Jaysome

iTrader: (1)

Hi, I seem to be having some alternator issues which I believe is just the fact that it is a cheap stock alternator from advance auto and I am using underdrive pullies on the alt and crank. I think due to a low idle in drive (about 600 rpms) and the alternator being rated at only 105 amps @ 2400 alt rpm it is turning too slow. I am about to upgrade to a 160 amp alternator from summit, but want to make sure that is going to pull enough amperage. I've looked all over for charts or something that will give me the amperage of my car, but ive only found charts that were generic I guess. So if anyone can help me find out what my car should be pulling, I would appreciate it very much.

1988 Firebird Formula 350 TPI

tavert91

iTrader: (1)

If your car is a standard setup without any audio amps or other mods that draw power, the 105 amp alternator is fine. More info on your car might help get better answers.

Jaysome

iTrader: (1)

I dont know what more info there is to provide really.. electronically there is nothing different. I have a cd player that replaced the stock casette player, but really thats it. The engine is mostly aftermarket performance. The previous owner had done a lot of upgrading on it, and i just had the engine rebuilt 1,000 miles ago with a new 105 amp alternator. After the rebuild I've seen the car idles "better". Since Ive owned it, I never recalled it idling anything lower than 800-900 rpms even in drive. But now it idles down to 600 and sometimes even 500. When it does that all lights dim, battery voltage drops down to what I am guessing is about 10.5 to maybe 11v from what the dash tells me, I can check with a voltmeter tomorrow if I really need to. So from that I am pretty sure its the alternators fault for not keeping the right amount of voltage to the battery, more than likely due to lower pulley ratio than a 3:1. I havent measured the crank pulley yet, but I know the alt pulley is 3 inches compared to the 2.5 so the alternator is definately turning slower than normal, reducing its amp output at lower idle.

Seafarer12

105 amp is plenty. You just aren't turning it fast enough. You might try putting the stock pulley back on the alternator. I bet a 160 amp wouldnt do much better than what you have now. Delta wound alternators (the newer high amp alternators) make more power but you have to spin them faster to get it. The older style wye wound alternators don't make as much power but they do it at a lot lower speed. I have a 63 amp 10si alternator on a tractor and it will do its job at at 800 rpm. You just have to figure out if the 5hp you get it worth an unreliable charging system.

What condition is your battery in?

tavert91

iTrader: (1)

I checked the specs for a '91 (didn't have '88 but I'm pretty sure they are close if not the same). The typical idle speed, I think, for a 350 is about 700-750 RPM's. 600 seems very low unless the engine mod's put in brought it down this far.

At idle, you should be getting a 42 amp output from the alternator and it should be turning at a 3.14 to 1 ratio relative to the crank. It sounds like the low RPM's combined with the different pulley are the cause. I would do as Seafarer12 suggests and swap the pulley back and see what happens. I would also check out why you are idling so low.

DJP87Z28

105 amp was standard on 305 & 350 TPI cars. The under drive pullies are your problem and there is nothing wrong with a 600RPM idle. Get rid of the underdrive junk and your problem is solved.

TreeFiddy

Simple. Get an ammeter and measure the load on the battery. Get a voltmeter and measure the charge voltage at the battery. Go from there.