how much cfm to cool 3.2 engine

[dfink]

Has anyone ever tested how much cfm is actually needed to cool a 3.2 engine at the same level as the factory fan. My thought is to place thermocouples in the fins and test the actual fin temperature. Not the head temp as that is a different animal. But the temps at the fins. Would like to see a horizontal electric fan mounted then see if an electric fan could cool the engine fins as well as the factory fan does. If the fins don't get any hotter than they did with the factory fan then the electric should work just as well. Then mount a small alternator off to the side. Anyone up for the challenge have a motor they are getting ready to rebuild anyhow just in case. Shouldn't really be dangerous because the thermocouples should warn if the cooling is not effective then just shut it down. Could be fun making a horizontal fan shroud.

[Joe Bob]

I tried a dual fan on a Type 4.....it sucked.

[tctnd]

Well,
You certainly could use a properly sized electric powered fan to cool the engine but it will be heavier, more complicated, less reliable, and less efficient than the stock setup.regards,
Phil

[Flat Six]

Much detailed discussion on this topic already; pretty interesting IMHO. Check here:

Cooling fan HP

for background.

[stuttgart46]

That would be great for a 914/6. I'm probably 8-10 months away from being a guinea pig.

[Tippy]

From "mpdevelopment":

Fan output is in liters/sec 3.0-3.2 engine 1500liters/sec = 3178cfm this takes approx 10hp. Have you ever seen a 10hp 12volt 10000rpm motor?

[dfink]

Went back to some of the earlier posts on this and you get the typical back and forth arguments. One question is the cfm mentioned the amount the fan puts out or is it the amount the engine needs. On a normal 3.2 engine you have generally two heat ducts that allow air to escape. This air goes away from the motor either to the heater or out the dumps. On my engine I have no heat tubes and so all the air is forced to go someplace. At a certain RPM I don't recall exactly right now but there is so much air being produced by the fan that is starts to blow right back out the fan itseld. Makes me think I can save a few HP by not blowing so much air. It is also likely that given some proper ducting that a car in motion would likely produce enough airflow to cool itself. Now if you are going to hook a fan to a belt and that fan must produce enough air to cool the engine at idle speeds. Well it is still hooked up at 7000rpm and has to be pushing more air than heat can ever transfer to. So it seems a fan that is adequately sized for low speed and idle should be enough provided adequate ducting is provided for the higher speed stuff.
So the reason for the original thought of placing thermocouples in the fins as multiple reading could be obtained. Perhaps reverse flow the air and pull it up and out the back where it doesn't get trapped. Wouldn't work well for heaters but should work for cooling the engine. A couple of scoops on the side perhaps. What the heck it's a big motorcycle engine put a big underbelly scoop on and open the top completely removing the shroud let the air go out the deck lid.
I have a box missing two ends. Thoughts tend to excape from one end, money never accumulates due to the other end.

[j911brick]

Originally Posted by dfink

Went back to some of the earlier posts on this and you get the typical back and forth arguments. One question is the cfm mentioned the amount the fan puts out or is it the amount the engine needs. On a normal 3.2 engine you have generally two heat ducts that allow air to escape. This air goes away from the motor either to the heater or out the dumps. On my engine I have no heat tubes and so all the air is forced to go someplace. At a certain RPM I don't recall exactly right now but there is so much air being produced by the fan that is starts to blow right back out the fan itseld. Makes me think I can save a few HP by not blowing so much air. It is also likely that given some proper ducting that a car in motion would likely produce enough airflow to cool itself. Now if you are going to hook a fan to a belt and that fan must produce enough air to cool the engine at idle speeds. Well it is still hooked up at 7000rpm and has to be pushing more air than heat can ever transfer to. So it seems a fan that is adequately sized for low speed and idle should be enough provided adequate ducting is provided for the higher speed stuff.
So the reason for the original thought of placing thermocouples in the fins as multiple reading could be obtained. Perhaps reverse flow the air and pull it up and out the back where it doesn't get trapped. Wouldn't work well for heaters but should work for cooling the engine. A couple of scoops on the side perhaps. What the heck it's a big motorcycle engine put a big underbelly scoop on and open the top completely removing the shroud let the air go out the deck lid.
I have a box missing two ends. Thoughts tend to excape from one end, money never accumulates due to the other end.

I think one of the problems might be is that if you suck the air it will take the path of least resistance and you may not get even cooling. It can probably be controlled to some degree, but not as easy as forced air. Also: if your fan is pushing back air at high RPM, are you sure the blades aren't just stalling? I always wondered that the stall speed was?

[RSTarga]

Why????

[mfyoung1086]

Originally Posted by dfink

Went back to some of the earlier posts on this and you get the typical back and forth arguments. One question is the cfm mentioned the amount the fan puts out or is it the amount the engine needs. On a normal 3.2 engine you have generally two heat ducts that allow air to escape. This air goes away from the motor either to the heater or out the dumps. On my engine I have no heat tubes and so all the air is forced to go someplace. At a certain RPM I don't recall exactly right now but there is so much air being produced by the fan that is starts to blow right back out the fan itseld. Makes me think I can save a few HP by not blowing so much air. It is also likely that given some proper ducting that a car in motion would likely produce enough airflow to cool itself. Now if you are going to hook a fan to a belt and that fan must produce enough air to cool the engine at idle speeds. Well it is still hooked up at 7000rpm and has to be pushing more air than heat can ever transfer to. So it seems a fan that is adequately sized for low speed and idle should be enough provided adequate ducting is provided for the higher speed stuff.
So the reason for the original thought of placing thermocouples in the fins as multiple reading could be obtained. Perhaps reverse flow the air and pull it up and out the back where it doesn't get trapped. Wouldn't work well for heaters but should work for cooling the engine. A couple of scoops on the side perhaps. What the heck it's a big motorcycle engine put a big underbelly scoop on and open the top completely removing the shroud let the air go out the deck lid.
I have a box missing two ends. Thoughts tend to excape from one end, money never accumulates due to the other end.

I think the assumpation that a car in motion has the airflow to properly cool itself is a BIG assumption, the thermal trasnfer of heat to the barrels are 7,000 rpm is significantly greater than it is at idle... To add to that thinking, remember porsche went to a horizontal fan for more even cooling on their race cars, I think that some force induction cooling of the motor would be spotty at best

I geuss at the end of the day is it possible? To some extent it might be. Are the risks, time, and money worth it? Well thats a personal decision

[Grady Clay]

The 911 Carrera 3.2 is speced at 1500 liters/second at 6000 rpm crank speed using the original 1.67:1 fan-to-crankshaft ratio.

There is a larger crankshaft pulley that gives a fan-to-crankshaft ratio of 1.8:1 used on some 911s in the ‘70s (’78-’79 911SC for example). This combination gives significant increase in engine cooling.

For track use, there is the Rubbermaid Solution.

Best,
Grady