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!

Hardcore ordered the K1 Technologies 4340 forged-steel crankshaft with a 4.0-inch stroke and standard small block main and rod journals. Petralia also double-keyed the crank snout to ensure the dampener won’t spin off under high boost loads. The crank turns inside Clevite H-series bearings. ARP assembly lube ensures accurate torque on the fasteners. (Photos courtesy of Hardcore Horsepower)

Here’s a topside look at the Dart 4-bolt block. All the machine work was handled at the Dart factory, including the line honing, decking, 4.125-inch bore and final cylinder hone. Wiseco GFX rings were gapped as follows: .023 for top and .025 for middle. Wiseco’s 2618 forged blower pistons feature a 25cc dish, thicker material on top and pins with heavier walls to withstand boost abuse.

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. 

Bottom-end strategy

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.

The multi-layer steel head gaskets from Mr. Gasket can be stubborn when sliding over the head studs, so Hardcore positions the gasket on the block deck first, then installs the head studs.

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 4-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. 

Blown small-blocks have a sour reputation for spinning the harmonic dampers off the crank. A “rat snout” isn’t always necessary but Hardcore took the added precaution of double keying the K1 crank. A double-chain drive turns the Comp custom grind that was degreed at 110 intake centerline. Tech tip: Sometimes the lifters are so tall that they can’t be installed with heads in place, so double check before tightening down the heads.

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.

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.

The unique vehicle chassis required a custom-fabricated oiling system, prompting Petralia to order a Titan oil pump and Billet Fabrication 7-quart pan. The Titan pump is constructed of billet aluminum and features a gerotor pump design with an integral pickup, which is handy on energetic engines where there’s a greater chance of a traditional pickup cracking or falling off. The custom Billet Fabrication pan rails are constructed of 6061 billet aluminum while the pan walls are made from .090-inch thick 3003-alloy. Other features include windage screen, crank scraper and passenger-side kickout to help keep oil off the crank. The short block was assembled with Mr. Gasket gaskets and ARP hardware.

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″ 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.

AFR’s CNC-ported aluminum cylinder heads come complete with 220cc intake runners, 75cc combustion chamber and 2.10 intake/1.60 exhaust stainless steel valves with 8mm stems. The larger diameter valve springs required a little grinding on the washer and nut for clearance between the 4-6 and 3-5 exhaust springs.

Boost retard is absolutely critical on pump gas.
     — Mike Petralia

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. He also likes the intake-to-exhaust flow ratio, which helps when running a blower. Plus, the small intake runners will keep low-speed response crisp off-idle when the blower is not producing boost. The heads feature AFR’s largest intake runner with standard valve location–that is, without having to utilize offset vavletrain parts–which makes field service or replacements much easier. 

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 1.65:1 roller rockers.

With the long block complete, the fun part comes from Weiand. The polished 8-71 supercharger features the traditional GMC 3-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. 

Comp Cams Ultra gold 1.65:1 roller rockers add a little more lift and duration at the valve than a standard 1.6:1 ratio. On the bottom, seven quarts of oil are maintained by a Billet Fabrication custom aluminum pan with built-in windage screen, crank scraper and passenger-side kickout. Pushing the 30-weight Royal Purple synthetic lubricant through the block is a Titan billet pump with integral pickup.

Hardcore ported and polished the ports in the Weiand blower manifold. Again, sometimes it’s easier to install the studs after positioning the gasket.

“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 on 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.

Assembling the dual-carb throttle linkage for the twin Holleys is one of the more time-consuming chores on such a project. But patience is necessary as far too many enthusiasts want to fire up before all the details are addressed. Weiand also offers a dual-carb fuel-line kit with stainless tubes and -6AN fittings that make final assembly much easier than fabricating individual lines. The Mr. Gasket screened top blower gasket came in handy immediately when a bit of packaging material fell out of one carb as it was being installed.

On the dyno, the blown 427 peaked at 755 horsepower with eight pounds of boost on pump gas. Checking the plugs after the early runs revealed a lean condition, so all eight jets were bumped up about 5 percent. Behind the drive pulley is a Powerbond harmonic balancer.

Initial dyno pulls were made with the Weiand blower running at 8-percent underdrive. A later pulley swap for more boost was unsuccessful as a faulty connection to the MAP sensor in the distributor limited the runs on race gas. Still, Hardcore did hit 820 horsepower on one pull.

The rubber connections to the Mallory distributor couldn’t handle much more than 11 psi during the dyno test. Back at the shop, Hardcore fabricated a billet aluminum “boost manifold” to support a network of high-pressure lines and connections. It could be mounted on the engine or firewall. The connections include (from left): in-dash boost gauge (currently empty), front carb power-valve reference, rear carb power-valve reference, distributor MAP sensor, and boost supply line from rear of intake manifold.

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 6600 rpm with 645.5 lb-ft peak torque coming at 4800 rpm. The blower was set at 8% 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.”

Final thoughts

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.