More woofer = more gas?

[masterchief819]

So some guy says that he's running a 1400w sub in his car and that it really drains his gas. He says that because it takes so much energy to run, that there is more stress on the alternator which makes his gas mileage go down. Is this possible?

Sry if this is a dumb q

 

[supra8903]

no, the weight mabey, but not the electric system

 

[born2drv]

I think he's pulling your chain.... think of the energy required to move a 3000lb car... and the energy required to move a subwoofer.... clearly comparing one to the other the subwoofer seems negligable. I could do the conversions and calculate engergy in kJ, figure out losses in engergy, etc... but lets be realistic, they don't compare to eachother.

He might lose 0.1-0.2mpg or something if that based on the load of the alternator. If the alternator has a clutch and robs the engine of more power like the AC does then sure it could have an affect. Or if he put in a very high performance alternator which requires much more torque to turn, I could see a small decrease in fuel efficiency.

The A/C drains power and therefor affects fuel milage but not based on the electric power consumed but by the drag effect caused by engaging the AC clutch which therefor puts a load on the engine. If he thinks his AC pulls 20 amps and decreases fuel milage 10% and so his stereo pulls 50 amps and decreases it 25% he is flat wrong.

 

[Syris]

You arent going to lose Mileage off exlectricity. He will however off of having too much weight in the trunk though.

 

[T34418L3ONE]

no hes losing gas milage from drag. Engine has to spin the alt. more load on the alt the harder it is to spin.

 

[born2drv]

T3441 - Load on alternator does not vary with electrical load of car audio or other accessories. There is no clutch on the alternator like there is with the AC system. Torque is constant regardless of how many electrical devices you are running. The only variable is the RPM's of the engine which determines load of the alternator.

So if this guy has to rev his engine more to get the alternator to get his alternator to produce more amperage, or he increases his idle RPM so his batteries charge faster, that's the only way he will consume more gasoline.

 

[figgie]

T3441 - Load on alternator does not vary with electrical load of car audio or other accessories. There is no clutch on the alternator like there is with the AC system. Torque is constant regardless of how many electrical devices you are running. The only variable is the RPM's of the engine which determines load of the alternator.

So if this guy has to rev his engine more to get the alternator to get his alternator to produce more amperage, or he increases his idle RPM so his batteries charge faster, that's the only way he will consume more gasoline.

hmm

Sorry to say but you are wrong. Toyota on the 7m-GTE/GE uses a feedback based alternator. What you stated is true for a 1 wire based alternator.

The feedback system that the toyota uses from OEM does in fact increase load as it detects a drop in voltage.

Toyota's high-speed alternator has the following terminals: "B", "IG", "S", "U" and "17".

When the ignition switch is "on", the battery current issupplied to the regulator through a wire connected between the switch and terminal "IG". When the alternator is charging, the charging current flows through a large wire connected between terminal "B" and the battery. At the same time, battery coltage is monitored for the MIC regulator through terminal "S". The regulator will increase or decrease rotor field strength as needed (read increases or cecreases LOAD). the indicator lamp curcuit is connected through terminal "U". If ther eis no output, the lamp will be lit. the rotor field coil is connected to terminal "P", which is accessible for testing purpose though a hole in the end frame.

 

[born2drv]

OK my bad....... perhaps load does change making the engine run less efficient to the wheels (ie. lower fuel mileage)... but I still can't see it making a significant difference. I could be wrong again though

 

[Facime]

ever need to jump someones car with your Supra? when you make that final connection do you notice your RPM drop? Thats increased load. however, I still think the guy is yankin your chain about his fuel milage.

 

[Jeff Lange]

More current draw = more load on the engine.

This is one of the reasons an electric fan isn't necessarily the best upgrade, you are now doing 2 conversions instead of none at all. (Mechanical to Electrical, then Electrical to Mechanical, instead of just Mechanical to Mechanical), when using an electric fan, you are draining more power from the engine than you would with a mechanical fan (stock). The only real advantage is that the electric fan is not always on, but if working properly, neither is a clutch fan.

A sub could reduce fuel economy by a noticable amount, if constantly pulling max current, which if will almost never do. I doubt you'd notice a drop in milage beyond acceptable error.

 

[Fuzz420]

When i had my stereo in i could tell a slight difference in power, when i had it pulling maximum current.The weight is more of a killer than anything. I had two 50lbs subs, in a mdf box, talk about a gas killer.

 

[Dirgle]

It is feasible for an 1400W amp to place a noticeable load on an alternator designed to properly support such a sound system. You would especially notice this drop if you had the headlights on, the stereo up with a bass CD, the heater, rear window defroster, windshield wipers on, and radiator fans(on newer cars)

To run the average (newer) car under those conditions requires 960~1000 watts to run(yes this is on the high but believe it, these newer cars with all the electronics pull a lot of juice. If you turn on everything in the car you approach this. This is an 80~83 amp draw and most newer cars have 120amp ALT for this very reason.) So you have this plus the 1400W draw off the audio amplifier.

Totaling an est. 2400watt draw at peak usage.

This breaks down to 200amps(a properly set up sound system will have an upgraded alternator to handle the extra load.)

Now it requires 1 HP to produce 25amps. So 200/25= 8hp

It requires 8hp to run this system at full tilt.

So if we remember our power to weight conversions for every 14 lbs our car loses we gain 1 hp. Reverse this and we get the power loss. So with everything turned on it’s like we added 112lbs to our car just by turning stuff on. Not to mention the added weight of the speaker box and amp.
On a big V8 the power loss and the MPG loss might not be noticeable but on the smaller V6 and four-banger it will defiantly be noticed.

*it requires 10~13hp to drive an AC compressor. So imagine the power loss and MPG hit you feel from turning on the AC.

 

[Jeff Lange]

A point worth noting, even if you have a max lets say 2000 watt setup, you will almost never hit 2000 watts, and if you do, it will probably be for less than a few seconds at best.

Now typically you will be producing more than 12V, but lets assume 12V for arguments sake.

1HP = 745W (62A in a 12V system, not 25A -- not sure where 25A came from). So 2400W = 3.2hp.

3.2hp isn't that much, especially in a Supra. Of course that's assuming perfect transfer to electrical energy from mechanical (alternator), so it might be a bit higher, but still, not all that much IMO.

 

[Dirgle]

You are correct that 1HP=745W.

However, and I'm not sure why, with automotive alternators it's a general rule of thumb that it takes 1HP to produce 25A. Maybe automotive alternators are really inefficient, the mechanical to electrical conversion is poor, I don’t know. If you can find any hard math as to why they say 1HP=25A I would love to see it. But I was just going by the general rule when I made those calculations.

 

[Shytheed Dumas]

I think this issue could be cleared up with a little effort. The claim is that the sub "really drains his gas". That verbage sounds a lot like saying that there is a (statistically) significant difference with and without. So go ahead and find out if we can reject the null hypothesis: that running the sub makes no difference. Find out if it's actually significant.

In daily driving to/from work, etc you could burn 10 tanks running the sub and 10 tanks not while making every effort to keep all other variables, like running a/c, idle time, etc the same (25 of each would be better and 32 of each ideal). Instead of doing all with and then all without in a row, randomize the order. Record the actual mileage on each tank of gas. Send the data to me. As a side study, if you're going to be logging some serious miles, do the to trip with the sub and the return trip without on a long trip, or do the entire thing this way if you do lots of long trips - less noise.

First thing would be to do a test for normality and an analysis of variance and see how you did on keeping your other mileage affecting variables constant and/or whether there is some reason why running a sub would cause you to have more erratic mileage, not just worse mileage.

Assuming your data sets are normally distributed and both have statistically similar variances you then run a T-Test. A P value <0.05 would mean that you are 95% certain that running the sub REALLY makes a difference. THEN go figure out why. :icon_bigg

 

[Jeff Lange]

You are correct that 1HP=745W.

However, and I'm not sure why, with automotive alternators it's a general rule of thumb that it takes 1HP to produce 25A. Maybe automotive alternators are really inefficient, the mechanical to electrical conversion is poor, I don&#8217;t know. If you can find any hard math as to why they say 1HP=25A I would love to see it. But I was just going by the general rule when I made those calculations.

Got a link to somewhere that has that info? I've been searching for a couple minutes and can't find anything.

EDIT: Actually I've found a couple places that say that, I've also seen a few others that quote it at about a 15% loss, so I dunno.

 

[Shytheed Dumas]

No love for the statistics? aaaahhhh!

 

[Jeff Lange]

There's no way I would be able to do any testing to get the statistics, so nope... no love, lol.

 

[Shimmy]

anyone know id Cruise control uses more gas?

 

[bigaaron]

T3441 - Load on alternator does not vary with electrical load of car audio or other accessories. There is no clutch on the alternator like there is with the AC system. Torque is constant regardless of how many electrical devices you are running. The only variable is the RPM's of the engine which determines load of the alternator.

So if this guy has to rev his engine more to get the alternator to get his alternator to produce more amperage, or he increases his idle RPM so his batteries charge faster, that's the only way he will consume more gasoline.

No way! The alternator IS harder to turn when there is a big load on it. People think that just because it spins freely by hand that it's the same under load. That is not true. An alternator creates power from turning mechanical energy into electrical energy, at about a 50% loss. An alternator is not a magical device that creates energy. I just posted this link a few days ago:
http://www.svapowerproducts.com/html/how_much_hp_does_an_alternator.html