Thermostat or No Thermostat?
I use a 2006-2007 Chevy HHR fan on my 22" radiator, it kept my 410 stroker (540+ hp) motor cool on the streets, and between rounds on the track on hot days. It has a full shroud, pulls over 3000 CFM, and for about 50 bucks it is one of the cheapest options. All you have to do to make it fit is a little trim off the tabs on the sides.
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What I like about this fan is the blade shape, which is what you see in OEM fans, both mechanical and electric. A lot of the after market fans that crap out in real use have the thin blades. Here is why they fail to move air:
- Pulling air through the radiator creates a low pressure are between the back of the rad and the fan. This pressure drop is usually around 0.5" + or -. And there is also a higher pressure area immediately behind the fan.
- The air tries to flow around the blade from the higher pressure exhaust side of the fan back to the intake side of the fan where there is lower pressure.
- With the thin blades, the air has an easy time of taking this 'sneak path' around the blade and back to the inlet. When this happens, then the fan just recirculates the same air around its blades and stops pulling as much fresh air through the rad.
- The fat blades make it harder for this 'sneak path' to occur. Again, look a the OEM fans, both electric and mechanical.
So that is why the flow ratings of fans is not the whole story. The above-pictured fat blade fan probably maintains over 2000 cfm of airflow in a real installation, as opposed to multiple 2000+ cfm crappy-bladed fans, whose flows actually drops to well under 1000 cfm each when mounted behind a rad.
One final thing: The HP used at low speeds to move the car is in the 50 HP range. And when idling, the output HP is 0. So there is no real relationship between peak HP and the heat shedding requirements. At idle, it has a lot more to do with displacement, because the waste heat generated is fairly proportional to that. (Plus compression and timing, etc.)