I am adding a 250W RMS (or maybe 400WRMS) MTX amp to my car. I know that MTX can be underrated and I have a small SUV so I am pretty sure that I am going to need something to make sure I don't drain my battery etc.
Can I get away with just adding a Cap to keep my lights from dimming(1 Farad probably)? I know that the only real way to solve that problem is with an HO Alt. but they are a lot more money. What will be the effects on my car if I don't add a new Alt?
If I need to add a Alt would 150 amps be sufficient?
First of all, you should just try hooking the amp up and see what happens. If when you turn the volume up to where the subs are hitting hard and you see that your headlight/internal lights are dimming then start to look into a HO alt. 400 wrms isnt a whole lot so you might be alright. As for a HO alt, 150 will be more than plenty for your application. But it will also leave a cushion if you ever want to upgrade to something more powerful in the future.
im pretty sure in SUV's they come with like 100 amp stock alt, and the rule of thumb is you need 15 amps for every 100 watts so you should be fine with the stock one, like isaac W. said, install it all and see if your lights dim on night when you bump it, remember 3/4ths the way up on the volumn usually pushes full loudness
I doubt SUVs come with 100A alternators. Every car manufacturers only include one that will power the car and few gadgets. They do NOT expect anyone to install powerful audio amps. You will find most stock alternators are around 75A. 200w rms is about the average most cars will handle before dimming lights. Reason for that is, they do expect people to install fog/extra head lights. They use approximately 10A per side. That is roughly 200w rms (since no amp is 100% efficient). The part that doesn't make any sense is, people willing to spend extra money for capacitors and 2nd or 3rd battery. If you add them up, it will cost as much as HO alternator or sometime even more. A decent 1.5F capacitor is around $65. 2nd battery (yellowtop) and isolator will cost more than $200. Now you purchase lower gauge wire and inline fuses. It'll cost even more. After all that, the sound is only as good as the battery's reserve power. After it's drained, it's as useless as a dead car. A battery will give you 12.5v. The bare minimum for an amp. HO alternator will give you 14.4v and can easily replace your stock without much modification. Sure you have to upgrade the engine to ground, battery to ground and alternator wire. But that, plus HO (average price for 150A is around $200), costs as much as the above setup or less. So don't take the easy way out and opt for capacitor and a 2nd battery. It is NOT cheaper. Heck, if I can make a HO alternator not meant for my car, fit without anyone's help, I am sure you are more than capable of installing one that is designed for your car. It really is not that hard. If you can hold a wrench and know how to use it, then you can replace an alternator. Don't let the moronic and idiotic mechanics scare you. Because they have no experience/knowledge, they love to scare people, and also charge $500 or more for an easy install. I've been there. They tried to rip me off. I didn't feel like spending all day installing my HO alternator, so I got a quote. The ba$tards wanted to charge me $100/hr for labour. I would've paid $200 tops. But the crooks wanted 4-5 hours of time, plus parts. Talk about a rip off. I spend a day replacing my stock alternator with HO version, smaller alternator belt, thicker wires for the engine/ground and battery. All it costed me was $15 for the new belt, $12 for 4gauge wires. The crooks wanted $22 for the belt to start. I really don't know how they get their license.
If I don't replace the alternator right away(a few months), and take it easy on the volume till then, will I still be draining the battery to the point where my car will die?...btw I am also putting in a 75x2W RMS for my 2-ways in front now.
Also, once I have the HO ALt, is a cap still good to have for the sub or can my Alt take care of the long bass hits by itself?
You won't need anything else after HO alternator. A cap will still help maintain smooth voltage curve. With stock alternator, the voltage will drop from 14.2v to 12.5v. With HO alternator under heavy bass, it'll go from 14.4v to 13.2v. This drop in voltage will still cause the lights to dim slightly but nothing like before. The power is still being produced from the alternator. A cap should help with short bursts of bass. Whether you can wait few months to replace your alternator will depend on itself. If it is already dead or producing no power, then your battery will die. Monitor the voltage with a DMM and see if it's working.
As for replacing the Alt. on my own, that is probably the option I would WANT to take b/c of money reasons like you said. Will that process be too much for someone like me who has never really worked on a car before, and has no knowledge of how the alternator or belt works? If the alt. comes with directions I could probably figure it out but I don't want to screw anythiing up either. Also did you get the smaller belt and larger wires at a store like auto zone or pep boys or whatever?
Dificulty level will depend on your car's make and model. Mine was Acura and those idiots seem to love making things very hard to replace. American cars seem to be much easier. For example, my Mustang was a snap. Just matter of few bolts and the alternator and the belt came off. Before you do anything, read up on your car. There are some online manuals in pdf format that you can download for free or a small fee. It's well worth it. If you were to go to the dealer, they'd charge thousands for the service manual. 4 gauge wires can be bought from most car shops for few dollars. Mine was a 4 gauge ground wire with the ends premade.
the "big 3" only helps IF you upgrade the alternator, or your factory wiring is corroded and damaged, reducing the current capability of the OEM wiring. the factory wiring is sufficient to handle the current your stock alternator can deliver. capacitors won't help voltage rails at all, since cap voltage follows circuit voltage anyway.
hook up the amp, and try it out. if you have problems with lights dimming of voltage dropping then worry about upgrading the electrical system.
PS I've seen a Honda SUV with a 65A alternator, and a Durango with a stock 165A alternator. That means nothing at all. What does matter is the amount of reserve current the alternator can supply when the car is using power as well. those big stock alternators are there for a reason... usually to power things like a tow winch, sunroof, power locks and windows, HVAC, wipers, lights, heated seats, and all of that jazz. A lot of cars with larger stock alternators have little reserve to spare for aftermarket stereo gear.