Posted in  Engine, Features, Tech Stories

The In Crowd

Stock car action at Wilmot.

Stock car action at Wilmot Raceway.

From cup to claimer, racers all have one thing in common – they all want more power.

Think about it. Have you ever heard a racer wish they had just a little less power?

Not bloody likely.

The problem with making the proverbial MORE power is that class rules often make finding the hidden horsepower difficult, if not downright impossible. The more restrictive the rules, the more difficult it is to gain the advantage over your competitors.

One area often overlooked by racers is the induction system. This is especially true when racers have been running the same combination year after year, and even more so when the rest of the field runs the identical set up.

If it works – don’t fix it, right? While there is something to be said for a successful combination, this line of reasoning also limits potential power gains.

To illustrate the power gains offered by modifications to the induction system, we yanked a class-legal, Street Stock motor from the engine bay of a local racer and subjected it to a series of dyno tests. Check out the results of the header test elsewhere in OneDirt, but this test session revolved around the intake system.


The test motor was a mild 350 Chevy equipped with a stock Vortec long block, COMP circle-track cam profile, matching valve springs and guided, roller-tip rockers. Also present was a Speedmaster, dual-plane Eliminator intake manifold.

Per the class rules, the motor was sporting a 500-cfm, 4412, 2-barrel Holley using a phenolic 2-4-barrel adapter. Naturally the 500-cfm 2-barrel limited the power production of the combination, but that doesn’t mean there wasn’t a little power hiding in the carburetor and/or carb adapter. As this test would show, there was actually more than a little.

The first order of business was to run the motor with the 4412 carburetor and phenolic adapter. The motor was configured for dyno use with an MSD distributor, 1 ¾-inch, long-tube, Sprint-car style headers and a Meziere electric water pump.

Run on the dyno, the modified 4412 provided a near-perfect air/fuel curve and allowed the 350 to produce 358 hp at 5,700 rpm and 384 lb-ft of torque at 4,300 rpm. The combo produced a solid and useful torque curve, but we hoped there was even more power hiding in the induction system.

First up for testing was the carb adapter, so off came the phenolic spacer to make room for the aluminum 2-4-barrel adapter from Wilson Manifolds. Available in different heights, the adapter provided a smooth transition from the 2-barrel carb to the opening in the 4-barrel manifold. Obviously Keith and the boys over at casa de Wilson knew a thing or two about airflow, as the adapter netted a quick and easy 9 horsepower. Running the Wilson piece, the power output jumped from 358 hp and 384 lb-ft of torque to 367 hp and 390 lb-ft.

The final induction test of the day was to replace the tried-and-true 4412 with the latest and greatest version, the 4412 XP. The new Ultra XP 2-barrel meets all the specifications (and measurements) for 500-cfm, 2-barrel classes, but offers a number of desirable new features.

The XP featured lightweight, aluminum construction, increased fuel capacity in the bowls and even internal baffling to help control fuel slosh. The hard anodized coating and billet metering block and base plate helped ensure a long service life.

The new carb certainly looked the part, but how did it perform compared to the old stand by? Run on the dyno with the new XP Holley, the power output of the 350 jumped from 367 hp and 390 lb-ft to 382 hp and 399 lb-ft. The new carb improved the power everywhere, and with no change in the air/fuel mixture.

If we look at the combination of the Holley carb and Wilson adapter, the induction tricks increase the power output by a solid 24 hp. Now go ask any racer what they would give to have an extra 24 hp?

Graph 2Graph 1: Carb Adapter Test-Phenloic vs Wilson Manifolds

 Is it obvious from the results that the boys at Wilson manifolds know a thing or two about airflow. Replacing the phenolic 2-4-barrel adapter used by many racers in the class with the aluminum Wilson spacer resulted in some serious power gains. Power gains in classes restricted by 2-barrel carbs are often difficult to come by, but the Wilson spacer improved the power output of the mild (class legal) 350 from 358 hp to 367 hp. Note that the gains increased with engine speed above 4,000 rpm.

Graph 2Graph 2: Carburetor Test-Modified 4412 Holley vs New 4412 Ultra XP

The power gains offered by the new 4412 XP 2-barrel carb from Holley over the original were impressive. The original 4412 used for the test was no slouch, having been modified previously and then jetted to perfection. The carb served well on the race motor for three solid years, but the dyno does not lie. Replacing the modified 4412 with the new Ultra version resulted in a jump in power from 367 hp and 390 lb-ft of torque at 382 hp and 399 lb-ft of torque.

Once again, tricks applied to the induction system netted serious improvements in power.

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