Edelbrock Pat Musi 555ci Crate Engine - Popular Hot Rodding Magazine

Over the years, Edelbrock has been known for its high-quality cylinder heads and intake manifolds, but as the company has grown older, it has diversified its lineup of go-fast components. In the past few years we have watched the Fun Team jump into the nitrous market, suspension components, and even superchargers for various applications. Its latest venture has been crate engines. This month, we poke and prod at one the company's most popular bullets, an Edelbrock Pat Musi 555ci Crate Engine that spun the dyno to 677 hp-on pump gas. Making that kind of power in naturally aspirated trim and still include a warranty would be troublesome for some companies. For Edelbrock, it was easy, and they turned to a longtime partner, and multiple Pro Street and Pro Modified champion, Pat Musi, to help design this outrageously powerful crate engine.

The two collaborated to bring forth a powerful-yet street-worthy-combination, and launched it in 2007. Edelbrock had released version 1.0 of the bullet, which made 675 hp (EFI version) and 650 hp (carburetor on top). The first public glance at the engine came during a popular weeklong cruise where Vic Edelbrock's '67 Chevelle-equipped with the Edelbrock/Musi 555-completed the entire tour without a hiccup. It proved the engine's ability to withstand the rigors of street cruising and still retain a healthy power curve. The EFI version in Vic's Chevelle knocked down 19 mpg during last year's cruise. The car was equipped with a Tremec TKO six-speed and 3.31 rear gears. The carburetor induction saw 4 mpg less, as Edelbrock observed 15 mpg in the same Chevelle during previous tours.

Pat Musi Performance builds a 555ci crate engine sold under the Edelbrock brand. These two powerhouses teamed up several years ago when Musi made the switch to Edelbrock Nitrous. Since that time, the two companies have collaborated on EFI systems, new nitrous components, cylinder heads, and now crate engines.

Since that time, the folks at Pat Musi Performance and Edelbrock continue to think of better ways to offer this popular 555ci powerplant. This year, the team decided to change the cylinder heads from the Victor 24-degree rectangular port, to the Performer RPM XT (Xtreme) heads. "The exhaust port is in the stock location with the XT heads. That means our customers won't have to rely on a custom set of headers when installing the 555," commented Smitty Smith of Edelbrock. Not only did the XT heads allow for off-the-shelf headers, but the heads also bumped the horsepower up. The CNC-ported heads swelled the big-block's rating from 675 hp to an even better 697 hp-on pump gas-with the EFI system.

Edelbrock used EFI induction for the big rating, but we saw impressive results from a carb setup on an engine at Pat Musi Performance during our photo shoot. They officially call the carbureted combo a 676hp package, and we saw one bullet produce 677 hp. It shows how dead-on the advertising numbers are in the company's literature. While we didn't do a back-to-back test with the different induction components, Edelbrock and Musi told us that the EFI was worth 20 to 25 hp more. Musi was quick to explain the power difference between the two induction systems on this engine: "The main reason the EFI makes more horsepower is that the throttle body flows more than the [800-cfm Thunder Series AVS] Edelbrock carb." He explained how the throttle body is similar in size, but doesn't have the restrictive venturis to block the air coming in. Musi equated it to adding a Dominator to the engine, which would make a lot of more power than the Thunder Series carb, but driveability would suffer. The EFI is void of that problem, thanks to it being able to control the fuel and spark delivery.

Musi selected a Scat 4.250-inch stroke crankshaft, made of cast steel, for this engine. The steel material makes it a tough unit capable of withstanding abuse-especially from nitrous. It is one of the reasons Edelbrock offers a two-year warranty with unlimited mileage. We're already familiar with the Scat 9000 cast-steel crank, as we've been abusing this very same crank in our '68 Chevelle's 496 big-block.

Pat Musi Performance builds all the engines, and each carries a two-year warranty with unlimited mileage during that time. The engine is based on a Dart M block with a standard 9.8-inch deck height, making it fit very well under the hood of most muscle cars. Musi then bores each cylinder hole to 4.560 inches. Mahle forged pistons have been chosen, and the flat tops contribute to the 10:1 compression ratio. Musi sourced Scat for a set of steel connecting rods and a steel crankshaft (4.250-inch stroke). A Moroso oil pump and pan keep everything lubricated.

We don't doubt the toughness of the short-block; Musi told us of several 555s in service that inhale nitrous on a regular basis. Musi tested an earlier version of this engine with an Edelbrock Performer RPM nitrous plate (110 jet) and blasted the 555 to 1,018 hp and a max torque output of 1,140 lb-ft. He was happy to report that the customer still runs the engine after two years of abuse. Musi was also working on a prototype Edelbrock throttle body during our visit. He started with Edelbrock's four-barrel-style universal throttle body and is adding a nitrous port in the center. The nozzle will spray down into the manifold and will be run as a dry-shot of nitrous. That means only nitrous will be sprayed through the system, and the EFI will add the additional fuel required. It will offer a clean look while packing a massive punch-up to 400 hp Musi says.

Scat H-beam steel rods complement the Scat stroker crankshaft. The rods check into this affair with a length of 6.385 inches. A coated set of Mahle pistons was also tapped for this bullet.

"We went with a triple nickel because the bigger bore helps the head breathe better. That bore also allows a guy to go with a bigger one if he wears out the engine. The 555 is a really nice combo," Musi says. There is nothing like a joint venture to bring two powerhouses together to deliver a powerful and reliable product.

A closer look at the Mahle pistons shows each slug has a dual coating. The initial coating is on the piston for protection, while the secondary coating is on the piston skirt. The coating on the skirt reduces wear and friction. Earlier Edelbrock big-block crate engines didn't have this upgrade. BY THE NUMBERS EDELBROCK/PAT MUSI PERFORMANCE 555CI RPM XT CRATE ENGINE Engine type: big-block Chevy Displacement: 555 ci Bore: 4.560 inches Stroke: 4.250 inches Block: Dart Big M 9.8-inch deck iron Crankshaft: Scat 4340 steel Connecting rods: Scat 4340 steel Pistons: Mahle forged Camshaft type: Edelbrock Rolling Thunder Performer RPM hydraulic roller Valve lift: .632-inch intake, .648-inch exhaust Camshaft duration: 248 intake/256 exhaust, at .050-inch lift Cylinder heads: Edelbrock Performer RPM XT (PN 51559), CNC-ported Intake valve diameter: 2.300 inches Exhaust valve diameter: 1.880 inches Compression ratio: 10:1 Rocker arm ratio: 1.7:1 Intake manifold: Edelbrock Victor Jr. 454-R (PN 2902) Recommended fuel: pump gas with minimum of 91 octane Max horsepower: 677 at 5,800 rpm Max torque: 649 lb-ft at 4,800 rpm Total cost, as-tested: $12,800 (carb) $16,500 (EFI) ON THE DYNO RPM TQ HP 4,000 603 470 4,100 609 489 4,200 615 501 4,300 620 518 4,400 626 533 4,500 631 550 4,600 636 562 4,700 641 578 4,800 647 597 4,900 649 615 5,000 653 629 5,100 655 643 5,200 657 655 5,300 653 664 5,400 648 668 5,500 642 674 5,600 633 675 5,700 622 675 5,800 613 677