Adding AC into my MGB

Scott Costanzo · November 28, 2025, 09:24:16 PM

I thought I'd share my current winter project, which is adding AC to my car. I've actually been working on this for a little over a year now, mostly planning and gathering parts. I will state in this first post that I think very few of you would take an approach like I am when there are "easier" ones out there. I have goals that aren't easy to reach with the available off the shelf units without spending a lot, which I don't like to do.

I won't go into all of those details now. I'll touch on them as the build proceeds. So the video below shows a proof of concept I built to verify the MGB fan (with a Fiero fan motor) can supply enough air for the HVAC system. I used cardboard and included the evaporator as well. It's rough but I was pleased enough to proceed with this phase of the build. I will update this thread as progress is made.

waterbucket · November 30, 2025, 02:00:21 PM

Scott, I am also trying to improve the heater and add air con to my conversion, due to it being a relatively rare engine swap I am doing as much as possible before the engine finally goes in. I think the factory was more interested in getting warm air to the footwells that the future installation of aircon. apart from the two openings that let air into the footwells there are just two small holes (11/2"-13/4"?) to let air into the rest of the cabin. I have enlarged those two holes into one single hole about 10" x 4". I have found a Land Rover Discovery heater matrix that fits in the gap between the engine and passenger sides of the bulkhead. Not really sure if the original matrix restricted the flow but this one certainly doesn't. I will add photos when I have figured how to. 

Scott Costanzo · November 30, 2025, 02:59:29 PM

Quote from: waterbucket on November 30, 2025, 02:00:21 PMScott, I am also trying to improve the heater and air con to my conversion, due to it being a relatively rare engine swap I am doing as much as possible before the engine finally goes in. I think the factory was more interested in getting warm air to the footwells that the future installation of aircon. apart from the two openings that let air into the footwells there are just two small holes (11/2"-13/4"?) to let air into the rest of the cabin. I have enlarged those two holes into one single hole about 10" x 4". I have found a Land Rover Discovery heater matrix that fits in the gap between the engine and passenger sides of the bulkhead. Not really sure if the original matrix restricted the flow but this one certainly doesn't. I will add photos when I have figured how to.

Hi Philip,

Yes, the original heater fan housing was somewhat restrictive. I'm also doing unusual things too. There isn't much room in an MGB. I'm removing the flap thing that directs the air flow to the floor/windshield and running the air out of the bottom of the fan housing to the evaporator housing. It's hard to explain so this picture may help.

There are a lot of details that will become clear as the project proceeds. The possibility of a spectacular failure exists as well!!

BlownMGB-V8 · December 01, 2025, 05:39:25 PM

Lots you can do. That Disco core sounds interesting. Do you have a picture of it by chance?

With the flap gone something is needed for defrost and how much really depends on airflow. That can get tricky, probably best to overdo the air to the windshield if you can.

Jim

Scott Costanzo · December 01, 2025, 06:44:19 PM

Quote from: BlownMGB-V8 on December 01, 2025, 05:39:25 PMWith the flap gone something is needed for defrost and how much really depends on airflow. That can get tricky, probably best to overdo the air to the windshield if you can.

Jim

Jim, I'm absolutely maintaining defrost capability. You never know when you're going to need it with the way we use our cars.

Scott

waterbucket · December 02, 2025, 03:27:37 PM

I can easily take a photo but I have as yet not managed to upload one. I have resized them to the old standard and dragged them onto my reply where it stays for about two seconds before disappearing back from whence it came. The solution is probably obvious to anyone bar me. 

BlownMGB-V8 · December 02, 2025, 10:36:25 PM

Carl can help you with that I imagine, but are you using the quick reply or the reply button? The full window has a big full width button below to attach files.

Jim

MGBV8 · December 03, 2025, 10:32:51 AM

Quote from: waterbucket on December 02, 2025, 03:27:37 PMI can easily take a photo but I have as yet not managed to upload one. I have resized them to the old standard and dragged them onto my reply where it stays for about two seconds before disappearing back from whence it came.

Philip,

As Jim B suggested, use the regular "REPLY" button not Quick Reply. Then click on the "Click or drag files here to attach them." bar. That will allow you to choose a pic from your computer/device.

On a mobile device, you may have to tap "User Actions" to access the regular reply button.

For now, it will still be a clickable thumbnail at eh bottom of the post

Scott Costanzo · December 03, 2025, 04:33:14 PM

Everyone, I'm assisting Philip with his attachment issues. I was going to edit one of his posts with this but decided a new post would be better. The following are Philip's words. Philip, correct me if any of this is not correct:

The one of the hole in the bulkhead is taken from the brace that goes from the top of the tunnel to the cross tube that is at the bottom of the dashboard. The hole in the foreground is for the evaporator drain, beyond that is the hole for the evaporator to sit against and beyond that is where I enlarged the two small circular holes to increase airflow.

The Discovery matrix fits in between. The general mess around the area is the paint polishing compound that got everywhere and was resistant to pressure washing.

Scott Costanzo · December 03, 2025, 04:56:15 PM

I'll add more later but believe it or not, Philip and I are following very similar paths. My next step, once I finish modifying the fan housing, which is taking much longer than expected, is to do the bulkhead modification Philip has done. Very strange. This should be very interesting!

Scott

BlownMGB-V8 · December 04, 2025, 10:32:42 PM

Not that much different from what I did really, same sort of enlargement of the air holes and such. I did manage to retain the cowl vent flap which reduced the area available for air to move up to the evaporator which I placed just above the square tubing brace. Your evap is wider but considerably thinner, I'm looking forward to seeing how you guys position that. Handling the condensate was an interesting challenge, collecting that while getting the air around the collection tray. I expect it could have been done better than what I ended up with.

Jim

waterbucket · December 05, 2025, 03:38:14 PM

Quote from: MGBV8 on December 03, 2025, 10:32:51 AM
Quote from: waterbucket on December 02, 2025, 03:27:37 PMI can easily take a photo but I have as yet not managed to upload one. I have resized them to the old standard and dragged them onto my reply where it stays for about two seconds before disappearing back from whence it came.

Philip,

As Jim B suggested, use the regular "REPLY" button not Quick Reply. Then click on the "Click or drag files here to attach them." bar. That will allow you to choose a pic from your computer/device.

On a mobile device, you may have to tap "User Actions" to access the regular reply button.

For now, it will still be a clickable thumbnail at eh bottom of the post

Carl, I have got it sorted now. As Jim correctly said it was a simple case of not using the quick reply. I will start updating the build journal.
I think one major difference between Scott's AC and my attempt is that he is retaining his OE vents in the dashboard whereas I am putting a double DIN stereo there with car play so that I can have an easy to read Google Maps. Being profoundly deaf I cannot rely on hearing the sat nav's verbal directions. We are also getting an increasing number of cameras that can detect any use of a mobile phone which is what I normally use to find my way around.

Scott Costanzo · December 05, 2025, 09:16:34 PM

Quote from: BlownMGB-V8 on December 04, 2025, 10:32:42 PMNot that much different from what I did really, same sort of enlargement of the air holes and such. I did manage to retain the cowl vent flap which reduced the area available for air to move up to the evaporator which I placed just above the square tubing brace. Your evap is wider but considerably thinner, I'm looking forward to seeing how you guys position that. Handling the condensate was an interesting challenge, collecting that while getting the air around the collection tray. I expect it could have been done better than what I ended up with.

Jim

Jim,

This is how I'm handling the condensate.

I took this from an HVAC box I got out of a Nissan Rogue. My evaporator will be living in this box.....well, not this box exactly. I'll be fabricating a box with HDPE using what you see below as a pattern. It will be sitting on the transmission tunnel between the radio console and the firewall.

BlownMGB-V8 · December 06, 2025, 08:31:27 AM

I'm a little surprised there is enough room for that, but we do have to make use of every possible inch. My tray being as large as it had to be sure complicated matters, running wide where yours goes tall. You probably have better packaging. When I bought my core I was really only thinking about maximizing capacity and giving little heed to drainage and airflow.

Jim

waterbucket · December 07, 2025, 04:01:59 AM

I won't add any more photos after these, I may start my own HVAC thread rather than distracting from this one. The first photo shows the Discovery heater matrix in the bulkhead gap it just squeezes past the steel tube mount for Toms knob.
The second photo shows the Discovery matrix in position, there are at least two variations of this matrix and only the shortest in height will slide in. You can also see the shelf for the evaporator/distribution box to sit on, I had to cut away that ridge underneath the bulkhead for gearbox clearance this shelf became a two in one solution. I also removed the brace from the cross rail tube down to the tunnel and replaced it with a new one which is angled more towards the back of the car, I can just squeeze in a a 8" cube that houses the Ford Fiesta evaporator and the outlet pipe to the vents.
The last photo shows the matrix itself, they are cheap to buy over here from Ebay.

Scott Costanzo · December 09, 2025, 03:26:21 PM

I've been modifying my fan housing in an attempt to make it more efficient. It worked pretty well with the setup I've been using for the past 8+ years and also with the mockup I showed in the first post above. I'm never satisfied it seems. Anyway, this first shot shows where I stand at the moment.

Nothing is welded in this shot, I'm still fabricating things but this gives you and idea of what my goal is. When I first reinstalled the heater in the car after the LS conversion, I went to a Fiero fan. In addition, I had to modify the housing because I modified the firewall which changed the size of the fan housing mount. I also had to place the fan so it didn't interfere with the engine head and intake...it's close. Anyway, all of this resulted in the fan being placed in a suboptimal position within the fan housing. Since I'm needing as much flow as possible, I decided to do what I could to make it the best I could. I have a HVAC box from a 2011 Nissan Rogue (long story) which happens to have a squirrel cage that is the same diameter as the Fiero squirrel cage. Here is a picture of the Rogue fan housing:

I decided to use it as a model to place the fan in the MG housing since it is likely much more efficient than what BMC did 60+ years ago. That required some modification to the fan housing itself which you can see in the next picture below:

You can see the cardboard template that I made to duplicate the dimensions of the Rogue fan housing. What it looks like I will end up with won't be perfect because of packaging issues (fitting 10 lbs. of poop in a 5 lb. bag

) but I'm pretty pleased with the way things are going. I can't wait to test it out. I'm guessing I have another couple of days to finish the fabrication.

Scott

BlownMGB-V8 · December 09, 2025, 04:50:34 PM

I applaud your efforts and hopefully that will do the trick. But if not, at least there is an option to use a high speed motor. The Fiero probably hits somewhere between 100 and 1200 rpm loaded. If you can double or triple that the airflow should go up significantly. A steel fan should be fine up to something close to 6k, (but test) plastic also probably.

Jim