Introduction
The stock mechanical fan has become something of a hot topic, especially when replacing it with an electric fan is in question. The problem has been that
no one knows how many CFM the stock fan actually pulls, and it has become a general belief that the stock fan pulls more CFM than any reasonable electric fan ever can. There is good reason for this belief, as electric fans have an innate efficiency handicap of perhaps 70%. This is due to changing mechanical energy to electrical in the alternator, and back to mechanical in the electric fan itself.
Please see
[The Electric Fan].
This guide first and foremost presents an estimation for the stock S4 clutch-type fan's CFM and HP requirements. Second, once the stock fan is parameterized, it is compared to a popular after market electric fan to see if indeed the electric fan can provide an equivalent cooling capacity on hot days and low driving speeds, a classically bad situation for cooling systems.
Estimated Performance
Note: These calculations are meant to give ballpark estimates, and were done using WingFan 5.0; not all fan options are available in the program, so the closest available values were chosen. These deviations are noted below.
Sanity Check
Confirming that we can rely on this calculation method, values for Flex-a-lite's Black Magic 150 were used to see how close to the company's claimed 2,800CFM at 0 inH20 (zero static pressure - i.e. when it is in the open and not attached to a radiator) we get. Using values from the Flex-a-lite website as well as direct measurements, the following parameters were used:
- Fan Diameter: 15"
- Number of Blades: 8
- Blade Shape: Aerofoil, type "P3H"
- Blade Angle: 25-degrees (actually 26, according to the website)
- Tip Clearance: 0.2"
- RPM: 2,200
- Temp: 90C
- Installation Type: Ducted Inlet, Free Outlet
The result was 2,312 CFM at 0 inH20. This seems close enough to 2,800CFM for this estimation since companies tend to be generous with their claims, and because this calculation was done at 90C, for reasons given in the next section, to make the comparison with the stock fan meaningful.
click to enlarge
- CFM: 2,312
- HP: 0.27 (0.35 from the engine due to 70% efficiency)
- inH20: 0
Peak Pressure Efficiency:- CFM: 1,760
- HP: 0.36 (0.47 from the engine due to 70% efficiency)
- inH20: 0.5
On to the Stock Fan
The stock fan varies its speed depending on engine RPM and ambient air temperature. Judging from the training manual's diagram (see image right), the fan's RPM is 25% of the engine's RPM below 62C, 80% of the engine's RPM above 85C, and otherwise somewhere in between; this appears to already take the 1:1.23 spin ratio between eccentric shaft and water pump into account.
So, on a hot day, at an idle engine speed of 750 RPM, the stock fan will spin at 600 RPM. When cruising at 2,000 RPM, the fan speed becomes 1,600 RPM, and with the engine at 3,250 RPM, fan speed becomes 2,600 RPM. These three fan speeds were used to compute the stock fan's CFM, along with the following parameters, mostly from the FSM and training manual:
- Fan Diameter: 15.35"
- Blade Shape: Sickle, type "S2Z"
- Number of Blades: 7 (should be 8, but the closest sickle-type fan has 7 blades)
- Blade Angle: 45-degrees (FSM says 44-degrees)
- Tip Clearance: 0.75" (middle of FSM clearance range)
- RPM: 600, 1,600, and 2,600
- Temp: 90C
- Installation Type: Ducted Inlet, Free Outlet (i.e. radiator shroud)
Interestingly, the stock fan's tip clearance is quite large at 0.75". This is bad for efficiency when trying to pull any kind of pressure, like through a thick radiator. The likely reason for such a large gap is to account for engine vibration, since the the shroud is attached to the radiator, while the fan is attached to the engine. This ensures that the fan blades don't collide with the shroud when the engine shakes around on its mounts.
Also, a fan with 7 blades was used for calculation here, instead of the actual 8 blades on the stock fan. WingFan has no 8-bladed sickle-type fan, and the blade shape turns out to matter more than the number of blades. A rough estimate of 8-blade vs. 7-blade might be to multiply CFM by 8/7, or 1.14.
click to enlarge
- CFM: 673
- HP: 0.01
- inH20: 0
Peak Pressure Efficiency:- CFM: 500
- HP: 0.01
- inH20: 0.02
click to enlarge
- CFM: 1,791
- HP: 0.21
- inH20: 0
Peak Pressure Efficiency:- CFM: 1,324
- HP: 0.26
- inH20: 0.17
click to enlarge
- CFM: 2,916
- HP: 0.9
- inH20: 0
Peak Pressure Efficiency:- CFM: 2,160
- HP: 1.12
- inH20: 0.43
Conclusion
The original goal of this guide was to provide an estimate of the stock fan's airflow and power consumption at idle. Assuming that Mazda engineered the fan to run at its peak efficiency, this estimation suggests that at idle,
the stock fan provides 500 CFM, costing 0.01 HP, at 0.02 in H2O. This perhaps the first estimate of the stock fan's airflow; the rest of the data is provided on the following graphs:
Blue: Stock fan
Red: Black Magic 150
values are at peak efficiency
The question of whether an electric fan can provide adequate cooling is also answered: it seems most electric fans are actually overkill. The Black Magic 150, for example, provides more than three times the CFM at idle as the stock fan, though it takes many times the horse power while doing so. However, somewhere around 2,200 engine RPM, the stock fan can supply more CFM than reasonable after market electric fans. In cases where airspeed is low yet RPM is high, like driving up a mountain, the stock fan provides a safety buffer that an electric fan cannot.
Furthermore, after market electric fans like the 150 provide maximum CFM and pressure potential regardless of engine RPM. At idle, when the stock fan provides 500 CFM at 0.02 inH2O, the Black Magic provides 1,760 CFM at 0.5 inH2O. Thus, cooling setups requiring high airflow, like drifting radiators, or high pressure, like racing radiators, might benefit from an after market fan. The stock fan was tuned for the stock cooling system.
Just keep in mind this is a rough comparison, because the values here are estimates, and because the exact fan speed (the "operating point") depends on knowing how pressure varies with CFM across the radiator and shroud. Finally, this guide presents a fan's potential CFM, and does not consider how that airflow is distributed across the radiator. For instance, the stock fan uses the entire radiator, while the Black Magic's shroud confines the airflow to a slightly smaller area.
Interesting Observations
These observations are relative to the stock fan at 2,100 RPM and at 0.43 inH2O.- Shroud Clearance #1: Increasing shroud clearance from 0.75" to 0.76" decreased CFM by 6% at the same pressure. It seems that not only is having a shroud important, but CFM is very sensitive to small changes in shroud clearance.
- Shroud Clearance #2: Decreasing shroud clearance to 0.2" increases CFM by 40-50% at the same pressure. The stock fan's necessarily large shroud clearance can more than make up for the efficiency handicap that electric fans have.
- Altitude: Increasing altitude from sea level to 1,000' decreased CFM by 1.7%.
- CFM Above Idle: The stock fan can pull more CFM than most electric fans when the engine is above approximately 2,200 RPM (see results graph). For instance, at 4,000 engine RPM, the stock fan pulls 2,600 CFM at peak pressure, while the Black Magic 150 is limited to 1,760 CFM. When driving up a steep hill on a hot day, the stock fan might provide enough cooling capacity, while an electric fan might not.
Resources
See Also...
Note: You must log in to contribute to or view the source of this how-to.
[admin version]
[community version]
[version history]
