Building a supercharged engine with a traditional Roots-style GMC blower is about the quickest way to add power and draw attention to your vehicle at the same time.
Other types of superchargers may be more efficient and fit comfortably under the hoods of most cars. But if you don’t mind cutting a little sheet metal to clear a fat blower case, and you don’t mind bending your neck occasionally to see around that obtrusive air scoop–then a “Jimmie” blower is for you.
The GMC-style blower was a bit temperamental if not unpredictable when it first appeared on street machines in the ‘60s. Over the years, numerous supercharger shops have refined their setups and developed parts with tighter tolerances and increased durability. Additional improvements in supporting players like camshaft design, ignition options and carburetor tuning have more than tamed the big beast. But challenges still remain to building a supercharged engine for the street, especially if you’re looking for 900 horsepower out of a small-block!
Mike Petralia’s Hardcore Horsepower in Franklin, Tennessee, took on such a project for an owner of a sleek, black ’39 Chevy street rod. The original goals for the 427ci small block included being able to run comfortably on the street with pump gas; then, with just a few changes, switch to race gas for an assault on the salt. You see, this owner has an ambitious game plan that calls for driving to Bonneville, making a top-speed run, and driving back home.
“While ‘trailer queen’ show cars are still very popular, we’ve seen a definite push of owners wanting to prove that there’s more than spit and polish under their hoods. We’re getting requests for a lot more 600- to 850-horsepower pump-gas engines from people who want the whole package,” says Petralia.
It’s relatively easy to bolt a supercharger kit to a stock or docile engine with a compression ratio around 8:1 or 8.5:1, as long as the boost levels are kept below 5 psi. Anymore aggressive on the boost, and a much stouter bottom end is required.
Hardcore started with a Dart “Little M” small-block Chevy cylinder block that was fully machined at Dart’s Michigan facility. Numerous options are available for this block. Petralia went with the standard 9.025 deck cut down to 9.000 and a 4.125 bore. The block also includes splayed 350-sized four-bolt billet steel main caps with ARP studs for added strength.
Filling up the block are a K1 Technologies 4-inch stroke crankshaft, K1 6-inch steel connecting rods, Clevitte 77 H-series bearings, Wiseco blower pistons and Comp Cams roller camshaft. The K1 crank is forged from 4340 steel, features a .125-inch fillet radii and are nitrided for improved bearing life. Also made from 4340 billet steel, the H-beam rods are shot-peened and come with bronze wrist-pin bushings.
To achieve a modest compression ratio of 9:1, Petralia ordered the Wiseco pistons with a 25cc dish. These pistons, forged from 2618 aluminum and feature thicker material for the crown. The wrist pins are constructed with heavier walls to withstand the higher cylinder pressures under boost. Wrapped around the pistons are Wiseco GFX rings: 1.2mm stainless steel gas-nitrided steel top, 1.2mm moly second and 3.0mm oil ring. The thin rings were utilized to cut down friction and heat in the cylinders walls, without going to a low-tension oil ring, which may leak when the engine isn’t equipped with a vacuum pump.
Tech Tip: Marking The Distributor
To maximize gear alignment, Hardcore shimmed the Mallory distributor using a hard plastic spacer between the two gaskets. The distributor’s position relative to the intake manifold was also marked, should the customer remove the distributor to make a cam swap or other engine servicing. The marks will speed up the reinstallation of the distributor.
Blown engines don’t need a lot of cam. In fact, a camshaft with long duration and an aggressive overlap can hurt performance as the boost simply pushes the fresh air and fuel out the exhaust valve. Petralia ordered a custom street roller grind from Comp Cams with .630-inch lift (full specs are supplied to engine buyers only). The cam is turned with a Comp double-roller adjustable timing set and matched with Comp’s Endur-X solid roller lifters with pressurized oiling to live on the street and 7.800-inch Hi-Tech pushrods.
The Competition CNC-ported 23-degree aluminum heads from Air Flow Research boast 220cc intake runners that flow better than some larger-runner, non-CNC ported heads on the market. While some builders might consider 220cc’s too small for a blown 427ci engine, Petralia says he chose these heads based on a number of factors beyond just flow and volume – the least of which are cost, ease of installation, quality, warranty and made in the USA.
Boost retard is absolutely critical when running pump gas.
— Mike Petralia
The Fun Part
The 75cc combustion chambers, combined with the large piston dish and 10.4cc head gasket volume, result in a 9:1 compression ratio. The heads also feature a 3/4-inch thick deck, which helps head gasket seal in supercharged applications, and hardened ductile-iron interlocking valve seats. Accompanying the fully assembled heads are lightweight 2.10 x 8mm stainless steel intake valves, 1.60 x 8mm stainless steel exhaust valves, 1.550-inch OD dual valve springs (225 pounds on the seat), 10-degree retainers and keepers, 7/16-inch rocker studs and bronze valve guides. Finishing off the heads are Comp Cams Ultra Gold 1.65 roller rockers.
With the long block complete, the fun part comes from Weiand. The polished 8-71 supercharger features the traditional GMC three-lobe rotor arrangement and a 3-inch x 8mm Gilmer drive belt. Hardcore ported and polished the Weiand intake manifold for improved top-end performance. Providing fuel on top of the blower case is a pair of Holley 750HP carbs designed for supercharged applications and feature boost-referenced power valves for street use.
“The blower pulls so hard that it can create a vacuum under the carbs at wide-open throttle, which would keep the power valves closed.” explains Petralia. “These carbs can reference the power valves to the pressure in the intake manifold so they can operate as designed.”
Power valves help prevent the engine from getting fat on fuel during idle and part-throttle cruising and provides additional fuel at wide-open-throttle and high-load conditions to prevent lean mixtures that could lead to detonation. Manifold vacuum normally holds the power valve closed. When the throttle is opened, manifold vacuum drops, allowing the power valve to open and feed extra needed fuel to the intake.
But, the constant vacuum created by the blower under the carbs would fool a standard power valve into thinking the engine is always at idle, even if it’s screaming at 6,000 rpm. By referencing the power valve with a vacuum line from the carb to the intake manifold below the blower, the power valve will remain closed at idle. As boost builds when the throttle is opened, manifold pressure will open the valve to enrichen the overall fuel mixture.
Getting Ready For The Dyno
Final consideration on a blown engine is the ignition timing.
“Boost retard is absolutely critical when running pump gas,” warns Petralia.
Hardcore installed a digital Mallory distributor that can be programmed to control ignition timing according to boost. The new distributor is all-inclusive, featuring a built-in MAP sensor and the equivalent of a CD ignition box contained within its small-diameter housing. The timing curve is dyno-programmed for good low-speed drivability and a safe maximum advance to stay out of detonation. Mallory’s Hand-Held ignition programmer is also included with this engine so the customer can tailor the curve to their needs, such as changing boost-retard values if they add race gas and swap blower pulleys for more boost.
The engine was buttoned up with a dual-keyed Powerbond harmonic balancer, Meziere billet water pump and Chevy Performance valve covers before hitting the dyno. An early lean condition was fixed by throwing a couple sizes at the eight jets. Best horsepower on pump gas was 754.5 at 6,600 rpm with 645.5 lb-ft peak torque coming at 4,800 rpm. The blower was set at 8-percent underdrive for the gas runs, then the pulleys were swapped to achieve more boost on 110-octane race fuel.
“Unfortunately, the vacuum line to the distributor’s built-in MAP sensor kept failing at higher boost levels and testing was cut short due to the lack of timing control this caused,” explains Petralia. “Before the line disintegrated, power reached 820 horsepower at 6,900 rpm and 717 pound-feet of torque at 5,500 rpm on race gas.”
Even though the days’ tests were over,” sums up Petralia, “it’s important to note that power also moved up in the RPM range when running higher boost and race gas. This will help to keep pulling the vehicle at the top end during its run on the salt.”
To alleviate the high-boost problem, Petralia fabricated a billet “boost manifold” with compression fittings and high-quality lines to support all the necessary feeds to the distributor, carbs, and boost gauge.
With an in-your-face persona, instant throttle response and gobs of useable torque, a supercharged engine is also quite credible on the street. It can be as docile as required, then easily turned up for higher performance tasks. The keys to a successful project include a stout bottom end and careful fuel-spark tuning.