Air Filter Heat Shields – Essential or Unnecessary?
If you have installed an open cone filter in your engine bay in place of the stock airbox you might wonder if air filter heat shields are necessary.
I’ve done two articles in the past on this subject, here and here and this article distills the testing results from those articles down to their key points.
Temperatures
From testing, I found open air filters/cone filters will ingest hot air when air is not moving through the engine bay. However when the car is moving the intake temps will drop quickly. The closer the filter is to the headlight the faster the temperature change.
At cruising speeds a sealed air filter will have lower intake temps than an open filter but the difference will be surprisingly small, perhaps as little at 10 degree Celsius.
In short, heat shields work best vs an open air filter, when the car is moving slowly. Which tends not to be full throttle, high RPM conditions.
Heat Shield Placement
If air filter heat shields are going to be used they should be placed between the air filter and the source of the heat first. Not over the filter, not under the filter, unless the heat source is above/below the filter.
When radiant heat is no longer reaching the filter, we can move on to eliminating convection heat transfer from the engine and exhaust to the air filter by placing shields above/below the filter.
Important
While blocking heat from the filter is a priority, so is not blocking cold air from reaching the air filter. There is no reason to place any sort of heat shield between an air filter and a source of ambient temperature air flow, unless we are concerned about induction noise.
Exception
The exception to this rule is ducting. If we have sealed the air filter from sources of above ambient air and if every horsepower is valuable, we can completely seal the air filter in an enclosure. But only if that enclosure is receiving fast moving, ambient temp air which also has the potential to make positive pressure in the intake before the throttle body/turbo. Realistically positive pressure will be less than one PSI in most circumstances. Again, if every single horsepower is valuable it may be worth the investment of completely sealing the intake. I would say the ducting feeding the air filter should have at least 50% more area than the air filter neck diameter. Otherwise the enclosure being open to air from behind the headlight and/or wheel well will probably be enough.
In Conclusion – Performance Air Filters & Heat Shields
Standard air boxes are very good, especially on modern cars. I would not change it unless power has been increased more than 20%. If power has been increased more than 20% and/or the stock intake on a specific car is known to be restrictive, then increasing the filter surface area is the next step, which leads us to cone filters and the advice detailed above.
Yes shielding an air filter from direct sources of heat will be beneficial but as the speed of the car increases, the difference in intake temps between a shielded air filter and open filter can be surprisingly small, assuming we do not have an obvious problem eg filter mounted at back of engine bay close to exhaust manifold.
Touch the different parts of your intake after driving hard, nothing should be too hot to touch and ideally nothing should be more than “a little warm”. If something is warm/hot, some heat shielding is probably called for, use the info above for placement ideas.
Testing for Restrictions
To find out if your air filter is restrictive, this video shows how you can test your intake for vacuum. If your intake is pulling a vacuum at high RPM and/or high load conditions it shows that there is a restriction somewhere in the system. Which may or may not be the filter.
