Bonnet Spacers

January 1, 2004

To reduce the temperature underneath the bonnet I tried raising the back of it to let some of the hot air escape. Quite a few cars at the drags employ this trick. I used a Cable Free Indoor/Outdoor Thermometer to measure the temperature difference caused by raising the rear of the bonnet and the results were unexpected.


Putting in bonnets spacers is reasonably straight forward. First you need to make some spacers to fit underneath the bonnet hinges and then install them. I have seem some people use a number of stacked washers as spacers, but a proper mounting plate spacer distributes the load much more evenly.

Bonnet spacers for Honda Civic SiR

Bonnet spacers for Honda Civic SiR

The mounting plate spacers are made out of 10mm aluminum plate cut to fit underneath the bonnet mounting plates. Some longer mounting bolts were needed - from memory I think they were M6 by 25mm. Some washers are also needed as the stock mounting bolts have the washers physically attached to them. The little round bit of tube in the photos is to raise the existing alarm bonnet sensor.


Installation is trivial, but you do need two people.

  1. Open the bonnet and hold it up with the normal bonnet holder.
  2. Remove the mounting bracket on the side that is not physically held up.
  3. Insert the new mounting plate spacer and do up the mounting bolts finger tight.
  4. Remove the mounting bracket on the side that is physically help up while the helper holds the bonnet.
  5. Insert the new mounting plate spacer and do up the mounting bolts finger tight.
  6. Close the bonnet and see which way it needs to be moved (if at all) to line it up correctly.
  7. Move the bonnet around as necessary - although the stock mounting positions don’t allow very much movement).
  8. Tighten both mounting points.

With the mounting plate spacers installed there is 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. UPDATE: this might have been an incorrect assumption. It’s actually quite likely that the base of the front windscreen is a positive pressure area due to the turbulence and eddies from the bonnet lip.


The temperature change results using the thermometer placed on the tower strut brace revealed that the modification 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. 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.

Cable free indoor/outdoor thermometer on tower strut

These results seemed counter-intuitive until I popped the hood and thought about why that result might have occurred. Raising the bonnet now allows for hot air to escape out the top of the engine bay (as was the plan). The hot air likely takes the path of least resistance and that path seems to have the intake manifold and throttle body in its way. Heat out of the hot air gets absorbed by those components - the very things that need to be kept cool!

With the bonnet securely shut, the very hot air doesn’t get much of a chance to get anywhere near them. The left hand side of the engine bay appears to get a lot hotter because there is a nice path for the very hot air from the custom B16A headers to travel around the side of the block, over the transmission, and then 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!

The mounting place spacers were promptly removed, but for other configurations, perhaps where the headers have been wrapped, raising the bonnet may still be beneficial.

This exercise is an example of where it pays to know what was happening beforehand and then comparing to the results obtained afterwards.

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