Air Filter Heat Shields

Heat shields are supposed to increase engine power by reducing engine air intake temperatures. But how do they perform compared to the cars original air box? Do heat shields make any difference to intake temperatures and how do they compare with using the cars original air box. Is the stock air box better?

We did a test comparing the intake temps with the standard air box against the intake temps of a cone filter. We tried no heat shields, basic heat shields, completely enclosed filter with cold air feed and an enclosed filter which was open to the rear of the headlight.

Cold Air Intake (CAI) Information

Cold air intakes are more expensive than a simple open filter. They can have air boxes, heat shields, relocate the air filter and more. This test evaluates the BHP benefit of using heat shields to isolate the air filter with the aim of identifying if the extra expense of a Cold Air Intake (CAI) is worth the money.

See Also: Engine Tuning Information
Performance Air Filter/Induction Kit Manufacturers
Performance Exhaust System Manufacturers
Performance Chips and 3D ECUs
High Performance Car Insurance Brokers

Heat Shields


This is a test we carried out to asses the effect heat shields have on the temperature of the air entering the engine and therefore horsepower.


Test Summary-

Heat Shields can increase power by 6.8hp on a 150hp engine due to reducing inlet air temperatures by 14C. Banks Power Figure, See references.

Induction kits, if not shielded from the heat of the engine can potentially cause power loss by drawing in hotter air into the engine.

Our best results when using heat shields cam from completely isolating the aftermarket Pipercross cone filter, with air being drawn from behind the headlight and from the fender area. Gains are in comparison to the standard air box.

Urban driving: hp gain max+ 6.8hp when using heat shields – (intake temps 14C lower with heat shields over standard air box).

Urban driving: average gain + 5.84hp with heat shields – (intake temps 12C lower with heat shields over standard air box).

Open road driving: average gain + 4.38hp with heat shields – (intake temps 9C lower with heat shields over standard air box.)

These gains solely take into consideration the hp gains due to lower temps and not the hp gains that the induction filter may also bring, in reality the gains are probably larger due to the use of a freer flowing air filter.

The figures used to calculate power gains due to the reduction in intake temperatures are taken from Banks Power, see link.

1.0 Background – Reducing the temperature of the air entering the engine will increase the horsepower.

1.1 References – a 3-5C drop should give 1hp, sources:
f150 forum,
viper club,
temp conversion calculator,
banks power,
tech line coatings

Most of these articles have temperature in Fahrenheit. (5/9 Celsius = 1 Fahrenheit), or just over half. The best figures from the articles above is from Banks Power which says that for a 5.5C drop power should rise by 1.8%.

2.0 How the test was carried out – Each test was carried out in the same manner.

The idle temperature was taken by running the engine up to operating temperature with the bonnet open.

The bonnet was then closed and the temperature of the intake charge was taken after five minutes.

To test the temperatures on the road, a route was devised which took in Urban Roads, Open Roads and Motorways.

The temperature was monitored on the route to determine the trends in the temperature changes, the max temperatures and minimum temperatures.

2.1 Equipment –

A digital thermometer.
A Pipercross performance air filter  was used, rated to 300hp.
Aluminium 3inch ducting was used for the cold air feed.
Aluminium sheet was used as the heat shield medium.
The car the test was carried out on was a Nissan Primera (P11) GT SR20DE.

3.0 Standard air box

– Idle Temp: 41C. Driving Min Temp: 36C. Max Temp: 44C. These temps will be taken as a baseline.

3.1 Trends – Temps very consistent once on the move. Temps rose slowly when stationary.

The temperatures for the test below will be + or – the standard air box temperatures.

4.0 Foam Induction Filter Alone (No heat shielding)

– Idle Temp +7C over standard air box. Driving: Min Temp +4C
over standard air box. Max Temp +10C over standard air box.

4.1 Trends – Temperature changed very quickly. Temps rose when stationary, fell sharply when on move. Higher intake temperatures than standard air box.

5.0 Foam Induction Filter With Heat Shield behind base of filter

– Idle Temp +6C over standard air box. Driving Min Temp -4C under standard air box. Max Temp +3C over standard air box.

5.1 Trends – Temp changed more slowly than with no heat shield. Temps lower in general compared to no heat shield. Temperatures generally higher than standard air box.

6.0 Enclosed Air Filter With Intake Solely From Cold Air Feed

– Idle Temp +14C over standard air box. Driving: Min Temp +3C over standard air box min temp. Max Temp -1C under standard air box max temp.

6.1 Trends – Temps generally higher. The cold air feed was routed to receive air from the bottom of the car.  The heat soak from the engine was making the feed incredibly hot and therefore heating the intake charge as it passed through it.  I would not recommend having a cold air feed routed in any part of the main engine compartment, and to keep the feed as  far away from the exhaust manifold and engine block as possible. Cold Air Intake not as effective as imagined.

7.0 Air Filter With Heat Shields Completely Isolating Air Filter From Engine Bay And Exhaust Heat – Air Flow In Around Headlight and From Inner Wing – No Ducting Was Used.

– Idle Temp -6C under standard air box. Driving: Min Temp -9C under standard air box min temp. Max Temp -14C under standard air box max temp.

7.1 Trends – Temps lower and steady. When the car was stopped at traffic lights the temperatures rose very slowly approx 1C every thirty seconds. Once on move  temps dropped very quickly.

8.0 Heat Sheild Test Conclusion

Heat Shields should be used with an induction kit to ensure the intake charge is not heated by the main engine bay. Cold Air Intakes should not be exposed to the main engine compartment, ie between the two suspension turrets and between the firewall and front bulkhead. If possible a feed should be taken from the wheel arch or from the head light and a shield should be located to completely isolate the filter from the heat of the engine and exhaust.

9.0 Extra Observations – While assembling the heat shields I noticed that the black rubber pipe that connected to air filter to the inlet manifold was getting extremely hot. Given the effect the heat had on the cold air feed, the next step will be to construct a heat shield for this pipe. This test will appear shortly.


About Air Filters

In general standard paper filters are very good and it is not easy to increase horsepower simply by replacing the filter.

Replacement panel filters will not increase the horsepower of your car over a standard paper filter (that is new), however paper filters become blocked with dust more quickly than performance replacement panel filters due to their smaller surface area and hence they can cause the power output of a car to be reduced over time. A performance panel filter will also flow less efficiently over time as it (absorbs dust) but it will not reduce horsepower as much as the standard filter.

Induction Kits

The main benefits of fitting an induction kit is increased induction roar and better low end torque, however out and out horsepower may be reduced by an induction filter due to it sucking in hot air from the engine bay. For every 10C increase in intake air temps, air density is reduced by 3% and therefore horsepower is reduced by 3%. So while an induction kit will make a better noise and make then engine bay look better it may not increase peak horsepower if it is not properly shielded from the heat of the engine bay.

Performance Air Filter Information

Viper Induction Kits/Typhoon Induction Kits and other Cold Air Intakes CAI.

Cold air intakes and sealed induction kits such as the Viper from Pipercross combine an induction kit with a sealed enclosure. This enables the engine to breathe in colder air than even the standard intake system as well as being freer flowing than the standard paper air filter element. However these sealed induction kits do tend to be more expensive than the open type induction kit but they will produce more power.


This page was last modified Jun 24, 2018 @ 5:13 pm

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