sees the load through the alternator instead.We do all realize what new cars are coming with, right? My JL came with 240A due to electric-over p/s pump and of course all the modules, heated seats, heated steering wheel etc.
I dunno if electric steering racks take even more?
ANYWAY the point is if it really affected MPG that much someone (like those weird people sitting in a CAFE making rules) would be up in arms. As it is we're running water for oil and ATF at ~240F claiming it gets some (miniscule) degree of improved MPG.
Now I realize this is far from a technical argument, I'm just saying step back and look at the view from 30,000 feet
Note that the alternator is having to adapt to constantly changing RPM. So if 200 watts are commanded, at low RPM it’s going to drag a ton of torque at it’s armature, whereas if the engine spins up, alternator RPMS will spin up and in exchange for more RPMs the effective torque will fall.
CCOT air conditioner compressors are the same way. They have to move BTU's per minute, not BTU's per revolution. My Saturn was a dog just coming off of idle for this reason.
In a car with less than 150hp, you'll notice it when you start drawing demand from an HO alternator. Over 200hp and torque, you probably won't notice it much at all except at idle if there's a large draw.
Not much. I've got a modified heavy duty truck alternator I use for welding. A Honda 6.5hp engine can make about 120 amps at 25 volts while welding.
Most of the replies seem to assume that either alternator outputs its maximum amperage, which is not the case. Only the supplied voltage is constant, the amperage and therefore load on the alternator varies to match the demand.
In short, only at maximum load will the bigger alternator require more from the engine, at similar loads the difference will be negligible.
In short, only at maximum load will the bigger alternator require more from the engine, at similar loads the difference will be negligible.
I had a Ram 5.7l 160amp that I was able to find a 2500 Cummins Alternator that fit right up. Only difference was that the B+ post was up instead of out the back. I can't remember if the pulley was the same size, but the Cummins alt had an overrun pulley on it, where the original did not. Worked like a charm. I have a Jeep with a 250amp and it did have a smaller pulley and required a shorter belt. It was pretty easy to figure out though using standard Bando information. The reason this came up is because I have a Tacoma which I wanted an HO. The vendor said that 250amp is no problem, however when you get to 275 or more the tensioner starts having issues keeping up. I wasn't sure if that was an issue all of the time, or just when you are really loading the alternator.
