A lot of people are really interested in what kind of horsepower penalty they will pay when upgrading to the increased airflow of an Evolution Two fan versus OEM fans. This is certainly understandable, as you cannot escape the physics (the fan laws) which clearly indicate that as you increase the airflow capabilities of a fan you will increase the horsepower required! There is simply no way around that. Now, what you can do, is improve the efficiency, which will help to have less of an increase in horsepower required compared to the increase in airflow. For example, the Evolution Two fan for late VW doghouse fan shrouds has a 22% increase in airflow over the OEM fan, but does it have a 22% increase in horsepower required? Well, we are about to answer that question, for both the early VW version of the Evolution Two and the late VW version of the Evolution Two. Here we go!
The above is for the early VW version of the fan. As you can see from above, the horsepower required is extremely close, while slightly higher. The improved efficiency really helps to offset the power requirements here, and you have to get above 5,500 RPM before you even have a single horsepower increase over the OEM fan, while achieving a 25% increase in airflow! Now let's look at the late VW doghouse fan comparison.
Looking at the above, we see the same story. You have to go above 5,500 RPM before you even have a single horsepower increase, and all this for a 22% increase in airflow. The dual inlet design, along with all the optimizations of the fan itself really pays off!
NOTES: Some clarification information is required here, when looking at these graphs. One, is the fact that both the early VW and late VW horsepower numbers are almost the same, and that may seem incorrect. Well, they are different, but the difference is so small (a .05 amp draw difference on my electric motor), that it just rounds out to the same at the lower RPM ranges. You can see them diverge as you go up the RPM range. There is another factor involved, even though the late VW doghouse fan does make more airflow, is the fact that the inlet size between the two fan shrouds are different, with the early VW being larger, so at the test RPM of the electric motor you have more air restriction bringing down the airflow and horsepower requirements. Another issue to talk about is belt slip.
These numbers are calculated off the electric motor driving the test, which is highly accurate (probably more accurate than a dyno), but they cannot account for belt slip. The standard v-belt starts to slip around 4,000 to 4,500 RPM, which would bend the horsepower curve down at that point and above. So, these numbers are based on zero belt slip, so represent a worse case scenario above 4,000 to 4,500 RPM. Below that RPM, they are completely accurate. If you are using a poly v-belt system (serpentine), belt slip is really not known, but I suspect that there is virtually zero, so these numbers are probably very accurate for a poly v-belt setup. It's very likely that a poly v-belt setup can transmit 27 to 28 horsepower through the belt and pulley system.
Finally, I published numbers on horsepower draw comparing the late VW doghouse fan to the original lightweight cooling fan, and these baseline numbers are different than what I published before. It turns out I had a mistake in my calculation, where I didn't have the electric motor efficiency in the calculation, which makes a big difference. The difference, percentage wise, between the two was accurate, but the specific horsepower numbers were off. These numbers are corrected for the electric motor efficiency, which turns out to be 73%.