When I bought my car, the stock airbox was gone and there was a K&N filter in open air. I went to the local metal supplier and bought a sheet of 20 gauge aluminum fo $7.50. I mocked up a heat shield with cardboard, traced it out to the aluminum, then cut the aluminum. I do have the advantage in that I have access to shears, a bending brake, and a nibbling tool at my work. I think it wouldn't be too much harder to do this with home tools but it would take a bit longer. $1.77 worth of hot water pipe insulation and a coating of black undercoat to improve insulation (I happened to have a can) and I'm done.
I know the argument that metal conducts heat so I tested the effectiveness by placing a thermometer on either side of the heat shield. Starting with a hot engine then idling for a few minutes, the filter side of the shield read 80 degrees and the engine side was unknown because the thermometer only goes up to 120. Outside temperature was 60. I expect even better performance when the car is moving but stationary performance is important to me for when I autocross.
In my opinion, if you have an exposed air filter and you race, this is an easy improvement.
I know the argument that metal conducts heat so I tested the effectiveness by placing a thermometer on either side of the heat shield. Starting with a hot engine then idling for a few minutes, the filter side of the shield read 80 degrees and the engine side was unknown because the thermometer only goes up to 120. Outside temperature was 60. I expect even better performance when the car is moving but stationary performance is important to me for when I autocross.
In my opinion, if you have an exposed air filter and you race, this is an easy improvement.
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