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.

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.

Supercharging our 406ci Small-Block for Super Power

The CHP Impersonator is all about making big power from a small package. We began this quest in the June 2003 issue of Chevy High Performance when we teamed Vortec iron cylinder heads with a Coast High Performance 406ci short-block. The bottom end got Probe Industries 23cc dished pistons and a 0.041-inch-thick head gasket in order to achieve a 91-octane pump-gas-friendly 9.5:1 compression ratio. When set up with a hydraulic flat-tappet single-pattern Lunati camshaft sporting 230 degrees of duration at 0.050-inch lift and 0.480 inch of maximum lift, an Edelbrock Performer RPM Air Gap Vortec intake manifold, and a 750-cfm HP Holley carburetor, the Impersonator punched out 525 lb.-ft. of torque at 3,500 rpm and 428 hp at 5,000 rpm. The small-block package delivered big-block torque and handily upheld the Impersonator theme.

We replaced the Vortec castings with Airflow Research aluminum heads featuring 180cc intake runners. In order to aid the small camshaft lobes, we added 1.6:1 Lunati roller-rocker arms. While this added $1,500 to the cost of our engine, it also provided an additional 62hp and 4 lb.-ft. of torque on 87 octane. A total of 529 lb.-ft. of torque and 490hp rattled the engine dyno. After making such favorable power numbers from reasonable, mass-production parts, we figured we'd lay our Impersonator to rest. However, the readers always write, and the Impersonator project was called out once again. Many of you wanted to see us humble the 500hp mark. Since Part II already noted that using a slightly larger camshaft would easily surpass that, we decided to take an alternate route. We had cubic inches on tap, the cylinder heads were flowing some serious air, and an improved camshaft design was obvious. Until this point we had stuck to a tight budget that would have led Part III down the nitrous-oxide road. Instead we got crazy and ordered a 6-71 Weiand supercharger. This was decidedly more expensive than nitrous oxide, but we wanted our power to be on tap all the time, and what looks cooler than a 6-71 on a street motor? At once, it's a ballsy, somewhat demented, retro statement.

We began prepping the Impersonator for its additional power. The supercharger manifold uses the same intake bolts as a standard manifold, so we were able to bolt the new cast-aluminum piece in place. But once the manifold is secured to the cylinder heads, the engine is open to harmful debris. We installed the intake supercharger studs and blower-case gasket on the intake manifold before placing the blower housing on top of the manifold. At this point, it's crucial to mention that all Weiand superchargers are shipped with seals on the top and bottom in order to protect the rotors. Be sure to remove both seals and carefully lower the blower over the aluminum intake studs. The blower must be tightened in a crisscross pattern not exceeding 10 lb.-in. Over-torquing the blower case may cause it to twist and bind the rotors. Checking rotor gap is important and should be done once the case is finally in place. The clearance between rotors for this application should be approximately 0.012 inch. Anything less and the rotors may bind; anything more and they may not create the desired amount of boost pressure.

Next, you'll need to install the lower crank pulley. The accessory spacer and V-belt pulley connect directly to the harmonic damper; six bolts attach the 8mm blower pulley to the V-belt accessory pulley. Looking at the installation process from a durability standpoint, it's easy to envision how numerous parts on the end of the crankshaft will eventually overload the snout when a supercharger is added. The increased leverage of accessories combined with the twisting force of a blower will eventually create enough pressure to tear the snout right off the crankshaft.

Another common problem with supercharged applications is that the woodruff key on the crankshaft snout has a tendency to break, so order a crank with a big-block-sized snout and double keyways placed opposite one another. The additional keyway and larger snout provide a tremendous amount of strength, allowing a more aggressive blower pulley ratio. The ratio between the upper pulley and the lower pulley will determine how fast the blower will spin in relation to the speed of the crankshaft. In our case, we installed a 54-tooth lower 8mm pulley that would allow us to easily underdrive or overdrive the engine depending on the tooth count of the upper pulley. We figured that a 61-tooth upper pulley would spin the blower slightly slower than the engine and yield an 11.5 percent underdriven condition. This would keep boost levels below 10 psi and allow our bottom end to live on 91-octane pump gas.

Once both pulleys were in place, we looked to the belt-tensioner bracket. When setting belt tension, make sure the pulley slides freely across its bracket when not tightened down. The tension pulley is supposed to slide outward against the blower belt and take up as much slack as possible. It's required because the heat of the engine will relax the rubber blower belt during operation and create slack. Once the pulleys are installed in conjunction with the tension pulley, the engine is ready for its induction system.

The blower kit included a carburetor adapter plate, and considering the amount of air our 180cc-intake-runner heads and small camshaft could move, we decided on boost-referenced 750-cfm Holley carburetors. Once they are in place, it's obvious how difficult it is to set up a throttle linkage around the two dual-feed carburetors. We took the easy way out and ordered Weiand throttle linkage and a stainless steel fuel-line kit. Homemade fabrication is a low-budget option, but be warned that building one from scratch isn't as easy as the Weiand kits make it look. Both kits came with everything needed to do the job but it still took the better part of an afternoon to assemble.

In a weekend's time, the Impersonator was ready to rumble and act like a big-block once again. This time it would do so on the Vrbancic Brothers engine dyno on 91-octane fuel. We strapped the Impersonator to the dyno and added the 13⁄4-inch-primary-pipe headers. The ideal testing parameter called for a dyno curve between 2,500 and 6,000 rpm, but we figured that without a high-dollar forged crankshaft, rods, or cylinder block, we would be pushing the limit of our existing bottom end, especially on 91 octane. We also knew that detonation might become a problem...so we decided to raise the initial testing point to 3,500 rpm to help keep low-speed load issues to a minimum. Up to this point, the Impersonator had been tested exclusively at 34 degrees of total engine timing. This time we pulled the total timing back to 27 degrees to fight detonation. Firing the spark later in the combustion cycle helps keep cylinder pressure from becoming too great too soon. Once the distributor was moved and the dyno was set, we brought the Impersonator to life. The engine was warmed and put to a steady testing temperature before receiving the green light. Dyno operator Bob Vrbancic let the throttle handle fly as we watched the little big-block crank out some serious torque. Within a few seconds, the calculated power numbers muscled their way onto the computer screen: 659 lb.-ft. of torque at 4,500 rpm and 651hp at 5,600 rpm.

What made these figures so cool was that they were accomplished with only 9.8 pounds of boost on 91-octane fuel. Before making another pull, we kicked around the options: adding race fuel, increasing the blower drive ratio, or bumping the timing. The discussion inside the dyno cell started to get a little carried away in the name of power, but we soon settled down and remembered that the Impersonator was about streetable torque and hefty horsepower on a pump-gas budget.

This led to our next test. We bumped the total timing to 30 degrees, figuring the motor had already dealt with an outrageous amount of power, so a few more horses shouldn't hurt. Wrong. The additional 3 degrees of timing was just enough to push a head gasket out of the even cylinder side. Apparently, a small-camshaft 406ci with 27 degrees of total timing and 91 octane running 9.8 pounds of boost is about all the Impersonator would take. Of course, we thought about replacing the head gasket and starting the engine back up with better fuel, but we decided that chasing a race-fuel burning engine is not at all realistic for the street. It would be cool to see 700hp from a 180cc-intake-runner cylinder head, but the overall engine stress wouldn't be worth the possibility of seriously hurting the engine. We called it a day and considered ourselves fortunate to walk away with an engine that made 651hp on pump fuel below 6,000 rpm.

Part one of our Impersonator series offered the best value in terms of dollar per pound-foot of torque. Part two defiantly upped the ante and delivered an 87-octane engine capable of high-10-second e.t.'s in a properly prepared chassis. Part three has been a tale of ultimate torque. We definitely broke the budget, but we got what we payed for--ultimate pump-gas power.

Note: At minimum, a recreation of this package would demand studs rather than head bolts, suitable head gaskets, and the ultimate protection afforded by O-ringed cylinder heads. We installed the assembly without any of these precautionary measures and paid the price, but just dig that fat, oozing torque curve and how thickly it is spread across the board.