Beating Exhaust Heat

The factory downpipe from the manifold on NA and NB Mazda Miatas is wrapped with shielding and packed with insulation to keep the heat contained within the pipe and away from the transmission tunnel, which is where the driver’s right foot rests on left-hand-drive cars. The gas pedal is just on the other side of the transmission tunnel.

If you remove it and add a header, you’re introducing heat to that area, and the driver’s foot likely is going to get hot. And, of course, racecars have no carpeting, so there’s no insulation inside, either.

To correct it, we turned to a couple of sources. Cool It Thermo Tec makes a nice wrap we could use to wrap the header. The second source was the helpful hardware pro at Ace Hardware. There we sourced sheet aluminum, screws, lock nuts and nylon spacers with which we could fabricate a heat shield and a mounting system.

The header wrap keeps the heat under wraps, and helps with exhaust scavenging, and the heat shield mimics the kind of equipment you find on cars from the factory. The nylon spacers are nonconductive, and allow for an air gap between the chassis and the shield, which helps ventilate heat when the car is in motion. At least, that’s the theory behind the whole project, so we’re going with it.

These principles apply to any car, and shielding heat is good practice for cold air intakes, too. The trick is to put the shielding between the source and whatever it is you are trying to protect. You want the shield to absorb and then shed the heat. Putting the shielding on the inside of the car, for example, doesn’t work nearly as well.

Putting heat shielding on the inside of the car is a bit like wearing a raincoat inside out. In this installation, the heat is absorbed through the back side of the adhesive and transferred to the reflective material and then to the driver’s foot. It’s probably better than nothing, but certainly not the best approach.

Start with a piece of sheet aluminum and a piece of poster board. To begin the project, I cut the poster board to the same size aluminum sheet.

Hold the poster board up to the area where you need to shield exhaust heat and rough out the shape with a pencil.

When you get a rough shape with poster board, go back to the car to size it up again. It’s a measure-twice-cut-once kind of thing.

The forward edge of the poster board template is where the driver’s foot would be on the inside of the car.

This is for the obsessive compulsives out there. I didn’t like wavy cut at the top of the poster board template, so I used a straight edge to make it acceptable.

Once you’re satisfied with the template, trace it with a Sharpie on the piece of sheet aluminum.

This simple shape matches the contours of the transmission tunnel.

Cut the shield along the lines just like you learned in kindergarten.

Sometimes cutting corners is a good thing. I snip the points of 90-degree corners so they don’t snag clothing or header wrap or cut the hand of the next person who works on the car.

A wire wheel deburs all the cuts you made. You should be able to rub the edge on the back of your hand and have it not scrape off any skin. My eighth grade metal shop teacher taught me that.

Line the shield up where you want it and use a spring punch to create the spots where you want to drill the mounting holes. The dents created by the spring punch prevent the drill from “walking” while you’re drilling.

Use your shield as a template to mark the transmission tunnel — or whatever you’re working with — with a black Sharpie to create a precise fit.

Sheet metal on the car is a bit thicker, so you might need to use a regular punch and hammer. Fun fact: I made this punch in eighth grade metal shop class.

Drill the holes in the chassis for mounting the heat shield.

This shows the inside of the car and the area to be protected from heat.

Push the screws through the metal from the inside.

The nylon spacers I sourced from Ace Hardware are great because they’re just the right thickness to create an air gap, and they aren’t good conductors of heat, so they won’t transfer heat to the transmission tunnel. If you have the room you can cut two panels and use another set of spacers and longer screws to create a double shield. Space is kind of tight here so we had to settle for one.

Here’s the finished installation with the Nylock nuts and nylon spacers in place. You’ll need a second set of hands to help you tighten the screws from the inside.

The second half of the job uses Cool It Thermo Tec header wrap.

A quick way to determine the length you need for the header wrap is to wrap it once, then fold it over itself. Each fold equals one time around the header. Then measure length of the area you want to wrap, accounting for overlap, then create as many folds as you need.

Roll the header wrap so you can place it over the header and wrap it without having to chase the whole length of it with each wrap.

With the wrap already looped over the header, it’s easier to wrap it. You sort of unspool it as you need it. Pull it tight with each wrap, while trying to avoid bunching.

By folding the end over, you prevent the frayed end from fraying even further when the header is mounted and in use.

Most of the header wrap companies make their own clamps, but I still prefer a worm-gear clamp. Tight but not too tight. There’s going to be a lot of heat here.

Each wrap is overlapped by half the width of the “tape.” I also wrapped it from rear to front so that gaps from one layer to another face to the rear of the car. Also make sure your clamp screws are located so that you can tighten them on the car.

The wrap keeps the header from emitting too much heat. The heat shield absorbs and sheds the heat, and the air gap helps carry heat away while the car is in motion.