Re:Re:Re: Question: watts needed to kill an engine

[StereoInstaller1]

Well I used to install at best buy and have done sone custom stuff for people on the side. I'm not too worried about finding an amp or installing. I'm worried about the current my amp would pull if I ran the sub @ 100% killing the engine. Have you played around with any high current systems in E30s? You seem to be the car audio guru on the board. I was originally going to throw this sub in a blazer and upgrade the alternator but my friends slapped me out of my stupidity and I bought an E30. Now I'm not really sure how I should approach this.

Sent from my SGH-i917 using Board Express

Well, since you have some technical background, you should already know that 750 watts basically equals 1 HP. Figure a water pump uses about 1 HP, power steering using about 8 HP (maybe? ish?) to go full lock on big tires at idle...so maybe a 10HP load might kill a bone stock M10, maybe a smooth load (not all spikey, no music, just test tones)of 30HP might not kill a fresh M30.

Honestly, I have no clue what it would take, but I think you would need to upgrade the drive belt if you tried it very ofter. That little V belt stopping a running motor? I wonder if it would do it twice!

So now we gotta talk about loss in that calculation: 50% on each end covers it pretty accurately IMO. Figure the alternator at 50% loss and figure the amp at 50% efficiency.

(This is what the industry I have spent 30 years of my life in seems to agree on. Argue away on this, guys. I would love to conclusively answer this.)

So, that means that just because the math is good that 1HP=750W (I know, not exactly, but close enough, so FOAD) that does not accurately recreate the condition of use that the "1 horsepower" is used in. Due to the fact the best of the best alternators over time have proven again and again is the 50% ratio. Takes 2 HP to put out 750 watts because half that (basically)just spins the shaft. The other half gets used to power stuff. The 750 watts used to make power requires 1500 watts to make is the consensus.

Now, on the other end is load. That amp that is putting out 750 watts is basically using 1500 to make it...remember, there ain't no free lunch.

So, by my calculations, it takes 4HP to produce 750 watts of actual speaker driving power.

If my guesses of fluid dynamics vs. power demands of a steering system are correct (again, please argue away, this might get good) I am gonna "guesstimate" you could might maybe kill a decent all-stock M20 by asking your amplifier to provide a steady test tone (like 40Hz), I am gonna guess 7500 watts, or roughly 40 HP.

So, lets hear your take on this, guys. This is too good a question to keep as PM.

Luke

[Das Himmelman]

I agree, thanks for creating a thread.

Here is how I'm seeing issues and how I understood it when I bought the sub.

Ohms law states that I=P/E. (Current=Watts/volts)
Following this, if I ran my sub at it's full rated 1800w RMS, the amp would pull 125 amps (1800/14.4=125).

I'm my subs peaked, and my amp put out 3600w, then it would be pulling a 250 amp draw on the battery. And while the power you use while driving comes from the alternator (the battery is used as a rectifier), a 80-90 amp (from what a quick Google search told me) stock alternator isn't going to be able to handle 125 amps, let alone 250. You see lights dim in lower power set ups when a sub hits hard, I'm worried about it dimming everything completely. =P

[StereoInstaller1]

Agreed, a full 1800 RMS coming out of an amp will dim the hell out of everything...but it won't kill the motor, unless you were using test tones.

I would love to see some conclusive data!

[Das Himmelman]

Ok, So I did some digging on the12volt.com and found a post asking pretty much what I am and the best reply was from their mod DYohn,

"Actually, an amplifier (or any other electrical load) will try to draw whatever current it needs at any time no matter if the supply can provide it or not. If the supply (your alternator) cannot give it all it needs, the system will heat up trying to supply the current (and potentially fry trying to do so) and the voltage will drop as the current load goes up. Same if your cables are too small: the amp will draw whatever it needs through whatever size cable you use. The cables will heat up and can fry if they are not large enough to support the demanded current flow. If you have a sound system that can demand 200 amps and an alternator that can only supply 54 amps, what will happen if you turn it up is the alt and/or cables will simply be overloaded, your voltage output will drop, your sound system and/or all other electrical loads in your vehicle will shut down, and potentially you can fry your alternator and/or your power cables and unless your system is properly protected you can burn the whole car down. It will have NOTHING to do with how "loud" the system can get. It will get as loud as it can - that is, right up to the point where the alt is overloaded, then everything will begin to drop off the table and you can even end up with a molten hunk of metal instead of a car. :) Not a good situation."

So it looks like if I try to draw too much current, it'll fry the alternator and cables and eventually shut everything down. =( The best solution I could find too looks like first doing the "Big 3" mod and then a high output alternator if that isn't enough.

To sum up the "Big 3" mod is upgrading the stock wires to 0 gauge wires in these locations:

*Battery positive (+) to alternator - From the positive terminal on your battery to the alternator positive (+) post
*Engine block to chassis ground - From engine block to chasis. This is to strengthen the ground.
*Battery ground (-) to chassis ground - From the negative terminal on your battery to your car's chassis.

[StereoInstaller1]

Got any 0 gauge? NAPA has "better than" 0 gauge terminals.

I have about 50 feet of 00 Monster brand wire...beastly stuff, WAY bigger than say, Stinger HPM. I am gonna run that front to back and dual Exide spiral cells...only to run 1800 watts RMS, total. I think 4 GA is big enough for my alternator, but I may bump that to zero as well, but not this monster stuff, it is too damn big.

[Das Himmelman]

No, I need to get all my wires still. Running 0 gauge front to back is going to be a fun task... Where do you throw your second battery?

[ck_taft325is]

In the front battery tray would be my first choice. Otherwise, start stacking 'em in the trunk?

[StereoInstaller1]

1 in each factory spot...custom terminals on each end though. 00 grounds, too.

My current setup:

[Das Himmelman]

That looks really clean man. Good job. I'll have to come see your setup in person when I move to SeaTac in 2 weeks.

[StereoInstaller1]

That looks really clean man. Good job. I'll have to come see your setup in person when I move to SeaTac in 2 weeks.

Car is currently torn apart, won't be back together until about June.

[Das Himmelman]

Unfortunate. Engine swap? =D

[StereoInstaller1]

S50 on a stand in my garage as we speak!

Building boxes instead though.

[Das Himmelman]

Well, if you need some help with boxes, I'll be right up the street from you and I'm down to work for the experience and to help out a fellow R3V member.