Hey all, I recently discovered a voltage drop at high engine loads. I've replaced a few things but I'm still unsure what it might be or how to fix it. It might be kind of a rabbit hole as the car has so many aftermarket parts on it. Any help would be appreciated, please let me know!
The car
* Old whipple 1.6L CTSC at 9psi
* AEM series 1 ECU (tried two different V1 ECUs)
* RC 550 peak/hold injectors with stock resistor box
* K-series coils with foundry 3 harness
* Water meth injection system (WMI) coming on in boost
* Aftermarket fuel pump, unknown make, unknown wiring setup
* Fuel pressure at 46psi. I don't have the sensor running into the ECU, so not sure how it behaves during these voltage drops.
* Still using the stock fuel pump relay (it clicks at 4500 rpm)
* Prelude form-factor alternator cause CTSC (Tried stock 90A denso and Singer 180A)
* Brand new Antigravity ATX-30HD battery
* No accessories running during these situations (no lights, no stereo, no climate control)
* Been through all the grounds on the car a couple times for other electrical issues
The ECU is seeing a voltage drop at high loads. Basically it reads 13.8V under light loads, then it drops under high loads until it hits about 12.5V.

My original thesis was that the 90A prelude alternator couldn't keep up with the addition of the WMI and maybe k-series coils. I bought a Singer high output alternator and installed it. It has both modifications _and_ runs a smaller pulley, but no help, still a volage drop. The data above is with the high output alternator.
I've been digging around in old data logs and did some experimentation last weekend. It seems like this voltage drop has been happening since I bought the car. Old logs show logged injector dead time indicating about 12.5V in boost, just like the logs above.
* Probably not the k-series coils, it happened with the stock coils
* Probably not WMI system, it happened without the system. I also had it turned off last weekend, note the air temps way high in logs above.
* I tried spinning the motor at highish RPM under light load, there was no voltage drop
* It happened with the previous AEM series 1 ECU and also with the current slightly-later-model series 1 ECU.
It seems like the drop is directly correlated with fuel demand. The more fuel necessary, the more the voltage drops. You can see in the logs that the voltage drops along with jumps in injector pulse width.
But even if the fuel pump or injectors have a high current demand, shouldn't the alternator cover it, especially a high output one?
* Belt slip? The belt is very tight already, but who knows. Maybe the current demand is super high and it's slipping? I plan to put the stock prelude pulley on the high output alternator for a little more leverage and maybe less slip.
* Would it be something to do with the factory dual-stage fuel pump wiring? Should I remove the fuel pump relay from the loop? Would my fuel pressure drop up high if I did? Should I get the SoS or some other pump wiring kit?
* Would the injectors cause this?
* Maybe the alt charging cable? Someone on an S2k forum replaced the charging cable and allegedly fixed it.
What am I missing? What else should I check?
The car
* Old whipple 1.6L CTSC at 9psi
* AEM series 1 ECU (tried two different V1 ECUs)
* RC 550 peak/hold injectors with stock resistor box
* K-series coils with foundry 3 harness
* Water meth injection system (WMI) coming on in boost
* Aftermarket fuel pump, unknown make, unknown wiring setup
* Fuel pressure at 46psi. I don't have the sensor running into the ECU, so not sure how it behaves during these voltage drops.
* Still using the stock fuel pump relay (it clicks at 4500 rpm)
* Prelude form-factor alternator cause CTSC (Tried stock 90A denso and Singer 180A)
* Brand new Antigravity ATX-30HD battery
* No accessories running during these situations (no lights, no stereo, no climate control)
* Been through all the grounds on the car a couple times for other electrical issues
The ECU is seeing a voltage drop at high loads. Basically it reads 13.8V under light loads, then it drops under high loads until it hits about 12.5V.

My original thesis was that the 90A prelude alternator couldn't keep up with the addition of the WMI and maybe k-series coils. I bought a Singer high output alternator and installed it. It has both modifications _and_ runs a smaller pulley, but no help, still a volage drop. The data above is with the high output alternator.
I've been digging around in old data logs and did some experimentation last weekend. It seems like this voltage drop has been happening since I bought the car. Old logs show logged injector dead time indicating about 12.5V in boost, just like the logs above.
* Probably not the k-series coils, it happened with the stock coils
* Probably not WMI system, it happened without the system. I also had it turned off last weekend, note the air temps way high in logs above.
* I tried spinning the motor at highish RPM under light load, there was no voltage drop
* It happened with the previous AEM series 1 ECU and also with the current slightly-later-model series 1 ECU.
It seems like the drop is directly correlated with fuel demand. The more fuel necessary, the more the voltage drops. You can see in the logs that the voltage drops along with jumps in injector pulse width.
But even if the fuel pump or injectors have a high current demand, shouldn't the alternator cover it, especially a high output one?
* Belt slip? The belt is very tight already, but who knows. Maybe the current demand is super high and it's slipping? I plan to put the stock prelude pulley on the high output alternator for a little more leverage and maybe less slip.
* Would it be something to do with the factory dual-stage fuel pump wiring? Should I remove the fuel pump relay from the loop? Would my fuel pressure drop up high if I did? Should I get the SoS or some other pump wiring kit?
* Would the injectors cause this?
* Maybe the alt charging cable? Someone on an S2k forum replaced the charging cable and allegedly fixed it.
What am I missing? What else should I check?