Amps + Watts = ?
Instructor
Joined: Jul 2007
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Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
Say you're sitting at a light driving at night. You're in gear and idling around 700. Your 10SI alt is putting out 30-34 amps.
Now, consider that you have you ignition system running, headlights, taillights, marker lights, stereo, and maybe even A/C or a heat blower.
If you happen to be using more than the 30-34 amps then your alternator is maxed out at that RPM and any power consumed on top of that would be supplied by the battery. Then, once the RPMs are raised by driving the alternator catches up and charges the battery back up.
If this happens too many times or for extended periods of time, the battery will degrade faster than with normal operation. With a higher output alternator, your battery voltage will never drop, and should reach advertised life expectancies.
Instructor
Joined: Jul 2007
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Say you're sitting at a light driving at night. You're in gear and idling around 700. Your 10SI alt is putting out 30-34 amps.
Now, consider that you have you ignition system running, headlights, taillights, marker lights, stereo, and maybe even A/C or a heat blower.
If you happen to be using more than the 30-34 amps then your alternator is maxed out at that RPM and any power consumed on top of that would be supplied by the battery. Then, once the RPMs are raised by driving the alternator catches up and charges the battery back up.
If this happens too many times or for extended periods of time, the battery will degrade faster than with normal operation. With a higher output alternator, your battery voltage will never drop, and should reach advertised life expectancies.
Now, consider that you have you ignition system running, headlights, taillights, marker lights, stereo, and maybe even A/C or a heat blower.
If you happen to be using more than the 30-34 amps then your alternator is maxed out at that RPM and any power consumed on top of that would be supplied by the battery. Then, once the RPMs are raised by driving the alternator catches up and charges the battery back up.
If this happens too many times or for extended periods of time, the battery will degrade faster than with normal operation. With a higher output alternator, your battery voltage will never drop, and should reach advertised life expectancies.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Who said anything about 200 watts? A normal 800 watt 4 channel amp at RMS outputs would draw 15-20 amps, up to 30 when working hard on a good song turned up.
Besides, that's how a lot of people listen to their music, mostly young kids with a statement to make, but that's just how it's done.
It's not the stop and go that's bad for the battery, it's letting the electrical needs of the accessories draw more than the output of the alternator.
Instructor
Joined: Jul 2007
Posts: 152
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Who said anything about 200 watts? A normal 800 watt 4 channel amp at RMS outputs would draw 15-20 amps, up to 30 when working hard on a good song turned up.
Besides, that's how a lot of people listen to their music, mostly young kids with a statement to make, but that's just how it's done.
It's not the stop and go that's bad for the battery, it's letting the electrical needs of the accessories draw more than the output of the alternator.
Besides, that's how a lot of people listen to their music, mostly young kids with a statement to make, but that's just how it's done.
It's not the stop and go that's bad for the battery, it's letting the electrical needs of the accessories draw more than the output of the alternator.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
120
Right.It's about having more than you need instead of less than you need.
If someone doesn't want to change from their 10SI that's fine, it won't hurt anything but the battery and it'll make your headlights look funny if you have the volume up. Generally if you go to the trouble of installing a good stereo in these cars you're going to use it.
Maybe the car has an electric fan pulling 20 amps.
Lights all the way around, stereo, and ignition will probably be just as much or more than a stock SI can handle. If it's an early 34 amp SI then for sure an upgrade is needed.
Seriously Bill, no one's asking you to change. It's just a good suggestion for someone with modifications or changes to the accessories.
Last edited by Durango_Boy; Sep 14, 2007 at 10:18 AM.
Burning Brakes
Joined: Dec 2002
Posts: 1,069
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From: Sherwood Park AB
Of course more is sometimes better, but sometimes it is not necessary.
I had an 86 S10 4 cyl with a stock original alternator. It had 2 amps in it running a 12" sub and a set of 6 1/2" front speakers. It went louder than I ever wanted it to go (115 db) and I never ever had a problem with the alternator not keeping up with the stereo. My wife has a 91 Mustang 5.0 with an amp running a 12" sub, again, she has never had a problem.
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
Correct.
Of course more is sometimes better, but sometimes it is not necessary.
I had an 86 S10 4 cyl with a stock original alternator. It had 2 amps in it running a 12" sub and a set of 6 1/2" front speakers. It went louder than I ever wanted it to go (115 db) and I never ever had a problem with the alternator not keeping up with the stereo. My wife has a 91 Mustang 5.0 with an amp running a 12" sub, again, she has never had a problem.
Of course more is sometimes better, but sometimes it is not necessary.
I had an 86 S10 4 cyl with a stock original alternator. It had 2 amps in it running a 12" sub and a set of 6 1/2" front speakers. It went louder than I ever wanted it to go (115 db) and I never ever had a problem with the alternator not keeping up with the stereo. My wife has a 91 Mustang 5.0 with an amp running a 12" sub, again, she has never had a problem.
That high a db level is not hard in a small cabin S10. What were the RMS ratings of each amp? That S10 had at the very least an 80 amp alternator.
Your wife's Mustang also had a similarly rated 80 amp alternator.
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Le Mans Master
Joined: Oct 2000
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From: Santa Ana CA
That's important because if you go with twice the rated resistance (same as impeadence)in a speaker, the power you will get out is only 1/4 the rated power.
eg: If your amp says it delivers 200 watts into a 2 ohm sub and you hook up a 4 ohm sub you get,, that's right, 50 watts of power out of it. nothing you can do about it either. just bear these facts in mind,, i'd say that 75 clean watts of power into each of 4 speakers would be plenty good enough for even the most jaded listener,, and will sound great if you use quality componants and position them right.
eg: If your amp says it delivers 200 watts into a 2 ohm sub and you hook up a 4 ohm sub you get,, that's right, 50 watts of power out of it. nothing you can do about it either. just bear these facts in mind,, i'd say that 75 clean watts of power into each of 4 speakers would be plenty good enough for even the most jaded listener,, and will sound great if you use quality componants and position them right.
If the amp delivers 200 watts into 2 ohms, it delivers 100 (not 50) watts into 4 ohm.
Double the impedience = half (not 1/4) the power:
Watts = voltage squared divided by the resistance.
12 volts x 12 volts = 144/4 = 36 watts
12 volts x 12 volts = 144/2 = 72 watts
Le Mans Master
Joined: Oct 2000
Posts: 8,897
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From: Santa Ana CA
The 15" E140 is the JBL bass speaker (previously a D140) and it does 100 dB @ 1 watt @ 1 meter. Their 15" E130 guitar speaker does 105 dB @ 1 watt @ 1 meter and the 12" E120 does 103 dB @ 1 watt @ 1 meter. (I've been a JBL fanatic since 1968.)
http://www.jblpro.com/pub/components/eseries.pdf
Last edited by toddalin; Sep 14, 2007 at 12:20 PM.
Melting Slicks
Joined: May 2007
Posts: 3,246
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From: Bay Area CA
Let's end some fallicies here and now, this being one.
If the amp delivers 200 watts into 2 ohms, it delivers 100 (not 50) watts into 4 ohm.
Double the impedience = half (not 1/4) the power:
Watts = voltage squared divided by the resistance.
12 volts x 12 volts = 144/4 = 36 watts
12 volts x 12 volts = 144/2 = 72 watts
If the amp delivers 200 watts into 2 ohms, it delivers 100 (not 50) watts into 4 ohm.
Double the impedience = half (not 1/4) the power:
Watts = voltage squared divided by the resistance.
12 volts x 12 volts = 144/4 = 36 watts
12 volts x 12 volts = 144/2 = 72 watts
Burning Brakes
Joined: Oct 2006
Posts: 1,044
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From: Henrico VA
I've got the orginal 42 Amp alternator and a 1500 Watt stereo with 14 speakers total. At night, I have probablems with the high beams after about half an hour, but that's it. You'll be fine.
-Adam
-Adam
Team Owner
Joined: Aug 2006
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From: Columbia Missouri
Thread Starter
Instructor
Joined: May 2007
Posts: 249
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From: Waterville Ohio
Me thinks this thread has turned into sort of a monster 
. I think I'll run the new system without an amp and see how it sounds 
. I'm still debating on how the new box will look in the rear, but I'll post pics when it's done 
. Thanks to everybody who help with this thread, and gave me many opinions on this matter
. I think I'll run the new system without an amp and see how it sounds . I'm still debating on how the new box will look in the rear, but I'll post pics when it's done . Thanks to everybody who help with this thread, and gave me many opinions on this matter
Race Director
Joined: Apr 2007
Posts: 14,062
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From: Henderson Nv-Rohnert Park/Sonoma C o. ca/born in NY Rockaway Beach.
I Just did the same stereo upgrade and added an 2 front 6.5 and a 10in sub with a 300w amp. After a few weeks I now need a new battery. It was a Die Hard 5 years old. Just may have went. Im replacing it this weekend and once i do im going to check the sysem tosee if i need to upgrade my alt aswell......EMT mike here is a link to my thread if you want to take a look at what i did... With DBoy's help
http://forums.corvetteforum.com/show...763851&page=12
http://forums.corvetteforum.com/show...763851&page=12
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
I asked about the RMS ratings of your amps. You yourself can figure how many amps your stereo system is pulling. I would also like to know where in the world you put 14 speakers. I'd put money on half of them canceling out another.
Melting Slicks
Joined: Sep 2007
Posts: 2,429
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From: Tucson AZ
BillPk, you can't seriously be arguing ofer what a 72 amp alternator puts out at idle, can you?? If you have updated electronic equipment in an older Vette, do you really think the old unit will put out enough power (amps) to operate that stuff?? In 196? or 197? there were no alternators that put out............never mind, ignorance is really bliss
Safety Car
Joined: Jun 2005
Posts: 3,775
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from what little i know, and have done...
ive got 2 4x6 pioneer 20wRMS speakers running off my kenwood head units, 22wRMS x 2, and 2 6x9 pioneer 40wRMS running off a pioneer GM-5100T amp, 2x125wRMS.
i can idle at 700 with my system cranked, high beams on, and the heater set to max and my lights barely dim. and this is on the stock 38 year old alternator.
i can also do 85-90mph with my top off and the radio is still loud enough that i cant even hear the wind.
ive got 2 4x6 pioneer 20wRMS speakers running off my kenwood head units, 22wRMS x 2, and 2 6x9 pioneer 40wRMS running off a pioneer GM-5100T amp, 2x125wRMS.
i can idle at 700 with my system cranked, high beams on, and the heater set to max and my lights barely dim. and this is on the stock 38 year old alternator.
i can also do 85-90mph with my top off and the radio is still loud enough that i cant even hear the wind.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
from what little i know, and have done...
ive got 2 4x6 pioneer 20wRMS speakers running off my kenwood head units, 22wRMS x 2, and 2 6x9 pioneer 40wRMS running off a pioneer GM-5100T amp, 2x125wRMS.
i can idle at 700 with my system cranked, high beams on, and the heater set to max and my lights barely dim. and this is on the stock 38 year old alternator.
i can also do 85-90mph with my top off and the radio is still loud enough that i cant even hear the wind.
ive got 2 4x6 pioneer 20wRMS speakers running off my kenwood head units, 22wRMS x 2, and 2 6x9 pioneer 40wRMS running off a pioneer GM-5100T amp, 2x125wRMS.
i can idle at 700 with my system cranked, high beams on, and the heater set to max and my lights barely dim. and this is on the stock 38 year old alternator.
i can also do 85-90mph with my top off and the radio is still loud enough that i cant even hear the wind.
Hey bud, I just wanted to let you in on some results of some testing we did at the audio shop. We had a guy come out with some nice expensive equipment and measure the RMS output of any head unit we had or anyone brought.
Just to put things in perspective, even the best Kenwoods, Eclipses, Alpines and Pioneers didn't have an output greater than 20 watts RMS, and the average was around 17 watts.
That head unit of your is probably giving you a total of 80 - 100 watts when the volume is cranked and around 75 or so for normal volume listening.
I really can't say without testing but that head unit, even cranked, is probably not drawing more than 5 amps. A head unit without external amps won't put too much of a strain on an alternator...even a stock one.