Electric fans: 2 smaller or 1 bigger?

[E30SPDFRK]

The last autocross of last year a cone got wedged up behind my valence and broke my fan. It's been winter so I've been driving around fanless since then but it's starting to get warm again so it's time to fix it. I had a single 14" Spal before that worked ok, but I'm going to have a much hotter engine bay this year so I'm looking to upgrade.


My question is: what would cool better? A large single fan or 2 smaller fans? Say 12"ish for 2 or 16" for one. Would the stock aux. wires even be able to handle 2 fans?



Thaaaanks

[N2MYE30]

Subd for info. I was thinking about 2 pullers myself but still unsure.

BTW, 24v?

[CorvallisBMW]

I would lean towards 1 larger fan, say 16" or so. 2 small ones, even 12", probably wont fit on your rad. You'd have to make one a pusher on the front and the other a puller on the back, then offset them so they cover more of the actual radiator surface area. I'm also going to guess that 2 smaller fans will pull more amps than 1 larger unit.

[E30SPDFRK]

Yeah I just said 12, I'm sitting at work guessing and have no idea what size I would actually need, that was just to get the idea down. I need pushers too, I don't have room for a puller.

BTW, 24v?

That would get me less heat ;)

[fiftytakedowns]

on my 2002 I am going to run two smaller 9" fans, because frankly, it's all I can fit. That being said, it has an oil cooler as well, not sure if that matters.

[bimmerteck]

given 12.5mm H^2O resistance on the radiator/condenser core=


2x11" spal pusher fans pull 18.4 amps move 2160 cfm of air (costs $269.90 retail)
1x16" spal pusher fan pulls 13 amps and move 1720 cfm of air (costs 134.95 retail)


full speed the single 16" pulls 70% of the current while moving 79% of the air and weighing just under 3lbs less. as long as your using the stock controls there isn't really any advantage to 2 fans. . . now if you were to have two totally independent control circuits there could be and advantage in absolute reliability, but it would be at the cost of space, weight, expense, and draw on the electrical system. (if I wanted 2 fans I'd do 2 16" fans for the same cost and gain more efficiency. :) )

BTW- I believe I measured and figured out that 2 12" fans wouldn't fit side by side hence the 11" eval. ;)

[M-technik-3]

no mention of a shroud? Having one will increase your efficiency.

[bimmerteck]

no mention of a shroud? Having one will increase your efficiency.

It would if you had room for it. I haven't needed any more cooling efficiency than the single 16" provides in my current climate though so I haven't really tried too hard to evaluate adding one. Plus between the oil cooler, plumbing and still having A/C I prefer the space afforded by mounting the 16" fan directly to the condenser.

[M-technik-3]

You should have plenty of room for it. Look at Zionsville's and just replicate that.

[E30SPDFRK]

given 12.5mm H^2O resistance on the radiator/condenser core=


2x11" spal pusher fans pull 18.4 amps move 2160 cfm of air (costs $269.90 retail)
1x16" spal pusher fan pulls 13 amps and move 1720 cfm of air (costs 134.95 retail)


full speed the single 16" pulls 70% of the current while moving 79% of the air and weighing just under 3lbs less. as long as your using the stock controls there isn't really any advantage to 2 fans. . . now if you were to have two totally independent control circuits there could be and advantage in absolute reliability, but it would be at the cost of space, weight, expense, and draw on the electrical system. (if I wanted 2 fans I'd do 2 16" fans for the same cost and gain more efficiency. :) )

BTW- I believe I measured and figured out that 2 12" fans wouldn't fit side by side hence the 11" eval. ;)

Looking at those numbers I don't think the extra 400cfm is really worth it. I think I'll go with the single 16 for now and upgrade to 2 smaller if I end up needing it.

Thanks for the educated answer

You should have plenty of room for it. Look at Zionsville's and just replicate that.

I don't think shrouds work very well for pusher fans

[M-technik-3]

I don't think shrouds work very well for pusher fans

Agreed

[CorvallisBMW]

Agreed

+2, there isn't any advantage

[bimmerteck]

+2, there isn't any advantage

Well shrouds do technically increase efficiency when the fan is running but usually at the expense of airflow at speed.

[CorvallisBMW]

They won't increase airflow in a pusher, in fact the zionsville unit decreases it because it blocks off any air passing over the radiator due to the movement of the car. A puller fan (when spaced back a small distance) would be different since it is able to pull air from across the entire cross section of the radiator, where as a pusher moves air across only the area is sits directly on.

[bimmerteck]

Yes but a ducted pusher as I investigated for one of my builds is relatively efficient given the size of the duct intake is larger than the radiator to compensate for the efficiency loss of the air being forced through the pusher fan when off. The problem I encountered was to run an efficient duct-fan-shroud design for a 16" pusher fan with a radiator core the size of the e36 core I'm using you would need 8" or so of empty space in an area 3" larger than the e36 radiator core. Short of hacking off the front bumper, hood, valence it just won't happen in an e30 and doesn't need to b/c the single 16" pusher mounted to the condenser works fine provided you duct between the two cores with an non thermally conductive material to maximize air pressure into the radiator core at a dead stop.