"Saving Ford's hot hatch from becoming a fridge"
A resource site for owners of the Mk1 & 2 Fiesta

Project Bobcat Ford Fiesta resource site

Fitting a Bonnet Vent

Author: - 14, March, 2011

Firstly - if you're going to fit a large bonnet vent you need to bear the following things in mind.

  1. This is not a job for the faint hearted. You have to cut a huge hole into your bonnet with an electric saw!
  2. There's no going back, unless you're a brilliant welder and sprayer.
  3. You'll be able to see the vent from inside the car - a good thing I think.
  4. In very damp weather and with a recessed vent like the Subaru WRX job the extra cold air/damp can make it a bitch to drive in the early mornings
  5. And last, but most important, remember brittle plastic and steel both flex in different ways. Your bonnet will be a lot more bendy than the vent, so be a little bit more cautious when slamming it shut.

Below are the steps I took to fit my Subaru vent (which is a Genuine WRX one, so there !).

Subaru Bonnet Vent

Step 1: Sanding and Prepping the vent.

The first job is to sand the glossy surface off the existing paint on the vent and prime and spray it with the colour of your choice. Ideally if I had loads of money I'd fit it and blend it into the bonnet first and then get it all sprayed but I don't so I didn't! See my 'Removing Side Mouldings' guide for painting tips. Remember to use lots of Plastic Primer otherwise the top coat won't have anything to stick to. Also plastic primer is more flexible than standard primer.

Step 2: Creating a Template

Place the vent bottom side down on a large piece of paper. Some wallpaper or a piece of A3 would do.
Now draw around the vent with a pen/pencil. A thick marker is the easiest tool.
Once you have a traced outline prepared you now want to reduce the whole outline by about 10mm this is the distance of flat 'glueable' edge around the vent.
NOTE: This glueable area may be diferent depending on what sort of vent you fit.
Now cut out the template.

Step 3: Marking Up.

Measure your bonnet's width, find the centre and mark it with a pen (on top of a piece of masking tape). Now measure the vent and find the centre of that too.
Now you want to measure the distance between the centre of the vent and one end of it. Now do the same with the bonnet and mark it.
This will allow you to mark up where to put the vent and get it perfectly centered. Getting the vent centered is CRUCIAL.

A quick way of centering the vent is to fold the stencil/template exactly in half to find its centre then lay it over your bonnet so that the fold sits over the bonnet centre line.
Now you have to cover the bonnet in paper to avoid scratching it with the platter on the bottom of the jigsaw cutter. Another way would be to cover the metal platter in masking tape.
Affix the template to the bonnet with masking tape SO IT WON'T MOVE. The template edge should still be visible through/under the tape.

Step 4: Preparing the bonnet

Ideally you'd remove the bonnet from the vehicle. Only involves undoing four hinge bolts and a 'bonnet stay' bolt.
If you can't be arsed or you're too thick simply raise the bonnet up a little using wooden blocks along your wing tops.

Step 5: Cutting

Fit an electric jigsaw with a METAL CUTTING BLADE. If you can't even do this, remove all the sticky stuff and give up now before its too late.
Drill a hole at one of the corners of the new vent hole (probably about 10-12mm).
Stick the blade of the jigsaw through the drilled hole and make sure that it isn't long enough to cut through all your looms and cooling hoses!
The blade does need to be long enough to cut through a bonnet strengthening crossmember. You only need to cut one of these and because it's of angular contruction this does not affect your bonnet shape or rigidity.
Now that you have read all the bullshit... Start Cutting. Remember the safety goggles (yeah right) because this operation does involve lots of flying metal swarf and sharp splinters.
You'll need some way of keeping the cut-out section stable, either by attaching a sticky hook to it and then holding it up or by putting something underneath it. This stops it flapping about when you get to the final stages of cutting.

Step 6: Big F**king Hole.

Now that you have disgarded a large chunk of your bonnet you'll see a large hole (in your air box with coolant hoses pissing out water everywhere..heehee).
You need to lightly file around the hole to take off the sharp edges.
Sand about 10mm paint off the bonnet all around the hole (remember that glueable area).
Once all the sanding is finished use a vac to hoover up all the metal debris so it wont collect and rust your Fiesta. Clean off the sanded area.

Step 7: Glueing/Attaching.

Use some two-part glue like Araldite, Mastik or Halfords high strength glue (comes in 1000psi packs etc..) smear this around the hole and on the area of vent to be glued. Follow the instructions on the glue tube for the sticking instructions.
Once you have placed the vent into the hole wipe off any excess glue BEFORE IT DRIES and weight it down with a book or something. You could hold it if it's really quick drying and you've got nothing better to do.

NOTE: Some vents like the Subaru job may be a little to large for glue alone. Some sort of fixing job may need to be done to clamp it down mechanically.

Step 8: Finishing.

If you've got some sponduliks and you want to blend it all in and get it sprayed then now's the time.
Buy some ELASTIC bodyfiller and use it to blend away the edge between the vent and bonnet.
Go through all the general crap of sanding and filling, sanding and filling, sanding and filling, and yes, more sanding and filling, prepping and painting.
If you're broke like me use some silicon sealant (yes, that shite you use for bathrooms) and squirt and shape this around the vent. I've got a white car so I used white sealant and it looks really professional, surprisingly.

Step 9: Testing.

If you've got a direct intake filter like a K&N or Pipercross KK3000 take the car out for a blast. You'll notice the benefits of all that cold air getting to the element straight away.
The faster you go the more cold air you'll get and the faster you'll go (a vicious circle of speed!!)

Page Last Updated: Thursday 15th of August 2019