Chevrolet 406 CI Engine Build - Impersonator II

There is no one formula to follow in building the ultimate engine. Variables such as cost, parts availability, and intended use all play major factors in the horsepower equation. In the June '03 issue, we put a big-cubic-inch small-block together that pounded out big-block torque numbers. The engine featured a 91-octane-friendly 9.5:1 compression ratio, a mild hydraulic flat-tappet camshaft, and production GM iron Vortec cylinder heads. The 406ci beast soon earned itself the name Impersonator for its small-block size and big-block tendencies.

While stump-pulling is fun, we decided that this engine would offer more with high-performance aftermarket cylinder heads. We put our thoughts in motion, crunched a few airflow numbers, ran some hypothetical testbed equations, and decided that small-runner Air Flow Research (AFR) aluminum cylinder heads would be a perfect accompaniment.

The original engine featured a Coast High Performance crate bottom end outfitted with 22cc-dish, 4.155-inch bore forged pistons designed to achieve a pump-gas-friendly compression ratio of 9.5:1 when used with a 64cc (Vortec) combustion-chamber head. The induction system included a single-pattern Lunati camshaft sporting 0.480 inch of lift with 230 degrees of duration at 0.050-inch measurements, a Vortec RPM Performer Air Gap intake manifold, and modified Vortec iron cylinder heads with an improved valvetrain. When topped with a 750-cfm Holley mechanical-secondary carburetor, this combination made an amazing level of torque and respectable horsepower for less than $5,800.

When we decided to retest the Impersonator with better cylinder heads, the idea was to make about the same torque but more horsepower than with the Vortec castings. We also wanted to up the ante and do it on 87-octane fuel. This required aluminum cylinder heads, bumping the combustion chamber volume from 64 cc to 68 cc, and ports that could move a lot of air through a small runner.

The Vortec heads feature 174cc intake runners that promote outstanding volumetric efficiency to boost low-speed torque, while airflow past the valves has carried horsepower well beyond 400. We were drawn to the competition CNC-ported AFR 180cc cylinder heads, and their 68cc combustion chamber option was exactly what we were after. We also knew that the outstanding airflow numbers would support more than 500 hp.

A switch from the Vortec head design to a standard-style small-block head requires that the rest of the induction system be converted as well. Since the idea was to change only the cylinder heads, we did everything possible to keep other related variables the same. This called for a standard Edelbrock RPM Performer Air Gap intake manifold and matching Fel Pro gaskets, standard intake manifold bolts, and standard 1.5:1 roller rockers. We had also specified them for a street-driven 400 small-block, which required steam holes. AFR offers this option so we took advantage of it.

As we strapped the engine to the dyno, we noticed that the No. 5 spark plug sat slightly closer to the No. 5 primary pipe. This required a little hammering to clearance the header tube away from the spark-plug boot in order not to burn it. Once complete, we topped the engine off with a 750-cfm mechanical-secondary carburetor and prepared for its first pull by jetting the orifice size to 0.069 on the primary side and 0.076 on the secondary side. We set the timing at 34 degrees. After a few pulls, the AFR cylinder heads with 180cc intake runners made nearly the same amount of torque as the original configuration, but it did so 700 rpm higher in the power curve, adding 49 hp to the best Vortec dyno run. Remember, these gains came from a motor with less compression, and it put a smile on the face of everyone inside the dyno room.

After making 477 hp at 5,500 rpm so easily, we wanted to see more, so we bolted on Comp Cams Pro Magnum 1.6:1 roller rockers. We knew the camshaft was relatively mild and figured that a little more lift and duration from a higher-ratio rocker would help reveal the true potential of the AFR cylinder heads. The 1.6 ratio increased lift from 0.480 inch to 0.512 inch and the duration by approximately 2-3 degrees. As expected, the increased valve lift and duration afforded another 13 hp and 4 more lb-ft of torque. With a larger-grind camshaft, there is no doubt that the 180cc AFR-equipped 406 could have made well over 500 hp and the same, if not more, torque.

We finally called it quits with a torque peak of 529 lb-ft at 4,100 rpm and 490 hp at 5,600 rpm. With power like this and a few gallons of low-octane fuel, almost any medium-weight high-performance Chevy could run an 11-second quarter-mile. The added cost of the project is the price difference between the complete Vortec heads ($640) and the AFR 180 heads ($1,924). Basically, the initial price difference of $1,248 is worth an additional 4 lb-ft of torque and 62 hp, and you can run on 87-octane fuel. If low-octane big-block torque from a small-block package gets you hot, then our Impersonator II is the engine for you.

Meaningful Power

A look at the chart shows that the AFR heads shifted the power curve up by nearly 1,000 rpm. Look closely and you will notice that the jump in power is much better than it first appears. Down low, power differences are somewhere around 15 to 25, while upper rpm differences are more like 40 to 50. If you are building a tow rig that never sees action above 4,000 rpm, the Vortec heads keep the powerband low and torquey. However, if you plan on strip time or even a blast with the guy in the next lane, you see the AFR combination will clearly outrun the Vortec engine because its power curve is more within the realm of where your engine will operate at full-throttle. The next time you are in your car, stab the throttle and look to see where the engine does most of the pulling. It will most likely be somewhere upward of 4,000 rpm, even with a stock converter.

Make It Fit

When converting from a Vortec cylinder head top-end assembly to a conventional small-block induction design, make sure to have several things on hand before you begin. You will need standard intake manifold gaskets, as well as a standard-bolt-pattern intake manifold. The standard intake requires 12 bolts, but the Vortec intake only uses 8. The Vortec cylinder heads do not use guideplates because they incorporate self-guided rockers. Conventional performance cylinder heads typically use guideplates (like our AFR's), which means that guideplates and self-guiding rockers cannot be used together. The last detail is perimeter-style bolt-down rocker covers. All pieces are relatively the same price between Vortec and non-Vortec designs.