Is 75 amps enough?

[Xavier]

Well, after diagnosing some electrical problems, I found out the 100amp alternator I thought I had is actually a 75amp...great.

Anyhow, I am funning dual fans from Finishline, a single Spal 17" fan and an MSD ignition - no other goodies, other than brake lights etc...

I did a search on the internet, but everything was kind of vague.

I would like to get some feedback, thank you.

[Excaliber]

75 is more than enough.

[Xavier]

I figured, I remember reading it on another post, but better to be sure!

[RICK LAKE]

Xavier You would be better to have a 100 amp alt over a 70 amp setup. Here are basic amps for things on your car
cooling fans 6-15 amps each for draw
msd system 8-36 amps depending on system. I don't believe in the story they tell on the 6al setup of 2-3 amps. This doesn't include the coil.
low beam head lights 8-10 amps
high beam lights 10-15 amps
ipc and gauges 2-5 amps with lights
tail lamps 3-7 amps
fuel pump depending on size 3-10 amps per unit
FI system 5-15 amps.
battery size and recharging ability 1-30 amps depending on loads and temp
Are you running under size pulleys on the motor?
What is the rpm idle of your motor?
At idle the motor is not making 100 amps, maybe 40-60 amps, you can add up the numbers and see the problem.
I ran a 70 amp system and had to switch to a 130 amp system because of the under size pulleys not spinning the alt fast enough and keep a full charge on the battery as well as 2 fuel pumps,msd setup, 3 cooling fans, lights. Idle with 750 rpm.
You also need more amps from the alt if the cables are under size for the load or you have weak grounds. 2 grounds min on the car, I have 3 grounds for my car because of the extra curcuits.
There is a 10% fudge factor for alt outputs. Just because it said 100 amps doesn't mean it can mantain this rate all the time without cooking the regulator. You also have to include possible belt slippageof 1-3%.
Remember that is the alt can't supply ALL the power to the car running, after a while your battery will be low on CCA and not able to start the car because of the drain. The battery will not be fully charged either and shorten the life of the battery. That pretty much covers alt and charging 101. In this case bigger is better at 100. Rick L.

[Excaliber]

The likelyhood of running ALL electrical systems at once, for an extended period of time, is quite small. The likelyhood of running them all long enough to actually drain the battery enough to be a problem is even more unlikely.

But, theoritically, it could happen, in a worst case scenairio.

[RICK LAKE]

Excaliper Ernie at idle with a 70 amp alt putout about 45 amps the load was this 3 coolant fans 22 amps
msd system 9 amps
ecu for fi 6 amps
fuel pumps (2) 14amps
lights ipc and gauges 5amps.
under drive pulleys cut 10% of out put at idle
the battery is not being charged because amps needed is more than ALT output. 56 amps drain, 45 amps charging at idle. This doesn't include the 5-10% fudge factor of the charging system. Max power of my 70 amp alt was 61-65 amps when hot. I have had to recharge the battery more than once. That was a 600cca battery I now run an 875 one and have less problems. New alt fixed all problems now. No theory here Ernie. Have also redone the alt cable #4awg and added a mega fuse to protect the wiring harness and replaced the amp meter with a voltage meter. Is not period correct but safer IMO. Rick L. Ps Ernie you have to remember that the orginals didn't need half of the power we need today.

[Excaliber]

The "theory" is that you would need or have to run all the systems at one time for any given length of time. If so, then yes, you will need more power. Is sounds like Rick has been there with this worst case scenario. All three fans, at night, with high beams on, in town, at low rpm (for an extended period of time), two fuel pumps clicking away, underdrive pulleys and a high amp draw ignition system.

If you at cruising speed on the highway you would not need three fans, or any for that matter. If your in town running at low rpm (with undersize pulleys) you wouldn't be running high beams. Hardly any of us are running two fuel pumps at the same time. While this scenairo could play out, I think it is highly unlikely for most of us.

In Xaviers case, based on his other thread, I think a far more likely scenario is that something is fundamentally wrong with his charging system or elecrical system. Changing the alternator out to a 100 amp or more I do not believe is warranted until further diagnosis rules out the 75 amp he is currently running.

[Rick Parker]

I've had a 42 amp alternator and used a group 55 Battery for 20+ years. 1 Spal fan, typical lighting, MSD, mechanical gauges, Tilton Mini Starter. NEVER had a problem. Knock on wood!

KISS!

[IMG][/IMG]

[RICK LAKE]

Rick Parker In the orginal car that all you needed. You still have enough charging system to add a low ampage fuel pump. Low wattage lights? Do you have a heater and blower fan? You will be close at idle. Watch the amp gauge and see where the needle sits at idle with all the power on. You could also have a 60amp alt in a 42 amp case. Rick L.

[RICK LAKE]

Excaliper Ernie my car spends 98% of the time on the track and autocross. It doesn't get the steady cruising that alot of other cars get. The other thing is that most guys running amp gauges in the car are running 42,60,65 even 70 amp output ALT's. This is the safety max limit for this -60-0-60+ amp gauge for a SW or even Autometer. I can't answer for Smith. The isolators on the ngauge will melt under high charging systems and cook the whole wiring harness because most cars have no fuses between the Alt and this gauge. I have seen a couple of cooked harness and the hours to repair them.
Ernie the 2 points I am trying to make are these, OVERBUILD your car for safety and abuse. Alot of people cut corners around this part of a build and some are lucky(not me) and others get bite in the AZZ. Xavier also has another post for tha same problem and the output wire is too small for the load and charging system. #10awg is not enought. #6 is min, #4awg is better, #2awg is perfect to handle all loads and not cook the ends. This is another reason to get rid of the amp gauge if the system is pushing 70amps total or more. Volt gauge is not period correct but sure better than burning up a dash. Just IMO reasons. Rick L.

[undy]

Just remember.... Your 75 amp alternator is putting out around 30 amps at idle, when you need it most (fans on and the motor's getting too warm).

[CowtownCobra]

I use a Ford 3rd Gen 95a alternator and have had very good results. Google 'Ford 3rd Gen alt' and you will get a lot of hits. They are being retrofitted into lots of different types of cars.

[Rick Parker]

RL:
The point I'm attempting to make is that unless the cars are overloaded with accessories, it is unnecessary to have a huge charging system.

[CHANMADD]

Most street cars should not run two pumps and heavy draw ignitions anyway.
A stock 60 amp with one electric fuel pump, heated seats and lights has never let me down..Oh... two cooling fans as well.
The other thing I do not understand is why you all want to use an amp guage as apposed to a Volt meter. The ampmeter like Rick says carries all the current through your dash.......definite fire hazard..I have also seen many burnt wires because of this antiquated guage. The volt meter gives you an exact read on your battery at all times.

[Excaliber]

Yup, I do agree Rick Lake. Add fuel injection and an ECU and that system would demand that voltage not be allowed to drop, under any extreme conditions. Like Rick Parker says, most Cobra owners will be fine with a moderate charging system, but there are always exceptions to a rule of thumb.