Jim Stuart Explains Installation of Air Conditioning in MGB Sports Cars
Jim Stuart (green shirt) explains installation of air conditioning in MGB sports cars

British V8 2006 Tech Session:   Air Conditioning Installation in MGB's

as published in British V8 Newsletter, Volume XIV Issue 2, August 2006

presented by: Jim Stuart
coverage by: Curtis Jacobson

Jim Stuart has a lot of experience with MGB V8 conversions. Boy, now there's an understatement! He's built four MGB conversions for himself, plus two for friends, and Jim's cars see very tough service because he uses them to commute daily into the District of Columbia. DC is easily one of loveliest cities in the world - but it gets REALLY HOT AND HUMID in the summer, and its traffic is a bear. I met Jim for the first time on a sweltering hot July day in 1998 when the V8 meet was in nearby Annapolis, and I realized immediately how generous and kind Jim is about sharing his knowledge and experience.
Jim's tech session was our fourth on Monday afternoon, and for it we moved out of the classroom and down to the parking lot. It was good to stretch our legs, and it was great to be able to look over Jim's car as he presented the components one-by-one.

In his preliminary remarks, Jim noted that there are at least four competitive companies in the aftermarket a/c kit business, and he doesn't have a strong preference between them. "Vintage Air" and "HotRod Air" are the two companies whose products Jim has used, and he recommends them both. They sell very similar products, and in many cases use the exact same major components.

Jim started with the compressor first. Jim uses and recommends Sanden's model "508". The 508 designation denotes that it has five pistons and a displacement of 8.4 cubic inches. Sanden air conditioning compressors are cylindrical, aluminum-bodied and mount like an alternator, so it may not be obvious how they're constructed: the five pistons are all parallel, and they're actuated by a wobble plate instead of a crankshaft. One characteristic of Sanden compressors is that the pistons have a very short stroke and low displacement per cycle. This design characteristically requires very low horsepower (especially at low speeds, compared to competitive compressors) and it's almost vibration free. Jim also noted that the compressor can be purchased with a V-belt or serpentine-belt drive, and it can be purchased in a polished or unpolished version.

(Editor's note: I'm going to insert a quasi-testimonial here. In my previous career as an engineer for Volvo Truck I overheard a lot about Sanden compressors because an engineer down the row from me was responsible for the decision to change to Sanden. Decisions like that aren't taken lightly by OEMs; the testing program was lengthy. The changed proved a huge success because compressor-related warranty claims seemed to drop immediately as the Sanden compressors went into production. I believe Sanden has become the most commonly used compressor on North American heavy trucks. Trucks use a larger model, of course.)

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From the compressor we followed the hoses. Jim made special note of them because hoses are very important. Two different types of hose fittings are available: "barb" fittings or "beadlock" fittings. Until a few years ago barb fittings were the industry standard, but now the OEM's have switched to beadlock so they're the new standard. You can buy either, and Jim has used both, but he definitely recommends the newer-style beadlock fittings. "They're so much easier!"

Whichever fitting type you use, you need a special crimping tool to secure the hose ends. Don't even think about using a crimper that's made for hydraulic fittings because it's very easy to over-crimp the hose-ends and split the barrier lining of the hose. (What barrier lining? Modern A/C hoses have a lining that keeps the refrigerant from leaking out. R134a refrigerant molecules are smaller than old, obsolete, ozone-depleting R12 refrigerant molecules so one must be even more vigilant to avoid leakage.)

A/C hose comes in three sizes. The smallest, #6 hose (5/16" inside diameter), is used for the liquid lines that travel from the condenser to drier to evaporator. Number 8 hose (13/32" ID) is used from the discharge line that travels from the compressor to the condenser. Number 10 hose (1/2" ID) is used for the suction line that travels from the evaporator to the compressor.

Jim Stuart's Air Conditioning System

Along the pressure line from the compressor Jim pointed out the "trinary" switch. This is a very, very important part of the system because it protects the compressor from damage. You need this, and it must function properly. The trinary switch monitors pressure on the high pressure line from the compressor. If the pressure goes too high it opens the compressor clutch's electrical control circuit until pressure returns to normal. It also turns on an electric fan on the condenser to bring the pressure down. Its third function is to cut off the compressor in the event of a refrigerant leak, which is indicated by a too-low line pressure. (Note: some people elect to fit a "binary" switch in lieu of a trinary switch. A binary switch doesn't turn on a condenser fan, so the system just waits for pressure to come down whenever.)

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The next part of the system Jim discussed was the evaporator. The evaporator assembly is nominally available in either "in dash" or "under dash" models, but these terms may be a little confusing in MGB application. Jim explained: "The units I have installed are designed for behind the dash (i.e. in dash), but the MG's do not have enough room behind the dash, hence I had to cut through the double fire wall to install the evaporator, barely having room for ducts behind the dash."

Under dash evaporator installations can potentially be much easier and more efficient, if not as discrete, but they do require maintaining the original heater/defrost unit. Jim indicated that this is what he'll probably try for his next installation.

Jim focused on two aspects of evaporator installation. The evaporator assembly's outlets must be ducted properly to louvers or vents. Air flow through ducts is a black art, but as a first rule of thumb duct walls should be smooth. (Flexible hose ducting really robs performance.) Ideally the ducts should be sculpted to maximize flow-rate and optimize cooling.

A second area of concern is insulation and sealing. Engine compartment and exhaust heat is obviously counter-productive to cooling. Insulation of the firewall and footbox area is critical, but so is the upper firewall. Sealing around anything that breaches the firewall (including evaporator lines, etc.) is critically important.

Regrettably I had to excuse myself at that point, so I missed the conclusion of the tech session.

Disclaimer: This page was researched and written by Curtis Jacobson. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.

Photos by Curtis Jacobson. All rights reserved.

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