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Old 01-22-2009, 04:17 PM   #1
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Default 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?
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Old 01-22-2009, 04:26 PM   #2
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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.
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Old 01-22-2009, 04:28 PM   #3
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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

Last edited by MazDilla; 01-22-2009 at 05:00 PM.
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Old 01-22-2009, 04:34 PM   #4
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Quote:
Originally Posted by Savington View Post
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.
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.
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Old 01-22-2009, 04:41 PM   #5
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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.

Last edited by patsmx5; 01-22-2009 at 05:08 PM.
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Old 01-22-2009, 05:39 PM   #6
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Pat, what did you use as your power source?
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Old 01-22-2009, 06:07 PM   #7
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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.
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Old 01-22-2009, 07:25 PM   #8
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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.
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Old 01-22-2009, 09:20 PM   #9
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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.
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Old 01-22-2009, 09:24 PM   #10
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Quote:
Originally Posted by MazDilla View Post
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.
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Old 01-22-2009, 10:05 PM   #11
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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.
Quote:
Originally Posted by aseer
I was driving home the other night in the rain (so everything was on) and when I got home I noticed my battery was running down (MSPNP log was at 10.5V)....

Here are the numbers; I used a clamp-on DC ammeter and measured at the alternator and turned on one component at a time and recorded the reading (rechecking the base current each time too). Listed is the excess current above the baseline when that component is turned on (I also continually checked the logged voltage which stayed at 14V during the measurements).
All measurements are done at idle.


Baseline current: 13A
Fans (FM dual Spal package) 27.6A (18A for the big fan, 9.6A for the small fan)
PIAA Headlights (Xtreme white) 18.6A
Blower 15.9A
PIAA Foglights 11.4A
Aftermarket sound system on full 7.4A
Radar 0.3A

Any thoughts would be great (and if this post belongs in a different section, please feel free to point me in the right direction). The car is a 91 with a turbo, MSPNP, big injectors and other stuff...

vs. a stock system drawing ~57 amps
Quote:
Originally Posted by aseer
In case anyone is interested, I borrowed a stock 91 and did the same measurements, same conditions (though this time I measured the voltage at the battery for each measurement rather than reading my MSPNP since this car is stock, battery voltage barely moved once steady state was established).

Baseline ~11A (remeasured each time)
Excess above baseline.
Blower + Fans (AC on) 28.1A
Hi-Beams on 16.5A
Blower alone 15.5A
Headlights (hi-beams off) 12.7A
Drivers side fan 7.3A
Radio ~ 1A (it was moving around though)

From there you can infer the total fan draw is 12.6A
and the AC fan is 5.3A.

Again, if anyone was curious... (and I only checked one car besides mine)

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?

Last edited by MazDilla; 01-22-2009 at 10:18 PM.
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Old 01-22-2009, 10:35 PM   #12
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Quote:
Originally Posted by MazDilla View Post
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.
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