Next Generation Lightweight Cooling Fan - Part 1
Why a "Next Generation" Lightweight Cooling Fan?
My first generation lightweight cooling fan was a straight radial blade centrifugal design. The reason I chose that design was because the internal structure of the Doghouse fan shroud didn't allow for making a squirrel cage centrifugal fan with a larger blade. There simply is not the room due to the way the air inlet is designed, and the structure that catches and redirects air on the 1/2 cylinder side to feed the offset oil cooler. The stock fan really just barely fits to begin with, and I had a desire to keep as many stock parts as possible, making it very easy to just swap out the OEM fan with my lightweight cooling fan, and your done. While that desire to make as few changes as possible was laudable, it led to a design choice, the straight radial blade design, that suffers from one particular problem.
A straight radial blade centrifugal fan will degrade in performance more with static pressure than a squirrel cage centrifugal fan will. So, the higher the static pressure, which is the friction that air must overcome through the entire cooling system, the lower the performance of the straight radial blade design relative to the squirrel cage design. Having said that, there was no reason, based on my testing, to think it would be an issue, and for the vast majority of customers that have used the first generation lightweight cooling fan, this was not an issue at all. In fact, most customers that have talked to me directly love the fan, and have been getting great results from it. I had a customer that told me his engine temps were 45 degrees cooler with my fan versus the OEM fan. I have had many similar stories from customers all over the world, as I have shipped these to 16 different countries with all kinds of different climates. Generally speaking, the product has been very successful. So, why a change in approach with the second generation?
While this design has been successful, there were two cases that I was made aware of, through a post on The Samba, that had me concerned (there may be more that I am unaware of). The initial post was a customer with a Bay Window Bus. This customer was running a 2110 cc dual carburetor engine, and was using the OEM fan with a velocity ring, and a Porsche 356 alternator pulley. A good cooling combination, as the velocity ring adds about 5% additional airflow with the OEM fan, and the Porsche 356 pulley is smaller in diameter and changes the pulley ratio from around 1.6 to 1 to 1.8 to 1. This means the fan will be spinning faster, and based on the "Fan Laws", this means more cooling, as a fan spins faster it's output increases linearly with the increased speed. Of course, as output increases so does horsepower draw. In his post, he said that with the lightweight cooling fan his temps were actually better while driving around town on surface streets, and on the highway when he kept his speed down to 55 to 65 miles per hour. In the case of highway driving at 70 to 75 miles per hour, he would see his temps start to increase and increase to where he would be worried and slow down. My understanding is that if he slowed down the temps would come back down as well. The other person that had a problem was a Beetle owner, and his case is a little less clear.
The Beetle owner seemed to be trying to get his intake manifold to warm up better for some reason. He lived in San Francisco, so I'm not sure why the normal preheat connections from the exhaust were not good enough, as it doesn't get very cold in San Francisco, but it was hard to tell from his description what exactly he was doing, but it seemed like he was blocking off some of the engine bay ventilation slots in the decklid, or perhaps below the rear window. I may be wrong here, but that's the impression I had. It's very difficult to understand what is happening in these two cases without direct access to the vehicles in question, and being able to take some measurements, but I formed a theory about why they would be having the results they were having.
If you simply block off ventilation into the engine bay, the fan simply can only pull so much air through no matter what, so I'm not sure there is anything to be learned from the Beetle owner's experience, but if I'm wrong about what he was doing, it may fit into my theory the way the Bay Window Bus owner does. In the case of the Bay Window Bus, my theory is that these vehicles only had a top speed of 65 miles per hour, and that was full throttle on a flat highway with no wind. The engine was a 1600 cc (1584 cc) engine that had a maximum of 57 horsepower, and could only intake so much air, and the fan itself consumes the rest of the available air entering the engine bay through the ventilation. When you move to a much larger engine, that consumes a lot more air, and you add additional air flow through the combination of a higher performing fan, a velocity ring, and spinning that fan faster for a given RPM with a smaller diameter pulley, and then drive at speeds the vehicle was never intended to operate at, I think you get a number of issues.
The first issue, is that spinning a higher performing fan faster, means that you will encounter belt slip earlier, as a higher performing fan will use more horsepower. Based on my calculations, belt slip on a standard v-belt will start around 250 to 300 RPM sooner, and he was using a standard v-belt. Based on his gear ratios and tire size, he definitely was experiencing some belt slip at those 75 mile per hour speeds, but that would probably only mean that he was getting about the same cooling than he was getting before, so that doesn't really explain the entirety of the issue.
The second issue is the amount of air being used by the engine itself, and certainly that has some bearing, but again, based on the physics of the air flow into the engine, the engine was not likely to cause much of an issue, stealing airflow from the cooling system. That leads us to what I think is the primary culprit.
So, the third and final issue, is simply that the ventilation into the engine bay is effected by the higher speeds in such a way, that air flow has considerably more friction to overcome to enter the engine bay at those much higher speeds than the vehicle was ever intended to go. Given the nature of the straight radial blade centrifugal design, with that higher static pressure, the fans performance suffers more than the squirrel cage design, and airflow drops enough to cause cooling problems with the much higher performance engine, which requires considerably more cooling to begin with. That seems to be the only possible explanation that fits the data presented to me through the post. While I cannot be 100% sure, as I cannot take measurements from the vehicle and I don't own or have access to a Bay Window Bus for testing, this does seem to make sense, and possibly explain why this customer would get the results that he had, and why other customers were not having any problems. So, I felt like I needed to go back to the drawing board and rethink everything, because I don't want any particular use cases where the lightweight cooling fan doesn't work better than OEM. It's really just that simple, my desire is to ship a product that I have full confidence will always outperform OEM under all conditions.