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Insulating the Return

Stopping engine bay heat undoing the good effects of the intercooler.

by Julian Edgar

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No matter how good your intercooler is, if the return pipe to the throttle body passes through the engine bay, the air will be getting hotter all the way back from the 'cooler. This is especially the case at small throttle openings and when you're stopped in traffic. This is so for three reasons:

  • the small amount of combustion air is flowing more slowly through the plumbing and so has more time to get warmed;
  • the intercooler is working less effectively when the car is travelling slowly or stopped;
  • the slower that the car is moving, the higher are underbonnet temps.

Add that lot together and you can easily find that the temp of the air at the throttle body is 10 - 20 degrees C higher than the temp of the air exiting the intercooler! That's got major implications when you do actually nail it - instead of cool air, the engine takes a big lungful of hot air...

The theory was proved when I measured temps on my turbo intercooled car. The Nissan Maxima V6 turbo has an underbonnet intercooler (see the installation process at "DIY Budget Intercooler Fitment") and is equipped with a fast-response LCD temp probe that's mounted just in front of the throttle body (see "LCD Temp Display!".)

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I noticed that in cruise and when stopped, the intake air temp was relatively high. In fact on a 20 degree C day it was about 40 degrees C during off-boost cruise and then rose rapidly when the car was stationary. Since the intercooler is under the bonnet, I put these characteristics down to reduced intercooler efficiency, but then one day I discovered differently. As soon as I had stopped the car, I popped the bonnet and raced around the front to feel the temperatures. The outlet tank of the intercooler was cool to touch - barely above ambient. But the pipe that connected the intercooler to the throttle body was stinking hot!

Where was the heat coming from? The answer is that this section of pipe is placed directly behind the radiator, and the hot air passing through the rad and over the intercooler pipe was doing a great job of heating the air I had tried so hard to cool down! Despite this length of pipe (which includes some rubber bits as well as the steel sections) being only about 40cm long, there was enough exposed to the radiator heat to make a real difference to the measured intake air temps. Well, so it seemed.

The way around this problem is to insulate the pipe from underbonnet heat. That way, the air inside will stay cooler, at least until it gets to the intake manifold. (Heat shields can also be used but they're most effective against radiant heat eg from a hot exhaust manifold. In this case I mostly wanted to stop conductive heat transfer.)

Insulation

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In the past on intercooler return plumbing I have used fibreglass mat insulation, wrapped in aluminium tape. This is very effective, but as the aluminium tape won't stretch, it's very difficult to do a neat job. If you have just a straight length of pipe or very open radius bends, it works fine. But try going around corners - especially sharp bends - and the whole lot looks ugly very quickly.

What's really needed is a thin, flexible, stretch insulating material that can be wrapped around the pipe while conforming to the shape of the bends. After some research I found what I think is the perfect stuff for the job - though be warned, it is quite expensive. It would be nice if you could buy the material in its raw form, but AFAIK you can't - you need to buy things already made from it.

So what is this insulating material?

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It's a cross between wet suit material and a mouse mat, if I can put it like that! About 3mm thick, it has a greaseproof dimpled surface on one side and a coloured fabric finish on the other. It is suitable for insulating at up to 260 degrees C and is water and stain resistant. And where will you buy this stuff? At your local quality kitchen goods shop, sewn into the shape of high temperature oven mitts!

The ones we used are branded 'Duncan's Kitchen Grips' and are claimed to be 'the original and the best', implying that there are copies also available. For more on these mitts, see www.kitchengrips.com. I bought a 3-piece set from Coastal Kitchen and Cutlery (Gold Coast, Australia) for AUD$76.95. The three pieces comprise two mitts, each about 40cm long, plus a 'heat pad' - a flat piece of the material 15cm square.

Fitting

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Outside of the packaging the 3-piece set looks like this. Other sets are available that comprise just a single mitt, but they cost about two-thirds as much so unless you have only a very short section of pipe to insulate, better to spend the extra.

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This is the section of pipe that needs to be insulated. As can be seen, it is directly behind the radiator and is also in near contact with one of the radiator hoses. Other points to note include the fact that there's not much room (bulky insulation wouldn't fit) and that the plumbing includes rubber and steel sections, clamped together.

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Access to the pipe was easiest with the intercooler temporarily removed, as here. Note the two sections of black pipe which were responsible for most of the heat gain - not much of them, is there?

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Here is the probe that is being used to monitor temps. The full length of the probe is inside the pipe so that a realistic figure is gained - rather than just measuring pipe wall temp, for example. The insulation wrap needed to be able to go around this probe.

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One of the mitts was attacked with a pair of sharp scissors until it looked like this. The ends have been cut off and one of the sown seams cut, forming a rectangular piece of the material. Cut off as little as possible, remembering also that if the offcuts are usefully shaped, they may be able to be used in other insulation jobs.

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Start at one end, wrapping the material around the pipe and securing it in place with a cable tie. Don't use glue - you may want to be able to get the cover off without wrecking it!

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Work your way along, stretching and then cable-tie-ing. Especially when going around corners, it can be hard to keep the edges and seam straight, so sometimes it is useful to put a temporary cable tie in place further ahead, cutting it off when the permanent ties get to that point. Here the material from both oven mitts has been used.

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The material is flexible enough that it can be easily wrapped past plumbing, or as in the case here, around the temp probe.

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Here's what it looks like with the intercooler back in place. I deliberately made a feature of the seam, keeping it on the top of the pipe and as straight as I could. The material is quite tough and the surface won't tear easily when brushed against obstacles as the plumbing is being put back into place. That said, a sharp object like a knife will easily cut it.

The Results

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So, was it worth spending the time and money? In short - yes.

In identical conditions the measured intake air temp dropped 7 - 10 degrees C. Now at a constant 80 km/h cruise on a 20 degree day the intake air temp is 30 degrees C, and when the car is stopped, the intake temp rises much more slowly.

Problem solved!

Smoke then Flames?

The material that the gloves is made from is not a true high-temperature insulation. Expose the material to naked flame, for example, and with enough encouragement it will catch fire. So it is not suitable for use as a turbo or exhaust manifold heat shield. Use it where you'd be happy having rubber hoses, and if the location is too hot for that, look at using a different material rated for higher temperatures - for example, Pyrotek Pyrojacket as shown in "The Audi's DIY Boost Control - Part 2".

What About the Intake?

If it makes sense to insulate the pipe between the 'cooler and the throttle, what about the plumbing that connects the airfilter (or airflow meter) to the turbo? If it gets hot, then yes, by all means insulate it. Remember though the point above about proximity to the hot exhaust manifold, especially if the manifold is unshielded.

Colours?
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The oven mitts can be turned inside-out so that their coloured side is featured. The dimpled black side is easier to keep clean (and suits my aesthetics!) but if you want to make a feature of the pipe insulation, you can use it in this way. In addition to Spicy Red, the Kitchen Grips come in Royal Blue, Hunter Green and Charcoal Black.

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