1. ## Is my math correct?

I have a 2005 Grand Cherokee with the 4.7 V8, which as far as i have been able to learn has a stock alt that puts out 130'sh amps.

So...

130 - 30 (for stock accesories) X 14.4 =1440 watts right?

I can probably safely run an amp with up too 1400 watts? Is this the peak measure or the RMS measure?

The plan is to run a Type R 15" or something similar with one mono amp powering it to its max. The jeep only has 18,000 miles on it and don't want to over do the system and have to replace the alt sooner than expected.

Is this an OK idea?

Any suggestions other than the "big 3"?

2. ## Re: Is my math correct?

I had a ford with a 130 amp alty and safely ran 1400watts rms...

3. ## Re: Is my math correct?

Originally Posted by Cheap5.0
I have a 2005 Grand Cherokee with the 4.7 V8, which as far as i have been able to learn has a stock alt that puts out 130'sh amps.

So...

130 - 30 (for stock accesories) X 14.4 =1440 watts right?

I can probably safely run an amp with up too 1400 watts? Is this the peak measure or the RMS measure?

Any suggestions other than the "big 3"?
Your result is within reason, but getting there is a lot more complex.

From a couple very good and experienced mechanics I've dealt with you can generally assume about 40&#37; of a stock alt's output is head room.

So at 130A, about 50A is unused.

So let's say you run 1000w class D, and 400 class a/b
Class D at around 75% efficient, a/b at 60%, call it 1400w at 70% just for simplicity...
1400/.7 = 2000w
2000w/14 = 143A

Now the big variable - Crest Factor...
That 143A assumes the amplifiers are operated at full output playing sine waves. Music draws a LOT less current than sine waves -- 1/3 at the most. So even at full volume you'd be drawing about 45A. Depending on the music type and your typical listening volume you might not even draw 20A most of the time.

Crest factor (and low demand listening habits) explain why people can run 1000w+ for years on relatively small stock charging systems and never have a problem.

The big 3 is never a bad idea, and you might still see some dimming at high volume - but 1400w on a 130A alt shouldn't be a problem.

FWIW - I have a 2000 contour with a 130A alt. I'm running 1100w with no upgrades to the stock wiring and have only minor dimming at full volume.

4. ## Re: Is my math correct?

Your stock electrical accessories draw a lot more than 30A. The headlights will draw about that on their own. AC and ABS computer are a big draw also. Then factor in the ECU, fuel pump and ignition module. Don't forget the instrumentation and other misc stuff. The vehilce probably uses close to 80A when a lot of stuff is on. Chrysler didn't provide a 130A alt because they thought you would want to power a stereo, they installed it because the vehicle needed it. There's a bit of headroom factored in for duty cycle and aftermarket accessories but not 100A.

Does that mean that you'll have a problem? Probably not but don't think that you've got anywhere near 100A of excess current to play with.

5. ## Re: Is my math correct?

my rangers volt meter dips from just my head unit. shows how crappy mine is

6. ## Re: Is my math correct?

Originally Posted by toolfan89
my rangers volt meter dips from just my head unit. shows how crappy mine is
How crappy your .... install is?

7. ## Re: Is my math correct?

I posted incorrect info, for future ref. a 2005 Grand Cherokee with the 4.7L and heated leather will have a 160 amp alt.

So working with a 160 amp alt should ease things up a bit right? How much draw do you think the heated seats will pull?

Will HID lights require more amps than a stock (non-HID) light system?

I am aiming for that 15" Type R, and a "KICKER ZX1000.1" mono amp. My listening habits are not that extreme (lots of O&A + Ron & Fez on XM), and my wife does drive this from time to time and will not turn it up since she is prego. But when i want thump, i want it to hit.

8. ## Re: Is my math correct?

As someone already suggested, if your Jeep comes with a xxx amp alt. it NEEDS it, with that said 1400 watts will be no problem. If you listen with the vehicle off alot, you will work your alt. harder though. Wich could lead to premature failure.

9. ## Re: Is my math correct?

Originally Posted by 2critical
As someone already suggested, if your Jeep comes with a xxx amp alt. it NEEDS it, with that said 1400 watts will be no problem. If you listen with the vehicle off alot, you will work your alt. harder though. Wich could lead to premature failure.
Thanks for the help, everyone.

10. ## Re: Is my math correct?

Originally Posted by helotaxi
Chrysler didn't provide a 130A alt because they thought you would want to power a stereo, they installed it because the vehicle needed it.
Unless you were on the electrical system design team for that particualr vehicle I really don't think there's any way to know that.

Case in point - I have a 2000 contour - Other than essential accessories it has pwr windows, locks and abs. Do you think that NEEDS a 130A alt? Or do you think Ford just had a bazillion of them on hand so it was just as easy (and cheap) to use it than to spec something at 70 or 80A that would have been more than sufficient?

I'm sure a loaded cherokee has a pretty hefty electrical demand, but 160A is a HUGE alt no matter how you look at it. Unless EVERY accessory is used ALL the time I seriously doubt that 160A alt is NEEDED.

11. ## Re: Is my math correct?

Originally Posted by n2audio
Unless you were on the electrical system design team for that particualr vehicle I really don't think there's any way to know that.

Case in point - I have a 2000 contour - Other than essential accessories it has pwr windows, locks and abs. Do you think that NEEDS a 130A alt? Or do you think Ford just had a bazillion of them on hand so it was just as easy (and cheap) to use it than to spec something at 70 or 80A that would have been more than sufficient?

I'm sure a loaded cherokee has a pretty hefty electrical demand, but 160A is a HUGE alt no matter how you look at it. Unless EVERY accessory is used ALL the time I seriously doubt that 160A alt is NEEDED.
This is a good point, and also valid. I think this is the same alt they put on the hemis in most of there production cars/trucks.

All of this is a moot point now though, i bought a 12W6 and only plan on pushing 500-600 rms.

12. ## Re: Is my math correct?

Copper is expensive as hell. Bigger alt means more copper and more expensive to make. With car companies cutting cost in every way that they can, you think that they're just going to have a bunch of huge alternators laying around to throw in whatever vehicle it could be made to fit?

The nicer vehicles have bigger alts because all the options draw power, heated seats, power windows, HID lights, fog lamps, power sunroof, etc. My car has a 165A alt from the factory, and they typically bench at around 180A. The power windows draw enough current to make the vent fan motor slow down from the voltage drop. This was before I installed any stereo components and the vehicle was completely stock. Don't for a minute think that a car doesn't draw a ton of amperage and that there is a ton of excess to play with because your stock alt seems big.

13. ## Re: Is my math correct?

I disagree pretty strongly, but I just typed up a big post supporting my opinion and then my browser errored and I had to close it.
I'm not typing it again.
So -- whatever.

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