View Full Version : How many hp does the engine fan consume?



dcthompson
09-06-2014, 04:48 AM
Does anyone here know how much power (in hp) the engine driven fan on a B18 or B20 consumes? Let's assume a standard 4-blade fan (although I suspect that the number of blades makes very little difference), with the car operating at speed, with the engine spinning at 3,000 or 4,000 rpm. I am assuming that the number is large, but I'm curious to know what it actually is.

Phil Singher
09-06-2014, 02:31 PM
Dan, I for one do not know, and it's not easily researched. It's obviously going to vary quite a lot depending on the fan design as well, both in how much air the fan moves and how efficient it is in doing it.

The little four-bladed fan probably does not use more than a few HP. I'm more concerned about it being very heavy for the amount of air it moves -- power used in accelerating rotating mass is power that not available to accelerate the car -- and about it being adequate at idle in very hot weather.

Years ago when we lived in the San Fernando Valley (often 100+ F in summer, and lots of stop-and-go traffic), I ran a six-bladed flex fan. It moved lots of air and cooling was adequate... but there's no reduction in power loss. As the fan spins faster, power goes into flattening the blades.

Clutch fans are a little bit more efficient, but the objective with these is mostly to reduce noise. There isn't room for a clutch fan in an Amazon, 1800s that had them have the radiators moved forward almost an inch to accommodate them.

I've set up a number of old Volvo with thermostatically-controlled electric fans, using no mechanically driven fan at all. These consume power by loading the alternator, but only when they are running, and they are only going to run if you're stuck in traffic and not moving. Once you're going 20 MPH or so, there is no need for a fan of any sort. Who cares about power loss when you're sitting still?

gdill2
09-06-2014, 05:01 PM
Twin electric pusher fans are the way to go on an Amazon.

George Dill

dcthompson
09-06-2014, 05:45 PM
Hi Phil: The question came about because a fellow on the Facebook Amazon site showed a picture of his new engine, equipped with DCOE 45 Webers. Instead of velocity stacks, the Webers were fitted with 90 degree upward bends, with air filters atop them. The intent, of course, was to avoid having to cut the fenders to accommodate air filters or an air box. The same photo showed the engine with a 5-blade cooling fan. I of course asked him how much power he lost from the right angle bends, and the answer was 5 hp (he had run the engine on a dyno). I then suggested that he consider replacing the conventional fan with an electric one, since it wouldn't rob the engine of horsepower when the car was at speed--which led me to the question: just how much power does an unclutched fan consume at, for example, 3000 rpm and 60 mph? It turns out, as you say, that the answer is not at all straightforward. Factors that need to be considered include the effect that the forward motion of the car has on unloading the fan, the fact that fans aren't efficient under any circumstances, and that without any air to move (and in fact are trying to move air directly into a wall) they are a complete waste of energy.

If you look at the web, a lot of guys have tried to approximate what the fan consumes at speed by simply measuring the current to rotate an electric fan while the car is at rest. They forget a lot of things in the process, however--and their claims of (for example) precisely 0.11 hp are patently absurd.

So... The bottom line is that the only thing that makes sense is a thermostatically controlled electric fan. My guess is that the power savings should be at least 1 hp. I had not considered the effect of the rotational inertia of an additional flywheel on acceleration--and suspect that Phil is right that the inertial effect dominates over efficiency in the impact on overall performance.

Incidentally, the Norwegian guy with the 90 degree inlets is switching to electric fans, as am I.

Phil Singher
09-06-2014, 06:44 PM
Dan, I've set up several Amazons with a single 12" pusher, including a Carrera Panamericana race car, without problems. I had a single 12" pusher on my old hot rod 1800S, which dynoed at 194 BHP. It hardly even came on. I did use two 9" fans on a supercharged 1800E -- the forward placement of the radiator meant a 12" fan wouldn't allow the hood to open all the way on that one. You certainly could run dual fans on an Amazon, but I don't know that it would be an improvement (KISS, enough is enough).

90 degree air horns are definitely not the way to go. If the loss was only 5HP, that suggests that the baseline setup wasn't all that great either (the factory horns are a lot better than nothing, but...). We can discuss intake tuning off the forum if you like, although any decisions about that are not exactly urgent at this time.

dcthompson
09-06-2014, 07:24 PM
Approximate swept area of a 12" fan is 113 square inches. Approximate swept area of two 9" fans is 127 square inches. Actually, the 12" fan probably has more swept area than the two 9" fans, because of the extra area occupied by two motors. Assuming that the motors all run at the same speed, the volumetric capacity of the single 12" is necessarily greater than the two 9" fans, because of the higher tip velocities. Ergo, one 12" fan will give you greater capacity than two 9" fans. Cheaper too, I assume.

tmtalpey
09-06-2014, 09:02 PM
Back in the day, ipd sold a flex fan, made of a central flat steel hub with 4 stainless spring steel blades. it pulled like stink at idle, but went practically flat at rev. Only bad thing about it was that it sliced the heck out of your fingers when rotating the engine to adjust the valves, etc. :)

Oh, other good thing was it put far less load on the water pump bearings. Way less gyro effect under accel/decel.

Sigh, haven't seen one in years. Maybe ipd still has some way out back?

gdill2
09-06-2014, 09:24 PM
Twin (small) electric pusher fans on an Amazon can be offset sufficiently to minimize blocking the radiator.

If somebody reading this has the searchable ROLLING CD's look for Lee Cordner doing this installation on his Amazon with a detailed text/pic instructions.

George Dill

Phil Singher
09-07-2014, 01:00 PM
Back in the day, ipd sold a flex fan, made of a central flat steel hub with 4 stainless spring steel blades. it pulled like stink at idle, but went practically flat at rev. Only bad thing about it was that it sliced the heck out of your fingers when rotating the engine to adjust the valves, etc. :)

Oh, other good thing was it put far less load on the water pump bearings. Way less gyro effect under accel/decel.

Sigh, haven't seen one in years. Maybe ipd still has some way out back?

I've got one of those. And a scar where it nicked me with the engine running. It came with a 1/2" spacer to move it closer to the radiator, and you still had to cut the trailing edges of the tips for it to clear the generator or alternator. I can't remember the brand right now, but it was made by one of the main manufacturers of cooling accessories, including electric fans. It might very well still be available new.

M96
09-24-2014, 04:45 PM
This might work. https://www.flex-a-lite.com/belt-driven-fans/12-inch-flex-fan.html

The one I remember from the past was a 3 bladed Flex a lite