How many amps needed from my alternator?

[oldstv]

In the process of installing the Holley HP in my 1970 Buick GS. The car has a bb with alum heads, intake, big exhaust, solid roller cam, MSD 6AL box, no electric fans, big Magnafuel pump in a Tanks inc fuel tank.
Fuel pressure is holding fine until you really lean on it. It then drops to about 22lbs. I currently have the stock alt which puts out about 45 amps, and i am wondering if that could be my issue??

[GoodysGotaCuda]

Do you have a volt gauge, or some method to see how much voltage is getting to the pump? If not, you need to know what voltage your system is running on. A simple cigarette lighter voltmeter will tell you what's going on.

Your alternator is likely 45amps at 1500+rpm, at idle, you'll likely be at the edge of keeping it running at 13v+. 45amps is likely just barely enough without any lights on or the HVAC blower on.

[oldstv]

I checked the volts going to the fuel pump on the 10 gauge wire feeding it and it was seeing 12.5 volts with the car not being under a load.

[GoodysGotaCuda]

I checked the volts going to the fuel pump on the 10 gauge wire feeding it and it was seeing 12.5 volts with the car not being under a load.

Doesn't sound like it's charging, if the engine was running. You should be targeting 13.5v+ with the engine running at all times.

[nokones]

I believe the big Magnafuel pumps are drawing anywhere from 12 amps to 17 amps. I don't know what the Holley HP power draw is but, they do use a 40 amp fuse on the hot power lead for the HP module. A 45 amp alternator is probably not enough even with the headlights off. The problem may be that the pump may not be getting adequate electrical power when the pump is demanding more electrical power under a heavy load condition. You better leave the radio and headlights off.

[oldstv]

I haven't got turned anything on but the engine, it doesn't even have electric fans. My suspicion is that I don't have enough amps to the fuel pump and it sounds like you guys agree. Buick engines need and want gas so the pump has to put out at higher rpms.

[andrewb70]

I suspect your alternator is unable to keep up with the load.

Andrew

[oldstv]

Andrew, I hope that my issue ends there. That is my last resort before looking at the pump itself.

[andrewb70]

Andrew, I hope that my issue ends there. That is my last resort before looking at the pump itself.

You said the pressure drops to 22psi...what is it set to at idle?

Andrew

[oldstv]

I think it is 42psi?

[oldstv]

Correction, it is set to 55psi with no vacuum on the regulator.

[andrewb70]

Correction, it is set to 55psi with no vacuum on the regulator.

That's a substantial pressure drop. I would put a meter on the pump, run some long leads inside the car, and go for a drive. It would be easy to monitor voltage at the pump.

Andrew

[oldstv]

I am going to up the amps on the alternator first because it sounds like that needs to be done anyway.

[oldstv]

Update. Took the alt amps up to over 100 and still had a fp drop. Tested the pump "should have done that first" it was pumping 1/4 of a gallon in 60 seconds. New fuel pump now and no more problems with fp drop. I do however have a problem with tire spin, can't seem to keep it stuck to the pavement.....but that is a problem that I welcome!! Just wanted to let you know what I found..

[1989GTA]

Good news that you found the problem. Also I think it was the right move to up the amperage on your alternator.