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SL-C Manual: Heat and Air Conditioning


The SL-C has gone through several iterations of heat and air conditioning. The current system is from Vintage Air, but all are similar, varying mostly in the evaporator, and its placement. The system comprises an engine-mounted compressor, a condenser mounted in front of the radiator, an evaporator that is mounted inside the cabin and the necessary hoses, hose ends, controls and wiring.

Below is a simplified generic diagram that shows how air conditioning is implemented in most cars.

Installation Tips

Most of the parts placement is obvious, but there are a few tips, however:

  • The compressor mounting bracket is normally part of the front dress of the engine. If you don’t have a bracket to mount the compressor on the engine, Superlite Cars can supply one for the GM LS-series of engines available at extra cost. If you have another engine, most brackets designed for a Sanden 508-style compressors will work, but check the fitment carefully.
  • The standard compressor has a 6-rib pulley, and is designed to work with a regular GM serpentine belt. The compressor has a replaceable pulley for special applications if needed.
  • You should plan early for mounting the compressor and how it will work with the belt system you use. 
  • An alternative is to mount the compressor so that it can be driven from the inside set of grooves on the crank pulley. This requires a dedicated belt and one or more adjustable tensioners, as well as a custom bracket. The advantage to this mounting approach is that the compressor is moved back, and so allows more clearance for the water pump hose, and that a stuck compressor won’t affect the other accessories on the main belt. However, if you select this mounting option, be careful to ensure that you have adequate room for the AC lines, as the clearance between the compressor and the engine mounts can get tight.
  • The Corvette compressor, and some late GM AC compressors are variable speed, and not compatible with aftermarket AC systems, including the Vintage Air system for the SL-C. You must use the compressor in the kit, or a similar model unless you have validated that the compressor you have selected will work. If you use the Cadillac CTS-V front dress and the included compressor, everything fits (and the compressor is a single-speed one) but you will need to make custom hard lines from the compressor, using the (expensive) OE lines, cutting and welding them as needed. Any competent AC shop can do this.
  • The new radiators include one or more notches or other openings which are designed to allow the AC lines to pass by that area, making routing easier. If you have an older radiator with no holes or notches, run the lines above the radiator, and be sure to insulate them for maximum efficiency. If you have the new radiator type, you may find that running hard lines (as opposed to hoses) may make routing easier, especially if you choose to route both lines through the same opening in the radiator. If you run hoses, you will need to run one hose through each radiator opening as the openings are too small to run two hoses through just one.
  • Be sure you wire the trinary switch according to the instructions in the Vintage manual. 
  • The evaporator is designed to mount under the passenger foot well panel under the dash. Careful placement is essential. See the “Mounting the evaporator” section for more details.
  • Mount the AC condenser with a ½” to ¾” gap between the condenser and the radiator-- don’t put it right up against the engine radiator. This minimizes heat soak and greatly improves AC performance.
  • When mounting the radiator and condenser, make ducting to completely seal the airflow through these coolers. This will improve performance noticeably. Do not let air escape over the radiator- this area, as well as the bottom and sides of the radiator must be sealed so that all air coming into the front of the car passes through the radiator and the condenser. This is important for both AC performance, and for engine cooling.
  • The refrigerant lines need to be insulated for maximum efficiency. Be sure to cover and insulate not just the tubing and hoses, but also the fittings. Special AC foam tape or putty is available from Vintage and other automotive air conditioning sources for the fittings, and there are many sources for insulation for the lines.
  • You must accommodate condensate drainage from the evaporator. There are two drains that must be connected to rubber hoses that run outside the cabin. Failure to connect these drains will allow the evaporator to drip a significant amount of water into the cabin. These drains must be kept clean and clear of any obstructions that would cause the water to backup into the evaporator and then overflow into the interior.
  • And of course, follow the directions included in the kit from Vintage Air!

Careful placement of the refrigerant and heater hoses will minimize intrusion into the passenger foot well. Keep them as high as possible in the footwell.

The kit comes with air outlets for the dash. To install these in the dash, measure the OD of the outlet, and drill out the dash openings for the side outlets. Be sure you make the hole tight enough so that the outlet is a press fit into the dash. Hoses will attach to the back of the vents from the AC plenum. 
If you are planning to upholster the dash, take the thickness of the dash material into account when making the hole.

AC Refrigerant Lines

The refrigerant lines can be implemented using hard lines instead of flexible lines. These are lighter, cheaper and easier to insulate. The following pictures show how some builders have used and routed hard lines.

As with the brake lines, check for tire clearance over full lock, with full droop and compression of the suspension. You will normally have to remove the springs to verify clearance, so combining the checks with suspension and brake line installation can save some time.

The hoses in the picture above have not yet been insulated.

Modifying the Evaporator

Most problems with the AC system center on mounting the evaporator. It’s a little tricky, but following these instructions will make it easier. You may need to modify the evaporator to fit. If your evaporator has the #6 valve fitting angled pointing away from the evaporator, you’ll need to re-clock the fitting slightly to make clearance for the evaporator unit in the footwell.

Here’s a picture of an evaporator, looking at it from the rear. Note the valve and 90-degree fitting. This fitting needs to be re-clocked for clearance, as discussed below.

Also, note the attached rotary controller (with the blue wire bundle in the pic below) for controlling the cool function of the unit. In most cases, you will need to remove this from the evaporator case, and re-attach it later (possibly elsewhere, depending on your preference), in order to facilitate installation of the evaporator unit.

To begin, remove the sticky AC foam tape from the #6 90-degree fitting on the evaporator (shown at the lower left on the picture above) and loosen the fitting. Then rotate the fitting so the orientation of the fitting points horizontally towards the driver’s side. You need to move it almost 180 degrees from its position shown in the picture above. 

Tighten the fitting, and reapply the sticky AC tape carefully, making sure the assembly is tight and well-sealed. If you are careful taking off the tape, it can often be re-used. Otherwise, apply new tape. Be careful with the small copper tube coming from the valve next to the fitting- don’t kink this and try to keep the same shape and layout as originally delivered.
This is what the fitting looks like with the tape removed:

When the fitting is correctly rotated, it looks like this:

Tighten the fitting, being careful not to lose or crush the green o-ring shown in the picture. Be sure to reinstall the tape and carefully re-align the small copper tube attached to the valve. Replace the black sealing cap to keep debris from the unit.

Mounting the evaporator

Now that you’ve modified the evaporator to fit in the passenger footwell, it’s time to mount the evaporator on the car.

The mounting brackets bolt onto the sides of the evaporator with the flanges oriented to the inside and back.     The evaporator won’t fit if you angle the brackets outside.
Make sure you have the evaporator unit in place, and attached to the chassis for the next step: drilling the holes in the evaporator case to allow the cooled or heated air to come through to the plenum.

You’ll need to drill holes in the evaporator case that match up with the holes pre-drilled in the upper footbox. Do that only when the evaporator is in its final position. 
You can actually drill through the existing holes in the chassis (see pic below) once you have mounted the evaporator in its final position.

Be careful not to drill any holes in the evaporator case until you know the drill can’t accidentally hit anything inside—finding a hole in the coil after everything is buttoned up will make for an unhappy day. There is plenty of room in the case if the unit is correctly position. Check for clearance anyway, before you drill.

Use ½” foam insulation or other sealing system between the evaporator outlet and the bottom of the footbox to seal the connection between the footbox and the evaporator. This is a critical sealing interface and must be as airtight as possible.

The hoses can be run through the footbox, but you can run the #8 hose down the outside part of the chassis. 

The use of hard lines instead of the hose makes routing easier, and looks better at the expense of install time and additional cost for the hard lines and fittings.

Mounting the AC plenum

The plenum is the large anchor-shaped fiberglass part that distributes air from the evaporator to the two side vents in the dash, and to the defroster vents in the dash. It is shown in the pic below. Note that the plenum will need to be trimmed to fit around the roll bar pads, and that it is designed to fit just over the horizontal 2X6 under the dash, as shown below.

Before you begin mounting the plenum, prepare the dashboard by carefully routing out the defroster vent openings with a small Dremel or similar tool.

Then, install the dashboard on the chassis and locate the plenum (you should have already located the dash position by mounting the steering column and centering the dash on it, then locating the end tabs and marking their position.)

Once the plenum is trimmed to clear the roll cage and the dash is installed over it, go over the newly-opened defroster duct openings with a marker, outlining the area on the plenum underneath that must be removed to direct air to the dash defroster vents.

Remove the dashboard and then cut out the openings in the plenum with a Dremel or similar tool, and using foam tape or other sealant, seal the interface between the back of the plenum to the dashboard defroster duct.

The hoses to the dash vents can be attached by gluing 1” tall plastic or metal rings to the plenum after drilling out an approximately 2” hole in the plenum. The short hoses from the vent to the 1” tall rings connect the vents to the plenum. Alternatively, just “screw” in the hose to a carefully cut hole in the plenum. In either case, seal the connections so no air can escape if the push-on connections don’t seem airtight.

The AC plenum also needs to be sealed to the chassis. Note that it is designed to clear the round openings on the chassis as in the picture below:

We suggest using foam tape and small screws to attach the plenum to the chassis. It is vital for good heat and AC performance that the plenum be completely sealed to the chassis. Leaks here will have a deleterious effect on the overall performance of the unit, especially in terms of airflow to the occupants.

The same is true for the plenum to dash interface where the defroster vents are located- this must be perfectly sealed to maximize performance.

Mounting the Compressor

Superlite offers optional compressor brackets for the popular LS-series engines such that the provided compressor from Vintage Air can be used. If you are using such an engine, contact the factory for the bracket. Other engines will need to use existing bracketry or use a custom solution.

Some builders with LS-series engines have used the front dress from the Cadillac CTS-V as offered in the GMPP catalog. This uses the CTS-V alternator and compressor. While the alternator works well, it fouls the stock aluminum motor mount and so the motor mount will have to be modified or another alternator and bracket used.

Likewise, the CTS-V AC compressor is not the same as the Sanden 508-style compressor provided in the kit. The GM compressor does work, but the preferred solution is to use the included Vintage Air compressor, in order to avoid having to make custom hoses at the compressor end. 

The simplest approach is to use GM idlers and the Superlite brackets for the alternator and compressor.
When installing the compressor on the engine, observe the following suggestions:

  • Do not distort the compressor body when mounting, as this may cause an oil leak.
  • Use Grade 5 or better mounting hardware on the mounting bracket and compressor
  • Use a sturdy bracket to mount the compressor on the engine, as excessive vibration will damage the compressor. Use a sturdy bracket to mount the compressor on the engine, as excessive vibration will damage the compressor. The bracket available from the factory is strong and rigid and makes mounting compressors on LS engines easy. 
  • Align pullies such that there is no more than 1/32” of misalignment- zero is preferred
  • Mount the compressor such that there is at least 1/3 belt wrap on the pulley. Less than that may reduce AC performance, may reduce belt life due to slippage and subsequent heat buildup, and may increase noise.
  • Set clutch air gap to no less than .016” and no more than .031” The AC manual should show how to adjust this.
  • If the compressor can be rotated, make sure that the outlet ports on the compressor are clocked such that lines can be attached without clearance problems.

Mounting the Condenser

The condenser should be mounted in front of the engine radiator, with a ½ to ¾ inch gap. Don’t position the condenser against the radiator, as that will promote heat soak, and dramatically reduce performance of the AC system.

There are several ways to attach the condenser using straps, tabs, captured brackets, etc. Use whatever solution works for your car. 

When mounting the condenser, be sure to offset the condenser in such a way as to allow hard lines or hoses to be attached to the fittings such that they can be passed through the openings in the radiator.

Charging the System

After you have installed the AC system, it needs to be charged with refrigerant. Normally, you would take the car to an AC specialist, or have them come to your shop to charge the system and verify proper operation. It is possible to do this at home, but specialized gauges are required to do this, and the EPA now requires that refrigerant is sold only to licensed technicians, which means you’d have to take – and pass v an EPA-recognized course.


The provided system can also provide heat. If you need only AC, you can cap off the heater core inside the evaporator, and run a recirculating hose from one water pump heater barb to the other. This approach closely emulates the OEM heater arrangement that has coolant always flowing through the heater core.

The standard kit-provided heater valve is a variable shutoff valve. This is a traditional and proven valve design and works well with most engines. For LS-series engines, however, the 4-port valve below is a better solution as it allows the coolant to always circulate through the heater ports on the engine, as the coolant flow through the engine was originally designed. Don’t use the Vintage-provided heater control valve if you have selected an LS engine to power your SL-C, and don’t just cap off the heater barbs on LS engines if you are not using heat in your car. Either of those approaches can cause overheating and possible engine damage.

For LS-series engine, you can use this electronic 4-way heater control valve available from Chevs of the Forties shown in the picture below:

This valve allows coolant to be completely shut off to the heater core, while still recirculating the coolant back to the water pump as the engine cooling system was originally designed.
It also allows the operator to open the valve in small increments to control the amount of hot coolant delivered to the heater core in the evaporator. 

For heat you should run lines from the water pump heater outlets to the heater core by way of the coolant reservoir. This allows the heater circuit to self-purge air that might be trapped in the heater core or lines. Failure to plumb the car this way in an LS-engined car will make it more difficult to completely purge air from the heater circuit.

An example heater implementation on a generic LS-powered car, courtesy of the Pirate 4X4 site, is shown here: