The SL-C uses a front radiator, connected to the engine with stainless steel tubes.  This is pretty standard stuff for a mid-engined car.

Careful design of the cooling system is necessary for optimum results.  Most of the design is already done- the radiator is sized for engines up to 700 HP, the cooling lines are sized properly for even more capacity and flow rate, and the factory-provided fans are sufficient to keep large, hot V8s cool.

However, there is some work the builder must do, notably fabricating a shroud for the fans, and careful routing and insulation of the cooling pipes.

If you aren't familiar with cooling systems, this link is a great explainer.

Optimum Engine Temps

Many modern engines run at temps that would be abnormally hot in older, lower tech engines.  This often causes owners of new SL-Cs with LS engines to worry, and start to change things in an attempt to "fix" the "problem".  In most cases, it isn't a problem at all, and the worry and effort expended are a waste of time and resources.

An LS engine will develop more power at temps over 200 degrees, as many dyno runs have shown.  The factory fan turn-on temp is 226 for the low-temp  fan, and 235 for the high-temp fan in most LS-based cars with dual fans.

Think about that, and those numbers, before you panic and start to add bigger pumps, fans, exotic coolants, etc.

No one wants their engine to overheat- but modern engines, and LS engines in particular, are designed to run much hotter than older engines, and the old "190 is beginning to get too hot" thinking is wrong for these engines. Please do NOT run a 160 or 180 degree thermostat in these engines- you will actually accelerate wear and make less power.

There is plenty of capacity in the radiator, the pipes are adequately sized, and if you have all the air out of the coolant circuit, have a properly-designed shroud in use, and nothing is broken, you almost certainly don't have a cooling problem.

Let the engine run at the temp the computer wants it to- which is probably much higher than you may be used to.  Just be cool with the heat!

One last thing- the purpose of fans in automotive cooling systems is to bring down the temp of an engine while the car is at rest, or moving very slowly.  If your car runs "too hot" at normal speeds, it has nothing to do with fans, and changing them won't really solve whatever problem your car actually has.

The most common cause of insufficient cooling at rest is not insufficient fan size, but the lack of a properly-designed shroud or ducting, or air in the system.  Correct these problems first before you decide to change the factory fans.

Mounting the Fans

There are many ways to mount the fans on the radiator, and most of them will work fine.  But one mounting method that should be avoided is using ty-wraps or cable ties fished through the radiator fins to hold the fans to the radiator core. The reasons for this are two-fold: first, because the fans should be mounted on a shroud, and standing off the radiator core by an inch or so.  The second reason is even more important, which is that the warranty on the radiator is void if there is evidence that fans were ty-wrapped to the radiator core.  Here's a sign at the shop where the SL-C radiators are built.

The fans sometimes ship with special plastic devices that mount through the fins and have a quarter-size disc on the end to spread the load.  You should discard these or use them as a last resort, for an emergency fix on the road.  Note that they are provided by the fan manufacturer, not the radiator supplier!  

The best way to mount the fans is to fabricate a good shroud.  

However, if you don't make a shroud, you can mount the fans to tabs welded to the top and bottom of the radiator core, and mount them close to the core, but not on it, to avoid wear from chafing.  But your car needs a shroud for optimum cooling. :)

Another way to mount the fans that requires no welding on the radiator is to make a frame that rests against the core, and holds the shroud and or fans so that they are positioned correctly over the radiator.

To summarize, properly shrouded fans will always work more efficiently and cool better. Non-existent or poor shrouding can sometimes be partly compensated for with bigger fans, but the best solution is to use the smallest fans that, from an engineering standpoint, will do the job.  Let the shroud help the fans work, and don't waste engine power that has to drive an alternator to produce more power to run fans that are too big.  The stock fans are more than adequate for engines up to 700 HP or so, on the street or track.

Fan Shrouds- Build or Buy?

Every SL-C that ever idles or is in traffic needs a fan shroud.  All others can skip building one.  

Well, maybe that's a bit strong, but there's a reason why every OEM car you've ever seen has a shroud for the fan- it's just the most efficient way to use the fans.  You might be able to just throw on bigger fans, but that costs power, and extra money, and doesn't really address the problem of cooling at rest or low speeds (which is all the fans do) as efficiently as shrouded fans.

So how to design or purchase the right one?

Building one can be fun, and buying just the right one satisfies the hunter-gatherer instinct in us.  Most SL-C builders have made shrouds out of aluminum because it is easy to work, and is consistent with the rest of the chassis material.  Here's a link to one builders blog where he discusses an aluminum shroud he built for his Porsche-powered SL-C.

Other builders have used different materials.  A popular choice is composite, typically fiberglass or carbon fiber.  Most of these are made using the lost-foam method.  Essentially, you carve a male mold of the shroud using the pink foam available for insulating purposes at your local home hardware store.  When you are happy with the shape, cover it with sealing tape, and then lay resin and cloth layers until you are happy with the thickness of the piece.  When cured, use Acetone to etch out the foam.  What remains is the part.  Here's a link to an article that used this technique to make a fiberglass scoop.  The picture below shows a shroud for an LS7-based SL-C that was made using this technique.  Carbon fiber was the chosen material, because the builder had some left over, and because it was a chance to experiment with the technique in carbon (it's mostly done in fiberglass because of cost).

Shroud Design

Designing the optimal shroud is not as simple as it might seem.  The usual constraints or inputs are fan size, fan capacity (usually expressed in CFM or cubic feet per minute, generally at an unspecified pressure), radiator size, clearance behind the shroud, weight, ease of fabrication, ease of maintenance, and most important, optimal airflow through the radiator core from the fans, including making sure that the shroud allows the fans to pull from all areas of the core, not just the circular area just under the blades, and that the shroud allows adequate airflow through the core when the car is moving and the fans are not needed.  None of this is trivial, and the major OEMs have engineering staffs (or their Tier 1 suppliers do) to design just such shrouds.

That's why often the best solution is to find an existing shroud from a car in the junkyard that has a similar radiator size, etc, since the design work is largely done.  Your only remaining task is figuring out how to seal it against your radiator.

If you can't find one, or prefer to roll your own (and there is a lot of that feeling in any SL-C builder, we've discovered!), don't hesitate to try to make your own shroud.  

The easiest way to get going on that, if you don't have the engineering skills to start with a blank sheet, is to carefully study OEM shrouds in detail.  Luckily, you can do that from the comfort of your own garage or office, since the internet is full of pictures of OEM shrouds.  Google images is your friend here.  Note that all of them are designed to allow the fans to pull air from all parts of the radiator, not just the area right in front of the fans.  They also position the fan off the core, from 1/2 to 2 inches.  Finally, all of them have paid attention to good sealing so  all that design effort for the shroud isn't wasted by air that leaks around the shroud.

Here's an example of a well-designed OEM shroud:

Note the volume around most of the area where the fan is, and even a flap which stays shut during fan operation or slow speeds, but opens during high-speed (or high air flow, which is assumed to be the same thing) to open that part of the rad core to free flow.  This is a well-proven design and works well.  Though originally produced for a small under 2L engine, it has been used by drag racers for many years.


Here is an example of a relatively non-optimal shroud design, one that is common in the kit car world:

What's the matter with it?  The main problem is inadequate airflow from the corners of the rad core.  Almost all the air pulled through it by the fans is coming directly from under the fans, and so the radiator core area elsewhere, particularly in the corners, is not used optimally.  In other words, the shroud design is making the radiator less efficient than its design would indicate.  Also, note that the edges are not sealed around the core; this allows air to leak out of the shroud, further reducing efficiency.  This design is not all bad, as a substantial portion of the core is well covered with fans, but the flat design and poor edge sealing make for a rad/shroud/fan combo that is less efficient than it could be.

Also, don't forget that air that doesn't even flow through the radiator is wasted, and can kill the performance of even the best radiator, fan setup and shroud.  That means you need to seal the top of the radiator against the body, so air doesn't just flow over the radiator instead of through it.  This is an extremely common omission on SL-Cs, and easy to fix.

Bigger Fans

Bigger is better, right?  Well, not always.  

As pointed out above, the fans that ship with the kit are more than adequate, and are suitable for engines up to 700 HP according to the factory.  In normal operation, properly installed factory fans will bring engine temp down pretty quickly.

However, several builders have upgraded the original fans to the Maradyne M122 which are available here among other places. They are slightly larger but still fit. The picture below compares the early fans provided in the kit (left) with the Maradyne (right) which has a significantly larger motor.

It should be noted that the use of these Maradyne or similar fans will generally require a relay or relays to operate them, as they draw more current that one InfinityBox output can provide.

As of early 2015, somewhat larger capacity fans are being shipped with the kits.

We don't have pictures of the new 2015 fans yet.

Fan Operation Strategies

The factory wiring harness controls the two fans as separate entities:  One fan is controlled by a signal from the engine ECU, while the other is controlled from a signal from the AC system.  In other words, both fans are only on when both the engine ECU requests a fan on, and the AC system requests fan on.  

Some builders choose a slightly different approach, and elect to have both fans come on when either the AC system requests fans, or when the engine ECU requests it.  This has the benefit of getting the most cooling effect immediately for both AC and engine cooling, at the expense of slightly more noise for a longer period of time.  In quieter production cars, this may be a drawback, but our cars are generally so loud that the added noise of both fans running is generally not noticed.


The latter approach is the one used by Allan (a builder of over a dozen SL-Cs) and many others.  It's easy to change how the fans are wired (PM Will on the GT40s site if you need help here), and reversible if you decide it's too noisy.  But running both fans whenever cooling is needed is likely to result in faster cool down of the engine, as well as improved AC system performance.

Heater Control Valve

 This is covered in the Air Conditioning section, but you will need to plan for a heater control valve if you are installing the factory AC system (which has an integral heater core) or an aftermarket heater only. 

The heater control valve shipped as a standard part of the kit works fine with most engines, but not the LS series, so follow the guidance in the Air Conditioning section and use a valve that recirculates the coolant at all times, whether or not the heater is actually getting hot coolant because cockpit heat is required.

Cooling Lines

There are several ways to connect the lines, and this page shows several options: Cooling Lines

However you make the lines, most builders choose to insulate them, to keep heat in the lines and from radiating into the interior.  There are several approaches to this.

  1. An inexpensive way to insulate the pipes is using foam insulating tubes designed for residential or commercial plumbers.  These can be difficult to source, so try the plumbing supply houses instead of Lowes or Home Depot. 

  2. Some builders are using header wrap.  This is a bit more expensive than the foam tube solution above, but is smaller in diameter, useful in tight situations.  This is available from the usual sources, including Jegs and Summit.

  3. Occasionally, some builders don't insulate at all.  Even in a track or race car, substantial heat will penetrate the interior if the pipes are not insulated, though this is mitigated somewhat by the fact that the pipes are external to the car, in contrast to the GT40 (and many other kit cars), for example, which has these tubes running down the central tunnel.

Coolant Tanks

Coolant expands with temperature, and it has to go somewhere when it is heated. If allowed to overflow to the ground, the system will likely not have the full amount of coolant needed to properly cool the engine. And spilling coolant on the ground is sloppy, and will get you expelled from a lot of race trackThat's why pretty much every engine in a car has a coolant tank, or overflow tank. The purpose of such a tank is to provide a place where hot, expanded coolant can go, and then be drawn back into the engine cooling circuit when back at room temp. A secondary purpose it to de-aerate coolant in the system. This helps the engine to self-purge, and stay that way over time.

There are many possible solutions, from using a plastic tank from a production car, to buying generic tanks from places like Canton, to building your own tank. Any of these can work, but all of them need to follow a few basic guidelines:

1. Position the tank so it is the highest point in the cooling circuit.

2. Use the tank to de-areate the heater circuit as well

3. When mounting it, make sure you use standoffs, or a similar approach to minimize heat transfer from the tank to the rest of the car, especially the interior.

4. Plan to connect the tank to the purge port on the radiator in the front.

5. Consider connecting the tank to the steam ports if you have an LS-series engine.

6. Provide a port to an overflow catch tank as the pressure cap opens

7. Decide the proper pressure rating for the cap you want. Too little a PSI rating and the system will purge too easily, too much pressure and you risk the possibility of a burst hose or fitting.

Whether you choose a tank from a production car, an aftermarket piece, or make your own, before you buy, plan for everything you want to connect, and decide how you will do that. In other words, if you want to connect the steam ports, how will they actually connect to the tank, etc?

Here are some pictures of a custom tank made for the SL-C by one builder. The tank was designed to have adequate volume, as well as being fabricated to fit under the spider, on the left side (you can see it attached to the bulkhead).

The next picture shows a better view of the fittings:

This tank has a 5/8" hose barb for one side of the heater circuit, a 3/4" barb for the other side, a female -3 fitting for the steam vents (in this car, all 4 steam vents go a a manifold, which has an output connected to the fitting on the tank), and a 1/8" NPT fitting for the radiator bleed hose. Standoffs were added later when experience showed that too much heat was being transferred to the bulkhead, and then to the interior. The tank was eventually also wrapped with insulation to keep the heat in.


Not shown is the pressure release hose barb under the pressure cap on the tank- it goes to the recovery tank discussed below.


In addition to the coolant tank, you'll also need a recovery tank. These are typically a tall, thin cylinder with a tube running to the bottom from the tio, and often with a clear hose on the side to show fill level. Here's an example of a recovery tank from one source:

Note that the cap is not a pressurized cap- that's managed at the main coolant tank.

Many builders use off-the-shelf coolant tanks. If you go this way, be sure the tank has all the ports you need. Use the list above to help you plan your coolant tank properly. In most cases, existing tanks don't have all the ports you might want (for example, they typically don't have a port for steam vents), so you may end up getting a welding shop to add fittings to the tank you selected.

Note that the stock Dorman tank that comes with the kit doesn't have a port for overflow when the cap hits the pressure limit- it just leaks coolant down the sides of the tank. 

Discussion on the topic on the GT40s site here.

Steam vents

If you have an LS engine, there are one or two steam vents on most cylinder heads. The purpose of these vents is to allow steam to escape from the heads. Why is this important? Well, steam is a pretty poor conductor of heat, and when the heads gets localized hot spots that cause the coolant to boil (and turn into steam), Bad Things like detonation can happen. As most everyone knows, detonation kills engines very quickly.

So what to do about it? In some cars like the Corvette, the two front steam vents are installed with pipes that can be run back to the radiator, or in our case, back to the coolant header tank. This allows the steam to de-aerate, and separate back into air and coolant. The air can be stay in the header tank, or get expelled when the pressure cap opens.

If you want to be doubly safe, you can pipe all 4 vents back to the header tank. One simple way to do this is with a kit from Kurt Urban Racing shown here. In this product, all 4 vents are run to a common manifold, and a common line is run back to the header tank. You can make your own one of these, just use the front two, or leave them off altogether. Engines making real power benefit more from having all 4 vents output sent to the header tank, as you would expect.

You can make your own with these adapters, some AN fittings and some hose if you want to go the low-buck route