DIY - Aftermarket Evaporator Installed In A Coupe

Hi Team. I set out this year to find a replacement evaporator for the AC systems in our E9s and E3s (and E12s and E21s).

I found one (originally made for a 2007 Land Cruiser). It fits in the original box pretty well and it’s in my car working now. I’ll show you how I did it and then update you at some point in the near future as to whether there’s any advantage to it over the original parts. This is a work in progress and you are joining me in the experiment!

I want to emphasize that many members on this board have made their AC systems work pretty well using the original BMW parts and that if you are in need of better cooling in your car start with the basics by making sure your system is charged with the correct amount of oil and refrigerant; making sure that you’ve cleaned the bees out of your condenser; inspecting your switches and wires; checking if the seals are still there on the little flappy doors inside your heater box; and seeing to other correctable things.

On the outset, I wanted to find an evaporator upgrade method made from cheap, commercially available, and reliable parts that could be shared as a DIY guide that anyone could follow. How’d this turn out?

Is it an upgrade?: TBD
Is it cheap?: Pretty affordable I guess. See the spreadsheet below.
Is it reliable?: I already feel better knowing that the connections have o-rings, not copper washers.
Are the parts commercially available: Most yes, some no.
Can anyone do this?: Yes, except for a few parts which are hard to fabricate. Read on.

Here is the stock evaporator box with its big tube-and-fin evaporator.

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Here’s the old evaporator and associated plumbing.

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Here are the new evaporator, thermal expansion valve (“TXV”), and associated plumbing:

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I modeled all of these parts to make selecting an evaporator from the Internet easier. You can see here that this new evaporator isn’t quite as tall as the old one but it fits in the box snugly against the blower cavity despite not being quite as tall as the old one.

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Below are all the parts needed for this conversion as well as a source for them (in the spreadsheet). This assumes you have an original AC box with a working blower assembly and that you’re willing to modify the box.

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Here is the kit assembled and in the box (top view):

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Thoughts and reflections:

I’m pleased! It’s been cloudy but I got a sunny day today finally. I got a chance to run the system starting with an interior temperature of 107º. I was measuring temperature drops (measurement of air going in minus air coming back out) of as much as 35º as the cabin cooled down. I’m going to keep tweaking some things to see if I can get more cooling.

I didn’t make any modifications to the blower even though it is probably the biggest limitation in this system. I’ve determined that the blower (with whichever evaporator is in the box) makes about 190 cubic feet per minute of airflow. If I’d found a way to switch the blower and the evaporator at the same time in this experiment there’d be know way to know which made the bigger difference.

I wanted this to be made completely from parts anyone could source. The 3D printed stuff I’m kind of excusing here since there are probably dozens of ways to secure an evaporator in a plastic box and 3D printing was just the easy way for me. Also, I can share those files with anyone who wants them.

As far as the fit of the new system, it was kind of a bummer that the TXV interfered with the back of the box since it was almost a perfect match. Have a look at this cross section:

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View from the bottom of the valve and the box cutout:

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A window shaped like this needs to be cut out from the box. Aside from that, the only other modifications to the box is a few screw holes.

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The blue plate (part D4 on the illustration) is a shell that makes a little more clearance around the TXV (particularly at the bottom) and also collects dripping condensate and returns it to the bottom of the box using gravity. I have it installed pressed against a foam seal and I’ll be checking it periodically this summer to make sure it doesn’t leak. It could also be glued in with silicone.

The real missing link I couldn’t find on any website was an adaptor from that particular TXV to an O-ring-stye connector and a clamp to squeeze them on. I’m referring to parts C3, C4, and F on the diagram. I made the adaptors on my little lathe and brazed them to tubing.

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This was my first time brazing. It seems to have worked well but I know this option isn’t easily available to everyone at home.

Presumably I or someone more experienced than me could make this assembly into a kit if the DIY route is too confusing. Also, I took all the pictures needed for a full "how-to" guide. But I think anyone with enough ability to take their box out of their car and / or charge up an A/C system should be able to assemble this.

I have enough parts for another prototype if someone with time on their hands wants to reimburse me for just the parts and postage. All I’d ask is that you’d be able to compare this system with the previous one in your car and report back for everyone’s benefit.

Anyway, it can now be said that someone has used a non-original evaporator in an E9. I’ll post a critique of the function soon. So far so good. Thanks for following along!
 
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teahead

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Wow! That must be one of the best descriptive how-tos ever!

Congrats!

This thread is for maybe improving the fan blower:

 

teahead

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You can always create a google drive folder and share it out to everyone.

The metal fittings aforementioned that required lathe work however...that will be something maybe you can offer from you directly.
 

Blinkling

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WOW!! Most impressive.
I hereby nominate @Blinkling's A/C Evaporator Upgrade as the G.O.A.T. of the E9 forum's Wall of Fame posts.

~cheers to you for ALL of this work!
You're way to kind! Thanks.

Maybe for "F", this could work?



or:

Good eye and good eye! Those are both extremely close but not quite. I actually bought the first one to try but the two openings need to be 40 mm apart. The second one might but it has one centered hole for the fasteners instead of the pair.

Here is the closest I came: tubes and bracket from a Prius:

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The tubes aren't of much use (aside from allowing me to copy the design of the terminals) but if that bracket was offered by itself (not a kit) it would be be the perfect "Part F".

Here's an unexplored route that also almost solves the problem: Toyota Part 88710-60850


It has tubes and the brackets and should fit this TXV just perfectly. You'd flip the evaporator 180º and install it in the original box the other way. The tubes would cross the box instead of making a U-turn. Problem is, the tubes are probably too short. Maybe it's just a matter of cutting and brazing a little extra length. This is what I'd explore first if I was starting over.

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Almost, right? That's a screen grab from this auction:

 

Stevehose

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Does the Land Cruiser evap coil have more surface area or is a parallel design, what makes it more efficient than the stock one given the similarity in size?
 

Blinkling

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Thanks, fellas!

Does the Land Cruiser evap coil have more surface area or is a parallel design, what makes it more efficient than the stock one given the similarity in size?

I can't answer this definitively. Talking with my mechanical engineer brother I got the impression that the best chance for heat transfer happens when there is liquid refrigerant on one side of a metal membrane and moving air on the other. In looking at the parallel flow design of all the new evaporators (like this one) it seems like there's just a lot more hollow, refrigerant-filled structure per cubic inch than on the old ones which were just a snake of round tubes. The old design seems like it had to rely on an awful lot of thermal conduction along thin metal plates.

So, yes, all this is premised on the new design being quite a bit more efficient than the old design but I think the only way to know for sure would be to park two coupes in the sun and see which one cools itself down faster.
 

Blinkling

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Hi again. Here are a few resources to finish this up as a proper DIY write-up. Here is a directory where you can download the STL files for the 3D printed parts:


Here are the dimensions for the custom machined parts. Everything else should be available on order.

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Impressions on usage: It's working well! I'm about to add a little module that will turn on the front fan (which is the little stock thing) when the high side temperature (after the condenser) gets to 130ºF. I've recorded as much as a 50º degree temperature drop between return air and supply air out of the front duct. Here is one of my readings idling in the driveway:

Engine at ~1400 RPM
Refrigerant: R152a
Car is in full sunlight;

38% Relative humidity;
92ºF Cabin ambient air temperature;
46ºF Chilled air from front vent;
48ºF Temperature of suction line (back to the compressor);
26 PSI Low side pressure;
73ºF Outdoor ambient temperature;
140ºF High side line leaving compressor;
90ºF High side line leaving condenser;
130 PSI High side pressure;

I'll continue to measure as I'm out running errands. It's been overcast here so I'll find some more sunshine.
 

jjs2800cs

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Where does one get R152a. How does it come, and why did you use it, etc?

jjs2800cs
 

Ohmess

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Dave - thanks for sharing this. Two questions:

Why did you choose not to replace the fan in front of the condenser with a SPAL pusher (I used SPAL part number 30102056). This is relatively cheap and makes the condensor more efficient.

Also, have you done the heater core bypass so as to avoid running hot coolant through the interior in the summertime. https://e9coupe.com/forum/threads/heater-bypass-valve-diy-in-progress.10824/
 

Blinkling

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Where does one get R152a. How does it come, and why did you use it, etc?

jjs2800cs

It's a very common commercial / industrial refrigerant.

It's also the main (and only) ingredient in “canned air” or “keyboard dusters." It’s literally being used to remove bread crumbs from space bars all over the world right now. So most people who use it in their cars get about three cans of keyboard duster and a side-tap refrigerant can adaptor and charge their system that way. R152a is difluoroethane so you’d make sure the can says “contains difluoroethane” on it. Sometimes they even just say R152a on them.

As to the reason: it’s cheaper, it’s not harmful to the ozone layer like R134a and R12 are, it’s somewhere between R12 and R134a in efficiency, which is super nice. Lastly, my state has a cash-on-return charge for the stupid R134a cans at the auto part store so perhaps convenience (and spite?) is a factor as well.

Now, R152a is comparatively more flammable than R134a so please don't interpret my use of it as a recommendation from me. I just read posts on other forums and made my decision accordingly. I’ll note however that refrigerant 1234yf has similar flammability properties and it’s being used in new cars now. Here are a couple of official sources that you can look at before you decide to switch:



As to my personal risk assessment: driving a car partly comprised of bowls full of gasoline seems to overshadow flammability concerns of the other liquids circulating in the car. R152a isn't a fuel, after all. And I believe that a lot of the hoopla concerns the byproducts made when the refrigerant burns, not whether it's likely to burn in anything but the most unique circumstances. But I also made most of my high side refrigerant lines out of copper tubing and located my high pressure blow-off valve up at the very front of the car, away from hot parts and ignition sources. I consider this overkill.

Why did you choose not to replace the fan in front of the condenser with a SPAL pusher (I used SPAL part number 30102056). This is relatively cheap and makes the condensor more efficient.

https://e9coupe.com/forum/threads/heater-bypass-valve-diy-in-progress.10824/

You’re right that this is a great idea. I didn’t do it when I put the condenser in back in 2014 and I didn’t feel like taking the condenser and radiator out this time. You weren't able to get the SPAL in through one of the front grill openings, were you? I’m saving my dimes and nickels though and I’ll make this upgrade for sure next time I have a reason to take the radiator out. Good call!

Also, have you done the heater core bypass so as to avoid running hot coolant through the interior in the summertime. https://e9coupe.com/forum/threads/heater-bypass-valve-diy-in-progress.10824/

Also not yet but I did replace all the foam strips on the doors in the heater box a few years ago so my heat intrusion is pretty minimal, at least from my heater. I did grab a couple of electric coolant valves out of (I think) an E39 5er in a junk yard a few years ago so I may attempt that soon.

This is massively impressive.

But mostly nutty and fun. :)
 

Blinkling

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Also, have you done the heater core bypass so as to avoid running hot coolant through the interior in the summertime. https://e9coupe.com/forum/threads/heater-bypass-valve-diy-in-progress.10824/

I actually wanted to run another observation by the group here. It's crazy talk but now's the time.

The two real major drawbacks to the AC in these cars is that all the air comes out in a single place and that there isn't a very big volume of it.

During reassembly I got reacquainted with two doors in the heater box that serve different functions: one is the "Air" door (controlled by the lower left lever on the heater control panel), and the other is the "Vent" door (controlled by the lower left lever on the heater control panel). Theoretically, if the Air door is closed and the Vent door is open, some of the conditioned air is going into the heater box instead of out the normal way, right? But also none of it is being lost to the outside through the heater fan? Observationally, when I have the vent lever open when the AC is running I notice less air coming out onto my shifting hand.

Ok, so here's the crazy idea: IF the blower in this car were powerful enough AND vent door was left in the open position AND the heater core had been bypassed via one of the excellent methods explored on this board (@thehackmechanic, @Ohmess, @Stevehose).....

....then wouldn't cold air come out of the defroster vents and floor vents as well? Seems like that would make a bit of a dent in the air distribution issue.
 
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