Selecting the Right Air Cleaner for Your Car

Also interesting is that Westech's Steve Brule mentioned that on previous dyno tests with the XStream Air Flow Top that it seems to work best when combined with flat-base air cleaner assemblies. Conversely, he has seen a slight power loss when the same lid is combined with drop-base air filter assemblies. We did not test a drop-base air filter assembly to verify this point.

This is the entire K&N Flow Control air cleaner assembly complete with a 14x4-inch K&N filter. This combination made 575 for peak power that was 8 hp over the baseline, but also improved the torque throughout the entire curve.

The Flow Control system clearly worked, so we played around with various pieces until we tried a flat base, a 14x4-inch K&N filter, a Stub Stack, and topped off with a 14-inch-diameter X-Stream airflow top. This package really helped direct air into the engine, and the big Rat responded with 581 peak hp and more torque almost everywhere. The average power ended up very close to the Flow Control system in Test 3.

This innovative car crafter combined a 14x3-inch filter with an XStream lid and then added a plastic snorkle to the underside of the hood to pull air in from the cowl.

At the very end of our test, just for fun, we tried a small 9-inch-diameter air cleaner assembly equipped with a 2-inch-tall paper air filter element, which cost 35 hp over the best combination. This is an indication of what can occur when you do not pay attention to little details like air cleaners and filter elements when building your fast street car.

We learned that paying attention to air cleaner design and especially to how air enters the carburetor can pay huge horsepower benefits as power numbers continue to climb. The overall key appears to be ensuring sufficient space above the carburetor to allow air to enter the carb (or throttle-body) without creating turbulence that can cause power to suffer. It appears that maintaining roughly a 2- to 3-inch clearance above the top of the carburetor is a great way to offer your engine every opportunity to make power. This is not always possible with low hood levels and tight engine compartments. But the fact remains that should you choose to restrict the inlet air into your engine, then you can expect to lose power as a result of that compromise. It doesn't get any simpler than that.

Another way to look at this is with a simple question. How much money would you expect to pay to make an additional 14 hp?

Test 1 Baseline with no air cleaner, 0-4779 750-cfm Holley w/choke

Test 2 Flat base w/14x4-inch K&N filter and Flow Top

Test 3 K&N Flow Control air cleaner w/14x4-inch filter

Test 4 Flat base, 14x4-inch filter, Stub Stack, and Flow Top

Test 5 9-inch-diameter, 2-inch-tall air cleaner w/paper filter

For kicks, we also tried this 9-inch-diameter air filter equipped with a new paper element. We bolted this air cleaner on our Rat, and it immediately cost us 35 hp at the peak with torque losses all the way through the test! It clearly does make a difference what air cleaner you use, especially at 500 hp and above.	Certainly the paper filter cost airflow and became a restriction, but we've also noticed that this step on some air cleaner bases will kill airflow especially when used with an HP or Demon style carburetor with its radiused entry design. On a 550hp 420ci small-block with carb-style throttle-body, a 1-inch-tall step like this one cost us 10 hp at the peak.
This 351W small-block Ford could use a taller filter element to help clear the carb's choke housing.

Air cleaner assemblies or air filters don't make power, but the right combination of airflow-enhancing filter assemblies will help your engine make all the power it is capable of producing--and keeping that horsepower-devouring devil out of your details.

Parts List

All parts listed are K&N products

*With choke housing on carburetor