HVAC

[BlownMGB-V8]

Evaporator icing is the most serious issue with LBC air conditioning systems. This comes from too much cold and not enough air so the water that condenses out freezes instead of draining. This happens mostly because of limited space for the evaporator and fan. It's a matter of physical space and volume. Unfortunately the mini units aren't much larger than the old under-dash units in that area and the ones that are require the firewall to be partially cut out to make space. In short, the car wasn't designed for it. So you have to decide just how badly you want it and in many cases, minimal AC is no better than no AC. And of course once you've done the install there's a natural human tendency to say it's wonderful even if it isn't.

But it is possible to make room for a larger evaporator and several creative approaches have been tried. You CAN roll your own and 4 Seasons sells a pretty good variety of cores. Summit is a good source for generic condensers. It takes a little imagination to build the air handler box but it's construction isn't difficult as it's mostly hidden.

You will hear that the components have to be perfectly matched for it to work at all. Not true. You can so badly mis-match them that the system won't work well, the most obvious case being an iced up evaporator... oh wait that's our problem isn't it. Or that the output is low... huh, sounds familiar. Here's a little non-secret, the more heat you can remove the better so only physical limits constrain your condenser size. If it were possible to make that large enough you might slow down the initial blast of cold air a little until the condenser charged up but that still wouldn't take long. Size of the compressor? Well if it's too small the system capacity will be low and it also could have an icing problem or if it's too big the compressor will cycle more than it normally would so it might eventually wear out the clutch but that wouldn't affect cold air discharge in the least. And we know that a generic 6 rib Sanden works just fine for a wide variety of cars. Therefore the only thing keeping us from building our own system is the evaporator box itself.

And clearly we want to make that box just as large as we possibly can for best performance. This is because the freon discharges in a mixed state at the inlet and flows down the tubes evaporating as it goes so with enough freon volume the resulting cold will be spread across the entire evaporator before all the liquid turns to vapor. Ideally you would get some but very minimal spitting of liquid freon at the evaporator exit port. If that doesn't happen you get a cold spot at the evaporator inlet and ice begins to build there and spread across. The goal is to spread out the cold so that the tubes themselves stay below the frost point. It's a rather tricky balance but a larger evaporator means that the cold can spread to a larger area and lower the average temperature to avoid the frost problem. Increasing the airflow also helps by carrying away the cold faster. That's an inverted way of looking at it since what we're actually doing is removing heat but a useful construct I think for purposes of visualization. We can assume that our design will have compromises since most of us are not experts in fluid dynamics and a short study on that topic can have great benefits. I expect u-tube has some useful videos but the main issue for airflow is sharp corners and restrictive passages, particularly for the core inflow and outflow, the defroster ports, the vent ports, and the fan inlet and outlet. If those areas can be made generous it will help as will rounded corners and elimination of eddies or stagnant air.

So what about space? The options are the area behind the dash, the central area where the existing system is, the passenger's footwell, and the shelf above it in the engine compartment. Some of that space has to be used for other things which will narrow the design choices. If you start with a clean slate it would be much easier to design a large capacity system. So for a clean build some consideration can be given to moving other components to remote locations and there is much that can be done along those lines. For instance, can the main power distribution panel be in the trunk? There will be more wires running fore and aft but it should be possible. The dashboard can be moved to the rear a bit. In my '71 I moved it back 2-1/2" and if I was doing it again I'd go for 3". To do that I bought some 3/4" thick black nylon plate, cut it out on the bandsaw and beveled the edges on the router table. It has the advantage of making the attachment screws accessible from the top and gave me room for an evaporator out of a VW Vanagon which is practically full width and something like 3x4" in cross section. I think it would be tough to get more volume in the evaporator. However the rest of the box was a challenge and you don't want to compromise here any more than you absolutely have to as it will restrict airflow. For instance the defroster vents had to feed directly out of the top of the unit and there was little room for control valves. The ones I ended up making have less flow area than is optimal, meaning I have to manually close all of the other vents and crank up the fan to get good airflow at the windshield so this is one area where you should try to overdo it. I find that eyeball vents are very useful in the dashboard and the early flapper footwell vents can be remotely controlled with a wire running up to the dashboard. I also had to relocate the wiper motor to the driver's side to get room for the freon lines.

The existing airflow passage through the firewall can be enlarged and modified to suit and the big rubber plug can also be used as well as the unused hole for the pedal box as was done in the early days by Jim Stuart. It makes a convenient way to get the fan out of the way provided it can be well integrated. Also, the cowl vent door can be used. Let's say you use one of those cowl magnets to help with winter heat and btw, if the clips are removed the cowl trim can be lifted to place that underneath and the trim will sit there very nicely and securely as long as you don't turn the car upside down. When not used the magnet can be stored on the underside of the trunk lid. But let's say you want AC recirc. One option is to block the cowl vent and open the vent door. The fan will then draw air from the cabin through the cowl vent door and recirculate it back through the system. since most of us set up our environmental controls and then change them the least we can get by with, that should work fine. Of course this is with the fan in the original location.

It hardly makes sense to go to all this trouble and not improve the airflow volume and pressure, and for that matter the heater output as well and fortunately that's actually easier than the inside airbox. There are a wide variety of fan choices available once you start basically from scratch since the hole for the fan inlet can be enlarged an inch or so before you get into the discharge side of the sheet metal and even then a little creative welding gives even more room. More compact, powerful and faster motors are available as well and the standard squirrel cages are good for around 6K rpm. They generally run below 3K and above that they do start to whine but can blow a lot more air by simply being run faster. I have found RC airplane motors to be useful here, though they do draw a lot of juice. One caveat, if you use the common "test" board to control them try to get one without the start safety feature which requires the knob to be at zero when power is applied. Aside from that it is a good 3 phase system but needs to be heavily fused. Go oversized on the PWM converter and then oversized again on the breaker.

For airflow into the evaporator the existing passages through the firewall can be used although it is probably going to be helpful to enlarge them by removing the defroster panel and enlarging the opening as much as possible. Scott made his inside box so that it mated directly with the heater box outlet for instance, a nice touch that maintained flow areas about as well as possible I think, while eliminating eddy areas. My approach was basically similar but used the entire area behind the console as an air duct, so not as efficient a design and I may redo that later. However I'm still using my footwell vents and I'm not sure Scott is. Regardless a simple opening in that area along with a panel that mimics the defroster panel might do the trick if the flapper doors are fitted.

That mostly leaves the heater box. Again, lots of options to improvise especially if you can consider a dual fan design, one for heat and one for AC. I chose to build an enlarged version of the original box using a core for a 60/70's Chevy C10 non AC pickup truck. I have room behind the engine to do that and it was the obvious solution for me.

Now, I have yet to test the AC system as the new compressor for it is mounted on the engine that goes in the car this spring, but the lines and everything else is in place to hook that up and charge the system. I am using an oversized compressor but since the rest of the system is also oversized I expect it to work out well. I expect to report on that once warm weather gets here. In the meantime, the heater system works amazingly well. I get usable heat within a mile or two and can saturate the cockpit with heat to the point of full comfort within a few miles. I'm talking toasty toes and warm fingers in the coldest weather with the fan turned down and the water flow reduced. I don't think I can ask for better than that, Proper management of the vents even gets rid of the inevitable drafts from the soft top. I'm hopeful this summer of having an arctic blast with the top down.

I'll post a link to my earlier build thread here:

https://www.britishv8.org/Search-Results.htm?q=ac&search_selection=board&sd_brd=8&search2=Search&advanced=0&tab=forum#gsc.tab=0&gsc.q=ac&gsc.page=1

Post #3 below has info on my heater:

https://www.britishv8.org/Search-Results.htm?q=heat&tab=forum#gsc.tab=0&gsc.q=heat&gsc.page=1
And to Scott's build thread here:

https://www.britishv8.org/smf20/index.php/topic,75760.0.html

Also Jim Stuart's 300 GT with the fan mounted to the shelf:

https://www.britishv8.org/MG/JimStuart3.htm

These should be helpful to anybody interested in rolling their own system.

Jim

[BlownMGB-V8]

Here is a good link for AC work. We've gotten down into the gritty details and I think we've done a pretty good job of making it understandable to the average mechanic, which when you talk about AC you have to admit is something of an accomplishment. It's an enabling work of relatively high standards. The general MGB/MGB-GT forum is a totally appropriate place for such a discussion, may our two favorite sites continue to support and enhance each other!

Jim