Recommended amperage for an alternator

duster1256 · 06-13-2016, 05:14 PM

Hello,

Looking to replace an alternator and was wondering what amperage should I look at to
support a mopar p4876728 or MSD6AL with a magnafuel pump and cooling fans.

Thanks in advance

Keith 944 · 06-13-2016, 05:24 PM

I just changed a east coast electric to a 100 amp gm. the east coast was 75-80 amp but did not charge well at idle. the gm 100amp 1 wire charged well at idle but was constant 11.5-12.4 during run. today I ordered an msd 160 dynoforce to see if it will run about 12.8 13v during the run. I am not concerned about any horsepower loss as you might be. with that being said I will have the east coast setup listed for sale on here shortly.
My draws were from grid ign. data recorder, dual fans, electric water pump (csr), Weldon fuel pump, driver fan , light at night and stereo

Woodfin · 06-13-2016, 05:30 PM

Duster,

It is completely acceptable to have more alternator output than your system requires. An alternator only puts out the amperage required by the load.

It is common to see 85 amp - 100 amp rated alternators on drag race cars.

Personally, I chose pulley sizes that spin the alternator about 20% - 25% faster than the engine.

Example - 7200 engine RPM x 125% overdrive = 9000 alternator RPM.

Another example - 1200 engine idle RPM x 125% overdrive = 1500 alternator RPM. That is enough alternator speed at idle and does not over speed the alternator at high engine RPM

FireSale · 06-14-2016, 02:29 AM

Make a list of the amp rating of everything your electrical system might be driving and total that up to find a starting range.

The problem with alternators on dedicated drag cars is that they don't usually put out a full charge at idle rpm and that's where your engine tends to spend most of it's time at the track.

If you pick a one wire alternator be sure to set it up correctly with the jumper to the right terminal. Use the remaining wire for a warning light of some sort. It comes in handy when you lose your adjuster hardware and drive belt at the track like I did Sunday. The light popped on so I took a look and everything was just gone.

The short version: 85 amps for the strip. 120-140 if it's a street car with lots of electrical stuff.

Dale

06-13-2016, 05:14 PM	#1
duster1256 Junior Member Join Date: Feb 2008 Posts: 76 Likes: 21 Liked 20 Times in 3 Posts	Recommended amperage for an alternator Hello, Looking to replace an alternator and was wondering what amperage should I look at to support a mopar p4876728 or MSD6AL with a magnafuel pump and cooling fans. Thanks in advance

06-13-2016, 05:24 PM	#2
Keith 944 VIP Member Join Date: Feb 2006 Location: Dela-where? Posts: 2,192 Likes: 1,159 Liked 1,873 Times in 576 Posts	Re: Recommended amperage for an alternator I just changed a east coast electric to a 100 amp gm. the east coast was 75-80 amp but did not charge well at idle. the gm 100amp 1 wire charged well at idle but was constant 11.5-12.4 during run. today I ordered an msd 160 dynoforce to see if it will run about 12.8 13v during the run. I am not concerned about any horsepower loss as you might be. with that being said I will have the east coast setup listed for sale on here shortly. My draws were from grid ign. data recorder, dual fans, electric water pump (csr), Weldon fuel pump, driver fan , light at night and stereo __________________ KEITH MAYERS 2-1/2 X somebody Still many X nobody Last edited by Keith 944; 06-13-2016 at 05:27 PM.

06-13-2016, 05:30 PM	#3
Woodfin Senior Member Join Date: Sep 2006 Location: Kilgore TX Posts: 520 Likes: 8 Liked 231 Times in 91 Posts	Re: Recommended amperage for an alternator Duster, It is completely acceptable to have more alternator output than your system requires. An alternator only puts out the amperage required by the load. It is common to see 85 amp - 100 amp rated alternators on drag race cars. Personally, I chose pulley sizes that spin the alternator about 20% - 25% faster than the engine. Example - 7200 engine RPM x 125% overdrive = 9000 alternator RPM. Another example - 1200 engine idle RPM x 125% overdrive = 1500 alternator RPM. That is enough alternator speed at idle and does not over speed the alternator at high engine RPM

06-14-2016, 02:29 AM	#4
FireSale VIP Member Join Date: Feb 2014 Location: Lakewood Washington Posts: 1,407 Likes: 29 Liked 237 Times in 126 Posts	Re: Recommended amperage for an alternator Make a list of the amp rating of everything your electrical system might be driving and total that up to find a starting range. The problem with alternators on dedicated drag cars is that they don't usually put out a full charge at idle rpm and that's where your engine tends to spend most of it's time at the track. If you pick a one wire alternator be sure to set it up correctly with the jumper to the right terminal. Use the remaining wire for a warning light of some sort. It comes in handy when you lose your adjuster hardware and drive belt at the track like I did Sunday. The light popped on so I took a look and everything was just gone. The short version: 85 amps for the strip. 120-140 if it's a street car with lots of electrical stuff. Dale __________________ Dale Shearon 68 Mustang 6394