I'm planning on getting a new optima battery, I'm just wondering, should I get a new alternator too? And if so which should I get, is it worth it to spring for the 140 or 200 amp alternators? Also how many amps are on the stock alternator of an 87 Trans Am?

 

You only need a higher-current alt if you're going to be using (for extended periods) more power than the current one can supply. Your current one is probably either 90A or 105A. Lets assume you have an audio amp which draws a continuous 85A @ 14.4V. Also assume that normal day driving takes ~30A with any accessories you wish to run (A/C, wipers, lights, etc.). This adds up to 115A on (assuming) a 90A alt. What will happen is as you get to the limit the alt can supply, the voltage will begin to fall to ~12.5V and all the extra current will be supplied by the battery. This is a perfectly OK scenario as long as there is a time when the battery can recharge (i.e.: if you turn the amp off), but if not, then would will need a higher power alt to keep everything balanced. Otherwise, one day you may not be able to turn the engine over because the battery doesn't have enough power.

That's a crude explanation and makes a lot of assumption, but you get the point. In most cases, if you NEED to upgrade the battery, the alt & associated wiring is a good thing to do as well.