Honda Civic SiR

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Bonnet Vents Installation

This page contains detailed information about the installation of some bonnet vents into my Honda Civic SiR. The bonnets vents were obtained through Shore Performance Tyres and are replicas of the bonnet vents found in a Ford Sierra Cosworth.

Installation Steps - Problems - Results

Installation Steps

Below are is some notes and photos taken during the installation of the bonnet vents. You can click on each of the photos to get a full size version.

  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

Problems

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.

Results

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.