Heat Extraction: Installing a Hood Vent on an NC MX-5

It’s no secret that production-based racecars require a lot of aftermarket parts to make them work on track. It’s also no secret that OEM parts have their own allure because of the amount of testing and engineering that goes into them.

In this case, we’re going to take a different tack, and install an OEM part from a manufacturer that didn’t build the car. I admit to stealing this idea from someone who did it on a YouTube how-to video, but I still believe words and pictures are the superior way to convey how-to information, and that’s what this story is about.

The NC chassis MX-5 creates a fair amount of heat under the hood, which is sealed effectively and it doesn’t evacuate it well. It’s even more acute if you turbocharge them. I was looking for solutions when I stumbled on the video that took the front fascia vent from a Porsche 997 and used it on the hood of an NC MX-5. It’s easy enough to find the video with the right search terms, but the finished product looked really good, like an OEM solution to extracting heat from under the hood.

There also are aftermarket solutions to extracting heat, but I really liked the look of the 997 fascia vent. It didn’t come without challenges, chief among them that the 997 fascia is curved and the MX-5 hood is essentially flat.

To get it to work properly, you have to use a heat gun to warm the 997 vent and then set heavy things on it to get it to sit flat. I used a Harbor Freight heat gun and then set cinder block on top of the vent on the work bench. I had to do a few times to get it to finally stay flat. The method wasn’t pretty, but it worked. Of course, I got a bit too enthusiastic with the heat gun and melted the vent on the driver’s side, so if you do the job, be judicious with heat application.

This job requires patience and a fair amount of cutting. The hood on the NC is made of aluminum, so that made things easier. It was also pretty easy to find the pressure numbers across the hood of an NC online, which informed where to locate the hood extractor. Negative pressure numbers are your friend. Negative pressure numbers pull heat out from an opening, and I ended up locating it in the same place as the guy on YouTube. The only other place to locate it on the hood wasn’t flat, so that would have complicated things.

There’s no going back after you cut a flap out of your hood. Measure three and four times so you can cut with confidence.

For lack of a better word, “surveying” the hood with tape helps you sight things out before doing any cutting or marking. If you decide to do this on an MX-5, tape from side to side using seem between the fender and the front fascia. The centerline of the hood goes from the middle of the right-hand wiper pivot to the center of the emblem on the fascia.

The vent kit from Pelican Parts comes with everything you need to do the job correctly, including a template you use to mark the lines you will be cutting. Once you have the tape applied correctly, use the template to locate the opening for the vent. You can draw lines on the tape with a sharp pencil. The openings on the template are too skinny for a Sharpie. Later you will have to cut the center out of the template so you can use it on the underside of the hood for retainer plate.

The underside the hood is a bit tricky because you have to remove the “webbing.” From the underside, you can’t tell where to cut, so you kind of have to cut parts of the webbing from the top side of the hood and then connect those cuts when flip the hood over.

You also have to make the opening on the webbing larger than the opening for the grille on the top side. That’s so you can get the retainer plate to lay flat and to be able to access all the tabs and fasteners. The tricky part is cutting around all the recessed areas where the webbing gets bonded to the outer hood panel. I cut out the bonds. If I were to do it again, I’d probably cut around the bonded spots so the outer hood panel would retain the same factory support. You’ll see what I mean in the photos.

There’s a lot of trial and error with this kit. You have to go slowly and there’s a lot of going back and trimming some of your major cuts to get everything to line up and sit flat. Take your time. Here’s how the job comes together.

Use a punch at each end to locate the drilling points. These holes will create the outer edges of the opening for the vent.

Be meticulous with your planning and outlining. Once you cut the hood, there’s no going back. Check and recheck your work before cutting.

The first step is to drill the holes at each end.

Use a step drill to enlarge the drilled holes to match the radius of the outer tips of the vent.

Now for the moment of truth. Use a cutting wheel to cut out the outline where the vent will drop into the upper panel of the hood.

Because it’s impossible to outline the underside of the hood, cut through the top to the inner webbing as best you can so you can have an idea where to cut on the underside.

This is the rough shape of the hood vent. Your cuts to the underside webbing will need to be larger than the opening on top of the hood so you can mount the vent properly and be able to access all the tabs and fasteners.

Use  bur tool to create the circular cutouts in the opening for the mounting screw bosses on the edges of the plastic vent.

The opening on the underside will need to be larger than the opening on the top, so you can access all the fasteners and mounting tabs.

Use a flapper wheel and a file to debur all the edges and surfaces you just cut.

Flip the hood back over and remove the tape. You’ll also probably need to do some more deburring.

Test fit the vent to see if it drops in the hole you just cut. I had to do some trimming and enlarging of the mounting cutouts.

This shot shows a test fit of the vent and the screen that goes on the underside. You can see the rounded cutouts we added to the outline of the cut with the bur tool.

The steel template you used to mark the outline of the cuts on the top side of the hood is also the retainer plate for the underside. Cut the center of it away.

Here you can see the black steel template dropping in around the outer edge of the plastic vent. You also can see I need to cut more away from the end so I can attach the tabs and fasteners.

These black stays come in the kit. One leg is shorter than the other for mounting on the sloped 997 front fascia. The area where it’s mounted on the NC hood is also sloped, but less so than a 997 fascia. The longer leg goes toward the rear.

Place the mesh screen on top of the black mounting tabs.

The aluminum “I” tabs go over the screen to retain it — with the hood upside down — and are screwed into the underside of the plastic vent.

The screws fasten to the plastic grille, which creates the tension on the black mounting tabs and the retainer plate. It’s a clever attachment system. Thanks, Porsche.

Each end has its own retaining tab that holds the screen to the grille and the grille to the hood panel.

I probably cut too much away from the webbing on the underside of the hood. Were I to do another one, I’d cut away less, but still enough to give access to all the tabs and fasteners. Once the vent is attached properly, reinstall the hood.

With the hood back on the car, the vent has the look of an OEM component. That’s because it is, though not from the OEM that built the car.