Coming from Roundel Weekly? Read Part VI: The Choice Of Refrigerant And Oil

Last week, I fabricated the brackets for the universal parallel-flow condenser, test-fit everything about a dozen times, and was ready to snug the whole thing down for good with Nylock nuts when I stopped and said that, in truth, you need to deal with the selection and installation of a condenser fan in parallel with the condenser installation, because most of the time you mount the fan on the condenser, and thus need to verify that when you mark any holes to drill for any brackets, the fan doesn’t interfere with anything.

It may have been a bit awkward journalistically to stop like that, but the condenser fan really does deserve its own column. So here we are.

Just as a radiator needs a fan, the condenser needs to have a front-mounted electric cooling fan pushing air through it to help to dissipate the heat that the condenser is trying to dump. And, just like a radiator, the condenser fan is least important while driving on the highway, and most important when stuck in traffic in hot weather. My advice is to pair the biggest condenser that’ll fit with the biggest, most powerful fan that’ll fit (“a fan that can suck a schnauzer off the sidewalk” is how I’ve previously phrased it). You usually select a fan whose diameter is the same as, or slightly smaller than, the smaller dimension of the condenser, which is usually the height. So with the shark’s sixteen-inch-high condenser, I planned to use a sixteen-inch fan unless a test-fit showed that the space directly in front of the top or bottom of the condenser was occluded. Nothing looked like it was in the way, so sixteen inches it was.

But the selection of a fan requires considering parameters other than diameter. These include weight, depth, cost, and—probably most important—the number of cubic feet per minute (CFM) of air flow. This last one is the parameter most affecting whether a schnauzer can actually be sucked off the sidewalk, and it is here that I can be accused of not following my own advice. Here’s why.

Since I began doing air-conditioning retrofits and rejuvenations almost twenty years ago, I’ve used Spal Italian-made sealed-bearing pusher fans. They have an excellent reputation. I’ve yet to have one go bad. The Spal fans come two main styles: low-profile and high-power. The low-profile fans, as their name implies, are very thin (like two inches) front-to-back. Their main application is to be inserted into the small space between the engine and the radiator to replace or augment a mechanical fan. In that configuration, you want a “puller” fan that pulls the air through, whereas if you’re mounting one in front of a condenser, you want to be certain to order one in a “pusher” configuration. Some vendors claim that their fans are reversible. Spal says that their fans will certainly spin if wired backwards, but that the fan blades are designed for one-way operation.

To be clear, for a condenser fan, we usually don’t care about the actual profile (thickness) dimension, as the fan is thickest in the center where the motor is, and when it’s mounted, that’s in the center of the nose where there’s usually oodles of room. It’s neither the low profile nor even the moderate price that has me often using low-profile fans, it’s the weight. A sixteen-inch Spal low-profile pusher fan weighs just 3.2 pounds.

And the weight is a crucial issue affecting how you mount a condenser fan. The original condenser fans on vintage BMWs were heavy, about eight pounds, and, if memory serves me correctly, they were mounted to the nose of the car using brackets with rubber washers or bushings to cut down on the transmitted vibration. These brackets are usually very specific to the original fan. So if you wish to mount another heavy fan, you’re faced with the prospect of having to do what you just had to do with the condenser: fabricate brackets and either drill holes or try to re-use old ones.

There is, however, an easy alternative. If you use a lightweight fan like a Spal low-profile model, you can instead mount it directly to the condenser using what are essentially zip ties (search for “Derale 13001 Plastic Rod Mounting Kit”). The first time I used one of these, nearly twenty years ago on the E9, I was very suspicious, as you’re hanging the fan off the condenser’s cooling tubes, but it held up fine. Note that it’s advised that you don’t use this mounting technique for a vehicle that spends a lot of time driving off-road, as the jostling of the fan can loosen the mounting ties, and the subsequent motion can bend the cooling tubes.

Over the years, the deal I’ve made with myself is this: You can use the zip-mounting kit for an a/c condenser fan as long as it’s employed on a lightly-used vintage car, not a daily driver, and as long as it’s used to mount a lightweight fan.” So, lightweight low-profile Spal fan plus zip mounting kit equals fan mounted on condenser in two minutes. With all the ins and outs of a/c retrofit, it’s a pretty appealing shortcut. And the fact that the low-profile fans aren’t as expensive as some of the other models is an added benefit.

In contrast to the lightweight low-profile fans, Spal’s high-performance (full-profile) fans move about 50% more air, but they also are twice as heavy, draw more than twice as much current, and cost more. The salient parameters for the sixteen-inch pusher fans are summarized in the table below.

Spal Model Blades Weight (lbs) Depth (in) Air Flow (CFM) Current (amps) Price (Amazon)
30100401 (low profile) Straight pusher 3.2 2 1298 8.5 $78
30102048 (high performance) Curved pusher 6 3.9 1959 19 $136
30102047 (high performance) Straight pusher 5.8 3.5 2036 22.5 $146

Regarding the blade shape, according to Spal, all other factors being equal, straight-bladed fans move more air, but curved-bladed fans are quieter. Indeed, nearly all of the Amazon reviews for the Spal straight-bladed 20102047 say “great fan, moves a ton of air, but man, this sucker is loud.” Spal’s low-profile sixteen-inch fans are only available straight-bladed, but their high-performance pusher fans are available with either blade shape.

Note that in buying a name-brand fan like a Spal, there’s a decent chance that when they say the fan puts out 1,298 CFM, that spec is accurate. There are certainly other reputable manufacturers of high-quality fans (Mishimoto, for example). In my humble opinion, you should treat the CFM specifications of unbranded Chinese-made fans with a healthy degree of suspicion. Just because an ad says a $40 fan puts out 3,000 CFM doesn’t mean it’s actually so.

There is some middle ground between the two options of quickly using plastic-ties to attach the fan to the condenser’s cooling tubes and spending half a day elegantly mounting it independently to the nose. You can instead mount the fan to the brackets on the sides or tops of the condenser, which are considerably stronger than the cooling tubes. Spal sells mounting brackets with Erector Set-like multiple holes and plastic ends that snap into the circumference of the fan shroud. They’re about $20 for a set of four. In some cases, the brackets are included when you buy a new Spal fan; it depends on the vendor you’re buying it from. These brackets work fairly well, although the result often isn’t quite as clean as you’d expect. The process of mounting the condenser in the nose always takes up some of the mounting holes along the sides, so they’re all not readily available for the Spal brackets. And the condenser’s side brackets aren’t flush with its face, so spacers (or at least washers) need to be employed between them and the Spal brackets. The top and bottom brackets are flush with the face, but they’re not as strong as the side brackets. And once the long Spal brackets are installed, they need to be trimmed to length with a jigsaw or a Dremel tool. Below is a photo from another project (the Bavaria) of a Spal fan and its stock brackets mounted to the top and bottom of a condenser. Here, the fan is smaller than the condenser because there was a clearance issue at the top.

Another thing to consider is whether the fan will be employed only as a condenser fan or whether it also will perform double duty as an auxiliary fan for the radiator. Early 1970s-era BMWs like 2002s, E3s, and E9s had no aux cooling fan; if the car originally didn’t have air-conditioning, there was no electric fan. 1980s-era U.S-spec BMWs like E24s, E28s, and E30s all had air conditioning, and on those cars, I believe that the electric fan in front of the condenser was both a condenser fan and an aux cooling fan. A gray-market car like my Euro ’79 635CSi falls into an odd niche, as it did not have air-conditioning (the reason for my from-scratch retrofit), but it did, to my surprise, have an aux cooling fan.

The newer the car is, the greater the chance that the aux cooling fan is a multi-speed fan that, in addition to being turned on by a relay triggered by a temperature sensor, is being fed a speed signal by a fan-speed controller, in which case it’s more involved to replace it with an aftermarket fan.

Finally, when selecting a fan, keep in mind that the higher the CFM rating, the more current the fan will draw. The three that I’ve presented above draw 8.5, 19, and 22.5 amps respectively. We’ll get to the wiring issues in a later installment, but for now, the point is that you need to be mindful of the electrical limitations of vintage cars. The E24’s 65-amp alternator should be sufficient for any of these alternatives, but the original alternator on a round-taillight 2002 may only put out 40 amps. You can see that your selected fan may use up half of the headroom. So, sure, put a schnauzer-sucking fan on the condenser, but be aware of what the electrical ramifications are.

Which raises the question: How many CFM do you need for a condenser cooling fan? Instead of sucking a schnauzer, how about a Chihuahua? More is always better, but there is some guidance offered on the Spal website for the amount of CFM needed when a fan is employed as a primary radiator-cooling fan. They recommend minimum ratings of about 1,250 CFM for a 200 to 250-hp motor, about 2,000 CFM for 400 to 425-hp motors, and 2,500 CFM for more powerful motors. Granted, these are radiator-cooling numbers, not condenser-cooling numbers, but they provide some scale for the CFM numbers needed to dissipate certain volumes of heat.

Taking all of that into account, and trading off airflow, weight, cost, and ease of installation, I ordered the Spal 30100401 sixteen-inch low-profile fan that weighs 3.2 pounds, moves 1,298 CFM of air, wasn’t reviewed as sounding like a passing helicopter, and cost about $78. I test-fit it in the nose of the shark with the condenser behind it, made sure there were no clearance issues at the top or bottom, took everything out, then mounted the fan to the condenser with the plastic zip-tie kit. I then had a small surprise as the assembled condenser and fan nearly wouldn’t fit between the mechanical fan on the water pump and the sheet metal on the nose of the car; I eventually got them in with some careful positioning. Not a big deal; worst case would’ve been that I simply needed to temporarily remove the mechanical fan.

Once the condenser and fan were in the nose and all brackets were threaded with bolts and Nylock nuts, I snugged it all down, taking care to tighten the fasteners in stages so that the brackets could seek their best alignment and not twist the condenser.

So, after way too much work, the condenser and fan are installed and snugged down.

It looks great, but I’ll admit that I’m having second thoughts about my choices. When I start second-guessing things like this, I go back and look at my logic:

  • I probably could’ve moved up an inch in condenser width, from nineteen inches to twenty. One of the reasons I didn’t was that nearly all of the posts I’d read that said that even nineteen inches is tight, but they all showed the use of standard 90-degree fittings on the sides. I don’t think a lot of people know the trick of using short-drop fittings that allow for more clearance. Then again, going from nineteen to twenty inches, we’re only talking about a 5% increase in the total condenser surface area, and that’s unlikely to be a defining factor in the a/c performance. Yeah, it’s fine. Second thoughts largely banished.
  • When I ordered the low-profile 1,298-CFM fan, I hadn’t even removed the radiator yet, and I didn’t know that the car already had a stock (though dead) auxiliary cooling fan. Knowing that, I’m now thinking that I probably should’ve gone with one of the more powerful high-performance 2,000-CFM-ish fans, since the fan should act as both a condenser fan and an aux cooling fan. I’d perhaps feel better about my selection if I knew the CFM rating of the original fan and how close the new fan’s CFM rating was to it, but I can’t find this spec anywhere. Plus, I’m troubled by the fact that the original aux cooling fan was directly in front of the radiator, whereas the condenser fan I’ve installed is offset from the radiator by several inches plus the width of the condenser itself. On the one hand, the E24 is a lightly-driven car, and I’ll most want the a/c on long road trips where I’m unlikely to get snagged up in traffic, but the additional $60 in cost for a fan with 50% more airflow than the low-profile fan (or the even-lower $20 additional cost for the loud straight-bladed version) wouldn’t have killed me. The high-performance fan is heavier than the low-profile model, but I could’ve mounted it to the condenser’s side brackets. And the E24’s alternator should be able to handle the nearly twenty-amp load. Danger, Will Robinson; second thoughts persisting.

It’ll be quite a while before I complete the a/c retrofit and charge the system up with refrigerant, so I still have ample time to revisit and correct my decision, if that’s what I chose to do. Or I can just plan to drive the thing and see if the a/c bogs down and the coolant temperature goes up unacceptably when the car is idling in traffic. It’ll all probably be fine.

Next week: the Evaporator.—Rob Siegel

Rob’s new book, Ran When Parked: How I Resurrected a Decade-Dead 1972 BMW 2002tii and Road-Tripped it a Thousand Miles Back Home, and How You Can, Too, is now available on Amazon. Or you can order personally inscribed copies through Rob’s website: