Fan current draw... How much is too much?
 

Looking for a pair of Spals to mount on an FM twincooler.

This is on my 97 (soon-to-be turbo) DD that which sees track time. Therefore I'd like to run as much airflow as I can, but Spals really suck down the amps!

On a rainy muggy summer night I need to have enough juice to run the headlights, wipers, A/C defogger on full blast and not stall the car or freak out my engine management due to the voltage drop. (I plan to run a water-injection pump as well)

How much is too much?

FSM says the stock fans pull 6.5 amps. I don't know what the relays are rated for, probably somewhat close to that. I have twin 11" Spals running on the stock relays and I have no problems.

FWIW, most of the setups I've seen run a pair of 11 inch fans.

I would like to run a pair of the thick fans, similar to JasonC_SBB. However, FM's Stage 2 fan setup uses one thick/one thin (less draw) and I've seen at least one thread complaining that it overruns the alt and discharges the battery with the fans and several stock accessories on (with the measurements to prove it).

11" Paddle Bladed (Thick)
Stat Press...Airflow...Current
...in.H2O......CFM......AMPS
....0...........1390.......12.5
....0.2.........1230.......13.5
....0.4.........1070.......14
....0.6...........800.......14.5
....0.8...........540.......15
....1.0...........420.......16
....1.2...........270.......16.5
....1.4............---.......18

11" Straight Bladed Fan (Thin)
Stat Press...Airflow...Current
..in.H2O......CFM........AMPS
.....0...........970.........8.3
....0.1.........910..........8.7
....0.2.........850..........8.9
....0.3.........790..........9.1
....0.4.........720..........9.2
....0.5.........640..........9.2
....0.6.........550..........9.1
....0.7.........420..........9.1
....0.8.........280..........8.9
....1.0...........90......... 9.0
....1.2..........---...........9.2

Thanks, Sav no worries with the relays (on 1.8L cars) as FM install instructions use the stock wiring and relays with the fans above.

I'd still measure the voltage at the fans and compare to say, the alternator. See if you're loosing any. Any voltage you loose is gonna = less air anyways. If you can find the fan relay measure and see what the voltage drop across it is, as well as from the relay to the fan, etc. Find the weak link.

It's not overly hard to rewire the fans. I wired both of my fans on a 30A Bosch relay and they work great. I used a 10 gauge wire to feed the relay and a pair of 12 gauge wires to feed the fans. I swear I think the fans "start" harder than they did on the old wiring.

But I like rewired the entire motor. Oh, and I'm on stock fans too.

Pat, what did you use as your power source?

Whoops, left that out (typed it, then deleted the sentence because it was wordy and then ommited the info actually). I pulled power from the starter. Just a stranded #10 wire to a fuse holder with a 25A fuse, and then to the relay.

I did the same for my single 16" spal - but from the post in the center of the engine bay fuse box. My spal is rated at 21amps 2750cfm and I've got a 70amp alternator. It's quite obvious the fan comes on if I'm running the head lights. Otherwise it's ok.

Let me ask this in a different way... How much excess capacity is built into the charging system?

I do not want to appreciably discharge the battery while the car is running. i.e. slow cranking after being stuck in bumper to bumper traffic. The rewiring info is appreciated though.

It depends on many things. Your year of car, type of transmission, battery in the car, idle speed, load, etc. FWIW, the fans don't run all the time. Just to cool the engine. You could wire it up so that only 1 comes on at say 210, and the other at 220. That way only one runs at idle when needed.

I dug around and found the post I referenced earlier: High current draw (do I need a bigger Alternator?)

Pertinent details:

This guy piled on aftermarket stuff weighing in ~94 amps when everything was cranked up.

vs. a stock system drawing ~57 amps

I have a 65 amp alternator (97 5 speed). If you subtract the stock fan draw there are ~20 amps to play with. In his first post you can see the 11" thick Spal eats up 18 amps of that (despite being rated less). How far can I push it with fans and WI pump? What's the smart thing to do here electrical gurus?

Wel, the fans and WI pump won't run all the time. The WI pump will never run at idle anyways, so forget about it.

I think you're worrying too much. IIRC, the automatic alternator is 5 amps more. I think most have a problem with the engine stalling at high current draws, not the alternator not putting out. So if that's the case you could wire in a small solenoid that idled the engine up when say, the fans come ona at 25 A. Just like how the A/C idles up to compensate for the load placed by the A/C compressor and extra electrical load as the A/C electric clutch, cooling fans, and blower motor all come on too.