Aircooled engines

How much airflow does an aircooled engine need to keep it cool? I know that is a very broad question and I am sure depends on a lot of things. But I don't know where to start.

It Depends. For example, a 883 Sportster on a hot day idling will seriously climb in temp. I've logged cylinder heat temps of over 230. Then, just pointing a box fan at the bike a few feet away dropped the CHT down to 185.

I would say that how much HP you're making at that point is going to mostly determine the air flow requirements.

Would two intakes (one for each cylinder) with a fan in the tubing directed on to the cylinders provide enough air flow around the cylinder to keep the engine cool enough for a 90-100hp engine?

Fans would be something like this.

Cooling the heads in an AC engine is the primary concern.

If it is a ducted system, you not only have to worry about flow but also pressure and electric fans generally do not create a lot of pressure. The Corvair (2.7 L engine) fan can draw from around 1 hp to over 10 (depending on rpm) and the fan is a pretty efficient design. As you can imagine, even a 1hp electric motor is rather large. Conversions using electric fans have run into issues at anything over light driving.

My guess, based on the stock Corvair engine, which puts out a bit over 100 hp in standard form, would be that it would be very unlikely those fans would be enough for more than just idling (unless those fans are a LOT bigger then they look), The addition of ram air and how efficient the fins are on the engine could alter this of course.

The bigger one is 4" in diameter. Any ideas on adapting a fan on a Moto Guzzi engine then?

I always heard that about 1/3rd of an engine's power goes into the coolant. 746 watts per HP, so let's say a 200hp engine for a test case;

746 x .33 x 200 = 49236 watts required to cool this thing assuming 100% efficiency in the electric motor and generator on this rig.

Using a 12VDC system, we're talking about the following;

49236 / 12 = 4103 amps.

Let's say instead you want to use a 70 hp Guzzi;

(746 x .33 x 70) / 12 = 1436 amps.

Anyone want to check my math, or can we just assume that this isn't happening?

I have always heard that you need to dissipate about a 1/3 of engine power as well. The heat will be dissipated by the fins however. So the fan will be changing the velocity of the air going over the fins to cool the engine. I guess I could do a fin problem. I mean I have taken heat and mass transfer.

1. I doubt it's that high of a percentage.

2. You're confusing the amount of potential heat transfer rather than the amount of power needed to enable it.

A 500hp Corvette engine is cooled by a 15amp fan... because the work the fan does not equal the heat transfer itself.

In an air-cooled engine, the air itself is the coolant... with no fam at all it still removes a great deal of heat. And the amount of heat it removes as you move it is not linear with the amount of power needed to move it.

One consideration in a Corvair engine is pressure... it takes a lot of air pressure to force air through the cooling fins... a lot more than a radiator. And that's typically why the electric fans fail at all but the lightest loads.

In my Berkeley, running a 10" 8A fan reduces the head temps of my engine more than driving the engine down the road and letting air flow across the fins. (The car was designed with no cooling fan at all.) But it's only 18.2hp. ;-)

So that's 8 amps for every 18.2 hp? I know it's not a true representation of what amperage is actually needed to cool a Guzzi engine but I think there's still going to be a serious amperage requirement if we're talking about dropping it into full bodywork. Also, and this is just a hunch, but the ZR1 or whatever Corvette you're talking about probably has multiple fans and a very large multi-core radiator. There's a lot of difference here and I don't think you can use a radiator fan example like that on an air cooled engine in a car body.

I was using the Vette as an example to show why you couldn't simply convert 1/3 of the engine power into amps... air cooled engines work the same way, just not as efficiently (less surface area for heat transfer of a less efficient medium).

The 16" electric fan on the Jensenator draws 11 amps. But like wcelliott says the amp rating has no real bearing on cooling system heat rejection. That's a function of the volume of air movement across the surface to be cooled (duh) and that means the fan has to be sized appropriately.

I'm not seeing 4" diameter fans moving enough air to keep the heads on a Guzzi cool. Plus, you'll need a good bit of ducting to keep as much of the air as close to the engine as possible to make the fan as efficient as possible. Check out how a VW or Porsche engine is ducted to see what I'm talking about. Here's the deal with that: once the ducting is in place to make the fan efficient, the fan now becomes the only real source of cooling air for the engine, even at highway speeds. That means the fan has to now be pretty darn big (and draw more power) in order to keep the engine in the normal range. At this point you are now getting into the 1 hp motor range mentioned earlier.

Is there a way to add an engine driven fan to the Guzzi motor?

Have you considered a Honda CX500 / CX650 motor instead? Similar packaging to a Guzzi motor, water cooled.

It was just an idea (obviously becoming a bad one). The reason I liked the Moto Guzzi is the external transmission. I was thinking for VW Bug but the ease of some of the other swaps makes it not worth it.

Fan on an oil cooler?

I found that Corvairs and 914s don't cool as well when you remove all the sheet metal duct work from around the engine.

Dan

My real point with the amperage talk was about asking a Guzzi to feed enough juice to cooling fans may be a bit problematic.

Remember one point: if you do this, you want fans rated for continuous duty. I believe those Attwood fans are bilge blowers, which are meant to run for maybe a half hour to 45 min, max. So if you're going to do this, and the ducting is such that you can't turn off the fans when underway, shop accordingly.

And rated CFM is measured at the exhuast side right at the blower. You will have cfm loss the further the fan is away from the heads. Granted probably not very far on a bike, but, something to keep in mind.

914Driver wrote: I found that Corvairs and 914s don't cool as well when you remove all the sheet metal duct work from around the engine. Dan

In Corvairs that's mainly due to the recirulation of hot air (both exiting from the bottom of the engine and from the exhaust), mainly at rest.

If you keep the top sheet metal on (to stop the recirc), but remove the bottom sheet metal (heater boxes) to allow lower pressure air flow, cooling improves significantly.

In track car (that doesnt sit in traffic idling) the cooling even without the upper sheet metal is similar because on the move (in a car) the recirc issue is minimal.

But to your point... directed airflow is critical. It won't make any difference what you do with the external sheet metal if you forget to install the little individual cylinder sheet metal pieces that direct colling air into the cylinder fins... that is critical on a Corvair and should be a consideration for any forced air cooling modifcation. (I didn't bother with it on my Berkeley because I knew the big fan was massive overkill to start with...)

With my math: 200 bhp output, twice that is wasted (just to heat lets say; ie. 1/3 the of the fuel's energy gets turned into bhp), 150° temperature rise, you'd need 6300 cubic feet per minute to cool the engine. Thats just back of the napkin type of caculation.

Pres589, the math was correct, it's the assumption that is faulty. The job of the fan is to move enough air to remove the calculated amount of thermal energy. It does this by heating the air as it passes over the cooling fins. The amount of heat removed is the delta T (inlet air temp - outlet air temp) times the mass of air moved. The fan power is determined by the mass of air that has to be moved to remove enough heat to keep the engine temperature low enough. And that relates to efficiency of the fan, ducting, fins, etc.

Carter