A 'Pull' fan is stated to be more effienct than a 'pusher' for a couple of reasons.
1. Installing a fan on the front of the radiator actually blocks surface area for cooling.
2. Installing a fan on the rear of the radiator with a shroud actually increases shroud pressurization which increases core efficiency.
3. If using two fans, one on each side, they need to be the same fan with the same CFM rating AND
they need to be staggered/offset from each other a not to create a static pressure between the two fans themselves.
SPAL recommends the use of a single 16" Pull fan for HD applications (part# 30102002). The use of their 'Twin" fan with shroud only increases CFM rating by roughly 400 CFM. Depending on on your core dimentions and core efficiency you might consider running a 16" and an AUX 10" to mount on one side. Using the single 10" as a back up or for supplemental cooler.
Blade type is also something to consider. A single strait blade type fan is slightly less efficient as compared to a curved blade.
If you are looking for one fan to do the job, you might want to consider the use of (yeah I know.....) the fan off of the Lincon MKVIII. It is a single, curved blade, two speed fan that pulls over 5000CFM. That is more than everyone else on the market for one single fan. I have been using them in the most severe applications, ie, big block,ac,auto,high stall in Tucson Arizona daytime summer heat......Max effort Desert racing and in max effort quote-un-quote street cars (LOL). and I have found no better. It has a built in shroud and its dimentions are easy on the installation.
Food for thought..........
Hey look at that I found a picture on a friends site: http://soderblom.net/bronco/tech/fan/2.jpg
And another link on electric fans LOADED WITH INFO
[ 03-10-2003, 11:36 AM: Message edited by: chicane67 ]