Chevy Small Block Engine - Impersonator

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.