87 T TOP GT302

My GT does fine in the daytime but at night with the lights on the light go dim when i come to stoplight and the guage shows ont the low sidebelow 1/2 .I 'm wondering what amp alternator or do I need a stronger battery,currently 550cca, and if this will cure my problem? Starts right up when warm as well.

badlx87

Your underdrive pulleys are why your car loses charge at idle at a stoplight, it's very common. I don't know if a 130 amp alt would help this issue. I've know people to run stock alt with underdrive pulley's and have no issues. Some do, some don't i've learned by reading.

silverpony41

Go ahead and upgrade to the 130 amp alternator. I had the same problem, and at times when i wouldnt start my car for a week it would be dead. That sorry 75 stock alternator is crap. I too have underdrive pulleys and upgrading the alternator was one of the best things i did. Try having your lights on and turning on the ac your voltage will really drop all the way. I have the 130 amp alternator and my lights don't dim anymore. Its a common upgrade for these cars.

67mustang302

A larger alternator my make the problem worse. Many of the higher output alternators have more aggressive amperage curves, and since they ramp up quickly they may produce less power at idle. Either way, with a good battery and alt it shouldn't be a problem. Upgrading the alternator isn't a bad idea anyway just to have the extra juice that's needed for electrical stuff....CD type ignitions can pull a crapload of amps at peak ignition power for instance. That and the higher performance alternators are usually designed to handle the strain of running at higher rpms.

copracr

another thing to consider is that that extra amperage isn't free. it requires more hp to turn the higher powered alternators. Might want to try replacing the pulley on your curent alternator back to stock, and if that doesn't help you could go back to the stock pulleys.

It just doesn't seem economical to install UD pulleys for a few hp to turn around and install an alternator that requires a few more hp to turn.

Bubba 95SN

My 95 has 130 amp (stock) with underdrive and never had a problem.

67mustang302

It does NOT require more hp to turn a higher output alternator. It DOES require more hp to PRODUCE more amperage in the alternator. But hp cost is totally dependent upon the amperage draw on the electrical system. Alt efficiency plays a role as well, but a 200A alt producing 90A draws the same amount of hp to run as a 100A alt producing 90A. That's because the voltage regulator controls current flow into the electromagnet in the alt to maintain a preset voltage by increasing or decreasing field strength to meet the amperage demand...in other words as the car draws more current from the alt, the voltage regulator increases the strength of the electromagnetic field to spit out the require amperage to prevent a voltage drop. It's the field strength of the electromagnet that robs power to turn.