Bonnet Vents

January 1, 2004

The new headers that were installed during the B16A engine swap generate a lot more heat than the stock exhaust manifold and heatshield.

In another attempt to reduce the temperature underneath the bonnet I tried installing some bonnet vents into my Honda Civic SiR to remove some of the excess heat.

Choosing Bonnet Vents

Finding bonnet vents that would suit the Honda Civic SiR was very difficult. More difficult than I would have thought given the increasing import scene in New Zealand. Many local shops sold cheap fiberglass Mazda GTR lookalike bonnet vents. I found other bonnet vents that were similar in design to the Mazda GTR ones, but most of them required a large hole to be cut in the bonnet, which is then covered with a badly made fiberglass surround and aluminium mesh. That seemed a bit silly because you might as well just have cut a hole in the bonnet. The gaping hole probably wouldn’t help the aerodynamics and the would allow all sorts of stuff (like rain) to get directly into the engine bay.

Given my lack of finding some good bonnet vents locally started to look on the internet to see what I could find overseas. I luckily found many sites that sold really good looking functional bonnet vents, mostly in the UK. Of the companies I emailed for prices and shipping charges, not a single one replied (possibly because they couldn’t be bothered shipping to New Zealand). However, I did spy the vents that I wanted - some cut in vents designed for a Sierra Cosworth.

Sierra Cosworth cutout bonnet vents

With a view of what I now wanted I asked around some more of the local shops to see if they could get hold of the Sierra Cosworth bonnet vents for me. I eventually found one shop that could get them (Shore Performance Tyres). They took a couple of weeks to arrive but I eventually got what I wanted in black with a carbon fibre finish. The carbon fibre finish is a bit ricey, but it saved me the task of having to paint them because the plain black ones were simply the molded black plastic finish and looked a bit nasty.


Installation was a piece of cake given that I have access to a bunch of great sheet metal tools. The whole ordeal only took about an hour and was really straightforward.

  1. The bonnet masked off with masking tape so we could mark where the holes would have to be cut. Marking the holes was a little complicated as you have to make sure you are not going to have to cut through any major structural support for the bonnet. The vents here are as far forward as possible without cutting into anything substantial.

    Marking the holes to be cut in the bonnet

  2. The vents on top of the markings just to make sure that they are going to look OK. A bit further apart may have looked better, but we were constrained with where they would fit by the supports underneath.

    The bonnet vents in the dummy position

  3. Cutting the holes with an air-saw. The air-saw cut through the bonnet like a hot knife through butter. It’s easy with the right tools…

    Cutting the holes for the bonnet vents with a compressed air saw

  4. The holes revealing what the bonnet vents would be sitting over. As you can see there is quite a bit of the spark plug well and the rocker cover showing.

    The holes cut out ready for the bonnet vents

  5. The bonnet vents in the car after sticking them down with very industrial strength double sided tape and 5 minute epoxy underneath. There’s no way they are coming out now! They look quite good and make the front end look a lot more staunch than it used to. They somehow remind me of the snout at the front of the newer BMW’s. Definitely not a run of the mill Honda Civic SiR anymore.

    The bonnent vents in position


There was some problems with the spark plug well filling with water when it rained. This caused some pretty bad spark problems one day. Now there are some rain runoff bits under the vent furthest back to prevent rain from filling up the well up again.

After having the bonnet vents in for a little while, I also enlarged the original size of the vent slots using a craft knife (the plastic is quite thin) to let more air out.

The massive amount of heat that is given off by the headers has in fact warped the right hand bonnet vent. It’s not warped very much, but that’s a pretty good indication that some seriously hot air is getting sucked through them. The bonnet vents are also sucking more hot air over the rocker cover than usual and some of the stock black crackle paint on the rocker cover has started to flake off. Not really much of an issue though. I have another rocker cover from the engine swap that I might get polished.


I don’t have any underhood temperature readings prior to putting the bonnet vents in, but the intake manifold can now be held after a long drive whereas that was never possible before, so it has definitely reduced the ambient air temperature under the bonnet. The latest Underhood Temperature Readings show that the underhood temperatures don’t get too excessive.

The intake manifold still gets quite hot after the car has been sitting for a while, but I don’t think that there is too much that can be done. I may get around to raising the rear of the bonnet to let even more hot air escape at the rear of the bonnet as well.

After driving for a long while the car still feels quite strong and responsive so I think they are doing their job and keeping things cool. In fact when stopped at lights, you can see the heat haze coming out from the vents if it’s not too windy, so it’s getting pretty hot underneath them.

Of course if I had wrapped the Custom B16A Headers before installing them in the car, I might not have even needed bonnet vents. So, if you are going to put some new headers on your car I recommend wrapping them, or putting in some form of heat shield if you have room to.

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