If the devil is in the details, then he's certainly lurking around the carburetor on your thumper big-block. Horsepower and torque are all about moving air into an engine, especially on the way to the 500-plus horsepower level. The fact is that any engine, even the lowliest lawn-mower motor, can benefit from improved airflow management.
While we know better, many car crafters consider an air cleaner as more of a nuisance than a power asset. But today, when 400 to 500 hp is relatively easy to achieve, the engine is moving a tremendous amount of air. So we thought it would be a good idea to pay closer attention to the air cleaner design and test a few parts to see if there is power to be gained.
We could have tested with a 400hp small-block, but frankly significant power differentials may have been more difficult to achieve. So instead, we went with a GM Performance Parts 454 H.O. engine equipped with a set of World Products Merlin Jenkins iron heads and a healthy mechanical camshaft (see Engine Specs) with a matching Merlin single-plane intake and a Holley 750-cfm 0-4779 mechanical secondary carburetor. On the baseline test, this Rat proved it was no slouch, delivering a solid 567 hp at 6,200 rpm.
Our test outline was simple. We baselined the engine without an air cleaner, then tested a 14x3-inch air cleaner assembly with a flat base, then went to a Stub Stack without an air cleaner, and then tested one of K&N's Flow Control air cleaner assemblies. The test of the 14x3-inch air cleaner assembly is not in our test results since it made the same power as the baseline (with no air cleaner), which meant that the standard air cleaner assembly did not cost any power.
Overall, our 570hp Rat motor responded with more horsepower on virtually all these different tests. The best combination for this particular big-block engine configuration proved to be a 14x4-inch K&N air filter with a Stub Stack along with an XStream airflow top. This combination of K&N filter parts was worth a solid 14 hp over the "no air cleaner" baseline. It's important to mention that this combination made the best peak power, but comparing the average power between Test 3 and 4 revealed very similar results.
Engine Specs
* GM Performance Parts 454 H.O. short-block
* 10:1 compression ratio
* JE custom standard bore forged pistons
* Crane mechanical flat-tappet cam, 252/262 degrees duration at 0.050, 0.554/0.554-inch lift, 110-degree lobe separation angle, Crane 1.7:1 Gold Race roller rockers
* World Products iron Grumpy Jenkins Merlin heads, 2.300/1.88-inch valves, ARP head bolts, Crane dual valvesprings
* World Products Merlin 4150 style single-plane manifold
* Holley R4779-6 750-cfm mechanical secondary carb
* Hooker 13/4-inch headers, 3-inch exhaust with Flowmaster mufflers
* MSD billet distributor, MSD-7A CD ignition, MSD plug wires
An important point (which K&N offers in its instruction sheet with the Flow Control air cleaner assembly) is that if your engine is tuned to its best lean-power combination, adding the Flow Control or any other well-designed air cleaner assembly will require increased jetting to compensate for the increased airflow and better air quality across the top of the air cleaner. Our test big-block actually recorded slightly rich air-fuel ratio mixtures at peak power with just the bare carburetor. Once we added the Flow Control and other air cleaner assemblies, the additional airflow slightly leaned the air-fuel ratio by roughly a quarter of a ratio. This probably improved the power slightly, although earlier jet changes of more than half a ratio did not improve the power output more than a couple of horsepower.
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.
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
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
