I have just run into an issue , under load [ lights , a.c , power steering ] the voltage starts to drop to around 12.5V - 13V . As soon this happens the idle drops to 800 rpm. Just wanted to make sure if the car is electrically good before i start looking at the tuning aspect of it.

- Voltage drop has anything to do with Battery ground and ECU ground ?
- How good a ground is intake manifold ?

 

I encountered a ground that was a stud with a nut and star washer on it, the ground wire terminating in an eyelet was under the nut and washer, the nut was tight and there was zero electrical connection between the wire and the stud. I only once found a completely failed connection that looked perfectly fine like this but once was enough to convince me that anything is possible.

If you want to be assured the grounds are good, sand and grease them.

 

Doesnt the battery matter too?

 

the battery only supplies current if the voltage at the battery terminal drops below the 12.x resting voltage of the battery.

 

As well as being perfectly normal for the voltage to be low at idle, it's also perfectly normal for the voltage to drop at high RPM.
Some cars will be more noticeable either due to differences in electrical load or with a loose alternator belt or due to a battery with only a small capacity.
An overly loose alternator belt will slip at high load and/ or high RPM and an alternator belt that's too tight will make the crank pulley fail prematurely.

Most owners wouldn't know about the voltage because it's not something you see or notice unless you're looking for it.

 

90 amps is correct