It might seem like a daunting task to cut holes in your pristine bonnet, but after you have started you will see how relatively easy the task can be. The old adage measure twice cut once is mandatory I think unless you want to risk a possible disaster. Also having the right tools goes a long way towards making the job easy. I used a Dremel tool with a cut off wheel which worked well on the top and bottom side, but a jigsaw with the right blade would also be good.
The first task is to work out where the vents will be positioned. Points to consider are aesthetics , functionality and how a hole might affect any components underneath. It would probably not be a good idea to place a vent over an electrical component that might suffer from moisture. Once installed there will be water that enters through the vents during washing on when standing still in heavy rain but it is surprising how little water makes its way in. Most is instantly evaporated away by the hot engine bay temps (that are now somewhat cooler). During washing you could place an old towel under the vents, but so far I have not found it necessary.
I started by positioning the vents where I thought they looked best and then took some measurements. I then worked out where the holes would be on the underside and checked to see if there were any obstructions etc. Don’t forget to look at the position of your firewall as a venting past this area is probably of little use.
After some minor adjustments I worked out the exact measurements and taped the area with masking tape which serves as a surface to draw the cut lines and also protects the paintwork. The hole size I chose was 130mm x 180mm which is the minimum size with a couple of millimeters to spare. I measured in 141.75 mm from the side of the bonnet and 214mm from the back of the bonnet (centre of back edge of vent). Use the blank vent to trace the angle once the first line along the bonnet edge is in place.
Then I fired up the Dremel and steadily cut all 4 lines in around 20 minutes.
Some of the reinforcing subframe is removed on the intitial cut but the rest is probably best done underneath. I used a drill from above to mark all 4 corners then used masking tape on the underside to set up my cut line for the subframe. Working on the curved surfaces with the Dremel is a little tricky but a slow steady hand gets the job done. I simply cut the under bonnet lining with scissors.
So in about an hour the finished hole is ready for a file to remove burrs and then some paint added to the edges to prevent future rust.
The Dremel does a fairly neat job but remember the cut lines are covered by the installed vent and will hide any irregularity.
The next stage was to apply a paint finish to the vents, but before painting it pays to spend a little time forming the vent to fit the bonnet/hood snuggley. Most bonnets are not flat and by gently bending the raw vent to suit the shape of the bonnet, a better bond and less chance of paint cracking will be apparent at the installation stage. The laser cutter leaves behind a very small entry and exit burr on each opening which is easily removed with a small file. I also took off the sharp edges with the same file preferring a slightly rounded edge for the paint finish. The vents always have a “good side” and the underside which gets slightly scratched on the laser cutting table so be sure work out the best side for the top side. Then a couple of coats of metal etch primer from the spray can was applied.
I use a hair dryer and a warm room which greatly enhances the drying time which soon had the vent ready for the top coats. For my build I wanted to match the bonnet color which was easily supplied by my local Autobarn store. The paint code off the firewall was all they needed to mix a spray can in about 10 minutes. I applied 3 coats on the top and two underneath and then finished off with 2 coats of clear on the top. The clear coats are not mandatory as I found without matched perhaps slightly better in my case.
It is a good idea to let the paint cure for at least 24 hours before installing the vent as the paint will still be soft underneath. There are different methods of fixing the vents with glueing or bolting being the obvious. I was looking for the 280zx style which has no showing bolts but some nice fixing screws with fancy caps could also look good.
I used a simple builders glue which here is called “Selleys Liquid Nails” which is easy to use and provides a strong bond. I first laid out the vent in the finished position then laid down masking tape around the exterior line of the vent to work out how much room I had for the glue. It is important to apply an even bead close to the edge of the cut that is not too thick so as to escape out the sides onto the bonnet once the pressure is applied to the vent. I placed a towel over the vent and then placed several lead weights on top and left for 24 hours. You could smear something like butter around the edges where the bonnet meets the vent in the case of over application of the glue which is then easily removed later without affecting the paint finish.
The next step was to install an under grille which is up to the individual. I had some spare black expanded aluminum left over from my front end mod which I quickly trimmed up with scissors and then fixed on the underside with builders glue again. Once dry the glue can be painted by hand from underneath with black paint to achieve a better finish.
If I can find an appropriate rubber or plastic edging it would finish of the underside.
The remaining task is to polish the top side paint finish which I did with some cutting paste and then polish both done by hand with a soft cloth. The end result I think looks great and also will be useful when the hotter months approach.