Bonnet Spacers

In an attempt to reduce the the Underhood Temperature Readings, I thought that I would try raising the back of the bonnet to let some of the hot air escape. Quite a few cars at the drags employ this little trick so I thought I would try it out and use the Cable Free Indoor/Outdoor Thermometer to see what temperature difference it would actually make.

Fabrication - Installation - Results

Fabrication
Installing bonnets spacers is a pretty simple thing to do. You just need to make up some spacers to fit underneath the hinges for the bonnet and put them in place. I have seem some people simply use a handful of washers, but a full mounting plate spacer distributes the load more evenly. Below are some pictures of the mounting plate spacers that were made.

The mounting plate spacers are made out of 10mm aluminum plate cut to size to fit underneath the existing bonnet mounting plates. You will also need some longer mounting bolts. From memory I think the ones I used were M6 by 25mm. You will also need some washers since the stock mounting bolts have the washers physically attached to them. That little round bit of tube in the photos is to raise the existing alarm bonnet sensor.

Installation
Installation is trivial, but you do need two people.

Open the bonnet and hold it up with the normal bonnet holder.
Remove the mounting bracket on the side that is not physically held up.
Insert the new mounting plate spacer and do up the mounting bolts finger tight.
Remove the mounting bracket on the side that is physically help up. Note that someone will need to hold the bonnet up for this.
Insert the new mounting plate spacer and do up the mounting bolts finger tight.
Close the bonnet and see which way it needs to be moved (if at all) to line it up correctly.
Move the bonnet around as necessary (although the stock mounting positions don't really allow very much movement).
Tighten both mounting points.
With the mounting plate spacers installed there is about a 5-6mm gap between the sealing rubber at the back of the engine bay and the bonnet. This is just enough to allow some of the hot underhood air out of the engine bay. The negative pressure behind the lip of the bonnet should also help suck air out. Note that this assumes the space behind the bonnet is actually under negative pressure. Without a manometer to measure the pressure, this is just an educated guess. Turbulence and eddies from the bonnet lip may mean this is not actually the case...

Results
Well, the results were really quite interesting. I have a thermometer placed on the tower strut brace as shown in the picture below. I also have some previous readings of the temperature in that location before the bonnet was raised, which can be found on the Underhood Temperature Readings web page.

From the results I have now obtained with the rear of the bonnet raised it appears that the modification has actually made things worse, much worse! Previously the underhood temperature reading only ever got as high as ~45°C. Now with the spacers in place it is not unusual to see the underhood temperature breaking into the 60°C range. Also after a long drive the left hand side of the tower strut (looking into the engine bay) gets so hot you would burn yourself if you held onto it.

These results seemed counter-intuitive until I popped the hood and thought about it for a few minutes...

I suspect that raising the bonnet allows for hot air to not only escape out through the bottom of the engine bay, but also out the top (as was the plan). It would seems that the hot air takes the path of least resistance out the engine bay, and now that the rear of the bonnet is raised, that particular path of resistance is quite small. As the hot air off the headers (actually very hot since they aren't wrapped) passes by the intake manifold and anything else in its path, some of that heat gets absorbed by those components. Those components includes the intake, throttle body and intake manifold. With the bonnet securely shut, the very hot air doesn't get much chance to get anywhere near them.

In particular the left hand side of the engine bay gets a lot hotter because there is a nice path for the hot air off the Custom B16A Headers to travel around the side of the block, over the transmission, and out the top. On some days I could actually see the heat haze coming out the left hand side when driving forward very slowing (like in peak hour traffic here in Auckland).

So, with my particularly configuration it appears that raising the bonnet does exactly the opposite of what I intended! It actually increased the temperature of the components I wanted to keep cool. So, the mounting place spacers were prmptly removed. For other configurations, perhaps where the headers have been wrapped, raising the bonnet may actually be beneficial though. Unfortunately, not in my case though.