Fitting Bonnet vents to a XJ

[Jimmyb]

I was always getting complaints from Julie (my wife) about how hot the passenger side floor was. So I did a little bit of investigating, on air movement in and around the engine bay of my XJ, and found that due to the lack of space in the engine bay and combined with the front diff guard, I discovered that all air movement out of the engine bay was via the transmission tunnel.

Additional extra’s in engine bay consist of twin battery setup (driver’s side) and relocated washer bottle and snorkel inlet tube (passenger side). It really is amazing what you can learn using a little bit of knitting wool and gaffa tape, and a video camera mounted under the bonnet focused on the bits of wool.

I looked around for various types of vents everything from the Falcon vents (XR6 & through to real fancy expensive ones obtainable from Odyssey Auto Sports in Newcastle ($130.00 a pair). I called into AutoPro in Bendigo the other week and off handed asked them if they had or could get any, the short answer was yes we can and showed me a picture of them. It took them three days to arrive and $29.00 later, I took possession of them. The only thing you have to do with them is open up the holes in the vents. I started out with a variable speed drill and several sized bits and got stuck into it drilling lots of small holes (see picture 1) and you can see the multitude of hole that I initially drilled.



I later used a dremmel and a rasp bit to connect the holes (to open up the slots full width) this allowed an even higher airflow out of the engine bay.

Installation (ground work).

First off I made two templates (out of cardboard) the full size of the vent, and one the size of what would become the actual cut opening I then pierced both templates (when put together) with a drill bit so that when working with the templates both the inner and full size would line up correctly. Lifting the bonnet I next used a “sharp Stanley knife” to cut away the sound deadening material from the area where the vents would sit (see above photo) and consigned the cut out pieces to the bin.

Placing the inside template against the underside of the bonnet I tentatively marked the bonnet with a white wax pencil (chino graph) where the centre punched hole would be and got out the tape measure to take some measurements so that I could line up the upper template with the underside template Upper template sits 3 5/8 inches from the back of the bonnet and 6 3/8 inches from the side edge of the bonnet (unless you have a great memory write these measurement down as you will need them later), mark top of the bonnet once again through the centre hole with a wax pencil (or similar) and then use the wax pencil to draw round the full sized template making sure it matches up with the measurements. Now is the take the smaller template and place it on top of the bonnet and set it up so that it corresponds with and aligns with the pattern drawn on your bonnet. Now stand back and have a good look at it all so that you know it is positioned correctly.

When satisfied that every this is right get out your centre punch and where you put your centre mark on the top of the bonnet and give it a light whack with a hammer. Time for the drill and drill that centre hole using the same bit that you did your templates with, and drill a hole downwards in your bonnet (cover engine with a lump of fabric or something to catch all the swarf. I used a lump of 2 inch by 6 inch Oregon resting across the engine bay to lower the bonnet onto), this made a secure rest to work against.

Now is the time for some 2 inch wide masking tape and mask up the upper side of the bonnet (see picture below)



Now you get to go through the aligning bit all over again. Check the inner template against the under side of bonnet for all round clearance, the top photo shows my white lines underneath. ”Make sure that you use the small template”
Once you have everything lines up again draw you cutting line on the masking tape (hint: mask up wide enough round so that when saw is going round corners the saw plate stays on the tape). Now is the time for that nervous drink and a sit down, contemplate what you are about to do (a small hole is easy to fix but a whopper like you are about to make is not so easy).

Use a short metal blade in your Jig / Sabre saw 32TPI (teeth to the inch) and start cutting, let the saw do the work, in other words do not force it; this will prevent the saw from over heating and stressing the sheet metal. When finished go around and repeat the same on the other side, and when you are finished you will end up with several large holes in your pride and joy.See Pictures below

Now that the hard part is finished it is time to remove the leftover masking tape and clean up the bonnet, ensuring that all the metal swarf is removed and then a file to remove all the burrs round the edge of the holes and to make these edges nice and smooth, closely followed by some black kill rust paint to protect the raw edges.
As these vents are self adhesive, after the paint had dried I used methylated spirits to clean the paint where they were going to be placed, now is a good time to find those measurements that you wrote down. So that you can place vents onto the bonnet and align them (once that exposed tape hits the metal it is no good trying to move them as they will be STUCK fast).

Then using your wax pencil when you are satisfied that vents are right where you want them draw around the outline of them. (Hint: place vents where you think they should be and walk round and study them from all angles I even sat behind wheel and looked). Now you can lift the backing tape (after giving metal another light wipe over inside your drawn line) and carefully position the vent and press it into place, once done now do the other one the same. Now you can wipe off your pencil lines from around the edge.

When both are stuck down you can sit down and drool over your newly acquired addition ventilation of you engine bay. I ran my engine this morning (12 noon Sat 15th Nov) for 1 hour idling in neutral with air conditioner on max and temp gauge never moved from its normal position (outside temperature was 33 degrees), it used to gradually creep up between two and three pointer widths above the half way mark.



The vents are NIKON AUTO ACCESSORIES (AirFlow) P/No S7250 and sold through AUTOPRO.

Tools used:
Cordless variable speed drill
Jig / sabre saw
¼ inch and 3/8 inch drill bits.
2 inch wide painter’s masking tape
32 TPI metal jig saw blade
White wax Pencil (chino graph)
Black kill rust paint
Metho
Soft cloth
5 healthy shots of rum
Time taken 2 hours

By: Grant Jones