I tried air filter heat shields in 4 different designs to see which was the most effective at reducing inlet temps.

We want to know

How do they perform compared to the cars original air box?
Do heat shields make any difference to intake temperatures?
Is the stock air box better?

Quick Links: The Designs – Summary – The Test Procedure – Results: Standard Airbox – Cone Filter Only – Heat Shield at Base of Cone Filter – Totally Enclosed Filter, Air from Feed Only – Filter Isolated from Engine Bay Only, No Feed, Open To Front Of Car – Conclusion

The Designs

Design 1 – Cone filter with no heat shields (to get a baseline).

Design 2 – Cone filter with basic heat shields. One shield mounted at base of filter.

Design 3 – Cone filter completely enclosed filter with cold air feed tube feeding from under the car

Design 4 – Cone filter completely enclosed, but open to the rear of the headlight.

Test Results Summary

Design 4 gave the best results. Isolating filter completely from engine bay but leaving the filter exposed to the air entering the engine bay. No cold air feed was necessary.

Design 4 Compared to the Stock Air Box

The air filter heat shields can increase *power by 4.5% (6.8hp) on a 150hp engine. Inlet air temperatures dropped by 14C.
*Using Banks Power calculation of the effect of temperature drop on horsepower, See references.

Air filter heat shields completely isolating the filter from the engine bay while being open to the front of the car gave the best results in my case.

Design 4 vs Stock Airbox – The Numbers

These are the power gains versus 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.

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

The Test Procedure In Detail

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,
zx11,
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

4.2 Conclusion

An open cone filter could give less power than standard air filter, if standard air filter is not a restriction.

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.

5.2 Conclusion

Better results versus no heat shields but power could still be down versus stock airbox.

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.

6.2 Conclusion

The cold air feed was routed to receive air from the bottom of the car but the feed ran close to engine and exhaust.

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 feed not as effective as imagined, in fact, not effective at all!

7.0 Air Filter Heat Shields Completely Isolating Air Filter From Engine Bay

Air flow came from headlight and 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.

7.2 Conclusion

No need for a cold air feed. Enough air was coming around the headlight and through the fender.

8.0 Heat Shield 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 feeds should not be exposed to the main engine compartment.
Heat shields should surround the cone filter an ALL sides, except the side facing the front of the car so the filter can receive air when the car is moving. Isolate from warm air that has passed through the radiator if necessary.
If possible a feed could be taken from the wheel arch as well.
Ducting routed next to the engine should NOT be used as the ducting is heated by the engine/exhaust.

9.0 Extra Observations

While assembling the heat shields I noticed that the black rubber pipe connecting the air filter to the inlet manifold was getting extremely hot. Given the effect the heat had on the cold air feed, insulating this hose may be beneficial.

Stock Paper Air Filters vs Performance Filters

In general standard paper filters are very good and it is not easy to increase horsepower simply by replacing the filter, at least on stock engines.

Replacement panel filters will not increase the horsepower of your car over a new, standard paper filter, however paper filters can become blocked with dust more quickly than performance replacement panel filters due to catching smaller size dust particles. This 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 because it is not filtering as much dust from entering the engine.

Induction Kits

On a stock engine, the main benefits of fitting an induction kit is increased induction roar and better low end torque, however, if the filter is not shielded from the engine, out and out horsepower may be reduced 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.

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 should produce more power.

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This page was last modified Apr 14, 2022 @ 3:42 pm

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