Chevy Small Block Engine - Impersonator

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 2003 issue of Chevy High Performance magazine , 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 500hp.

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 49hp 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 477hp 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 13hp 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 500hp 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 490hp 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).

The initial price difference of $1,248 is worth an additional 4 lb.-ft. of torque and 62hp. If low-octane big-block torque from a small-block package gets you hot, then our Impersonator II is the engine for you.

There are two Edelbrock RPM...        read full caption There are two Edelbrock RPM Air Gap intake manifolds for the small-block Chevy. The PN 7516 (Vortec) piece uses eight attachment bolts, and the PN 7501 (standard) unit uses 12 bolts.

Notice that the rocker on...        read full caption Notice that the rocker on the left is self-guided and features two rails that hold the rocker over the valve stem. The rocker on the right is conventional. It doesn't need guide rails and the pushrods are steadied by guideplates designed to keep the rocker centered over the valve.

The Vortec heads feature a...        read full caption The Vortec heads feature a 174cc intake runner, whereas our AFR castings sported 180cc intake runners...

...Because these were relatively...        read full caption ...Because these were relatively close, the torque numbers remained the same, but at over 4,000 rpm, the AFR head port design shined over the Vortec.

The carbon buildup around...        read full caption The carbon buildup around the intake port is due to the intake-runner design inside the Vortec head and doesn't necessarily mean that the combustion chamber is inefficient...

...Don't worry about porting...        read full caption ...Don't worry about porting the spark-plug boss out of the chamber.

When AFR heads are assembled...        read full caption When AFR heads are assembled at the factory, the screw-in rocker studs are not torqued down. Before running the heads, we torqued our studs to 60 lb-ft.

With 527 lb-ft of torque and...        read full caption With 527 lb-ft of torque and 490 hp, you can make almost any small-block Chevy into an 87-octane- drinking, 11-second quarter-mile hero.

We swapped off our 1.5:1 aluminum-body...        read full caption We swapped off our 1.5:1 aluminum-body Comp Cams roller rockers for Comp Pro Magnum series 1.6:1 rockers. The fact that our engine needed more camshaft resulted in 13 more peak horsepower.

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.