If you do a deep dive on the Internet, the rule of thumb for ambient temperatures and horsepower figures on a V8 engine is for every 10-degree drop in intake air temperature, you gain 1 horsepower, about 1 percent for 30 degrees. That doesn’t sound like much of a gain — and it isn’t — but if the inverse is true, that for every 10 degree increase in temperature, you lose 1 horsepower, and for every 30 degrees in temperature you lose 1 percent of the total, well then you begin to understand why cold air intakes have such broad appeal.

Think about that next time you exit the last turn on the last lap right next to your closest competitor on a hot day.

Cold air intakes are among the first modifications people make to their cars. We’ve written about cold air intakes before, how to build your own, consulted expert sources to get the best information, and there are some gains to be made. This story is going to address how to keep the cold air coming into your cold-air intake as cool as can be.

We also learned from talking to those experts for those stories that the factory air boxes are getting better and better. It’s getting harder for the aftermarket to improve air flow over the stock stuff.

The Cobalt cold air intake scoops air from under the bumper, right behind the grille and feeds it to the factory air box behind the upper radiator support. But it’s black plastic, which likely absorbs and holds heat, which could raise air intake temperatures. You have to remove the front fascia to get to it.

Take the NC Mazda MX-5. Lots of companies make cold air intakes for the car, but it usually locates the cone-style air filter behind the front bumper fascia, where air flow is kind of static and can mix with the hot air that doesn’t get through the radiator. Yes, some people have drilled holes and scoops in the front fascia to feed air to the cone, but any racer will tell you that might disrupt airflow over the front the car.

Yes, we are talking about excruciating minutiae, but lap times and photo finishes are measured in minutiae.

For the project TT5 MX-5, we found a pretty neat solution from Cobalt, an air intake that connects to the stock airbox at the front, but pulls air in from behind the grille, just below the bumper bar, well ahead of any heat sources. It looked the best candidate to deliver genuine ambient air to the air box. It was also cheaper and would attract less attention at the smog station while the car still needs to be emissions compliant.

Here’s what the Cobalt intake looks like out of the car. Seems like a good idea that could be made better.

The rub was that all that black plastic likely could absorb heat, and so would the airbox located right above the ECU, which itself is cooled with a forward-facing vent tube. As an engine bay gets hot, the airbox would absorb all of that heat and hold it and inevitably warm the air passing through, right?

So, if there were a way to shield all that ambient air against heat, it also would guard against heat soak, right? It was at that point that an IMSA prototype came to mind. On the firewall between the engine bay and the driver compartment, the team or builder had covered it with adhesive gold foil to reflect heat and keep it away from the cabin.

Lots of vendors sell the gold foil with the adhesive backing to deflect heat. Seems like it would be perfect for shielding the air intake tract. You’ll need to do a lot of cutting and fitting to get it to cover everything.

That seemed like it might also work for keeping heat from soaking into the black plastic, but that prompted the idea of a redundant shield that might absorb heat, and to separate it from the box with an air gap. Brilliant! Time to gather the supplies.

Lots of different racing vendors sell the adhesive gold foil, and the hardware store down the street sells sheet aluminum and the fasteners, and spacers to do the job right. All totaled, the cold air intake cost $95 and the shielding project cost another $50. That is still cheaper than most of the other cold air intakes on the market, and it passes smog and pulls air from a cleaner source.

Unless your bump-drafting. Then all bets are off.

The gold foil tape is surprising easy to work with and it seems like it will hold up over time, although, well, time will tell. Fabrication time amounts to a few hours and it is kind of fun, but it does get a bit tedious cutting all the foil to fit the crevices and contours of the airbox. By the time you’re tired of that portion of the job, you should be about finished.

Once you get the rough shape, you can further enhance it by making cuts to fit the contours and bends. Make most of your cuts before you remove the backing paper.

Then it’s time to shape and cut the outer shield from the sheet aluminum, bend it around the contours of the airbox and drill the airbox for the fasteners used to attach the shield. We accomplish that task with the method we all learned watching “Orange County Choppers.” Cut the shape and form the approximate bends using posterboard, then transfer the shape and form to the sheet metal.

Once you’ve got it shaped and cut to fit, remove the backing paper and apply it to the surface you’re trying to protect. Once you remove the backing paper, the foil tape becomes easier to work with.

When the shapes are final, it’s time to drill the sheet metal and the airbox and bolt them together with the fasteners and the spacers to create the air gap between the sheet metal and the air box.

Because the nuts and bolts and washers are on the inside of the airbox and downstream of the air filter, it’s important to use Loctite on them so that they won’t vibrate loose and be drawn into the engine. That would probably create more than a 1 horsepower loss, which is what we were trying to avoid in the first place.

The gold foil tape sticks pretty well, but you can also use a heat gun to get it to lie flatter on the surface. Heat applied to the outside did not transfer to the inside, so it seems this gold foil stuff works!

The stock airbox has a lot of strengthening ribs on it, so you’ll need to cut the foil tape to fit around them.

If you miss one, you can cut it with a knife or razor to get it to slip over the ribs.

The foil tape is pretty pliable, so it’s not too difficult to get it to fit around corners and into the contours. Once it’s wrapped completely, it’s time to create the air gap heat shield.

We’ve all seen this on “Orange County Choppers,” right? Create the rough shape using poster board. It’s easier to start with than sheet metal.

Once you have the shape figured out on posterboard, trace it onto the sheet metal and cut it.

The first bend is easy to make on the corner of a workbench by hand. Before you make the second bend, size up how it appears to fit on the airbox.

The second bend gets a little tricker. A poor man’s sheet metal brake is two 2 x 4’s and two C clamps. As you can see, the second bend follows the angles of the airbox pretty closely.

Draw a straight line to mark the last bend and put it back in the sheet metal brake to bend it.

It’s a good idea to round the corners and finish off all the edges on the wire wheel so it’s less likely to cut you while you’re working on the car.

Now you can start marking and punching and drilling the holes through which you will attach the heat shield to the airbox.

Once you have the holes on the airbox marked, you can punch and drill them, too.

Nylon spacers between the airbox and the heat shield create an air gap and extra measure of protection to keep the plastic airbox from absorbing heat.

The hardware store didn’t have Nylock nuts in the size we were using, so we made sure to use Loctite on the bolts, which are downstream of the air filter. If they come loose, they could get sucked into the engine.

Here is the finished product, wrapped on all sides with gold foil, and shielded on the bottom, back and top with aluminum sheet metal with an air gap in between.

The entire cold air intake tract is wrapped in gold foil and the airbox is ensconced in foil and shielded on the sides exposed to engine bay heat. Seems like a fair amount of work for a minor gain, but racers will do anything to avoid power losses.

Images courtesy of Eric Green, Eric Green and Brett Becker

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