GMS Turbocharged Stroker Engine - 585-RWHP Three-Valve Mod-Motor Throwdown

Granatelli Motor Sports (GMS) has been a name synomous with Mustang performance for a long time. Not only is owner Joe "JR" Granatelli a prominent figure in Mustang drag racing and parts manufacturing, his family racing lore goes back decades.

GMS has been in the S197 market since the cars hit the streets and continues to develop products to enhance and improve the vehicle. The company started with suspension upgrades, and then released various electronic and digital controls. GMS recently expanded its reach by releasing a turbo system capable of 800-plus rwhp, with optional upgrades.

Keeping with diversity, GMS began selling CNC-ported cylinder heads for all types of modular engines last year. This year the company is keeping the aftermarket well-supplied and announced a new line of crate engines.

"We collaborated with Ken Duttweiler of Duttweiler Performance to bring modular crate-engines to the market," commented JR Granatelli. Not only can you buy virtually any bolt-on part for the Three-Valve engine from the company, but GMS can also supply a fully built engine to go along with its other parts. The engine ultimately found its way into one of the company's project Mustangs. We ventured to the GMS headquarters in Oxnard, California, to get the lowdown.

"Mod motors love boost," proclaimed a proud Granatelli. This is a guy who should know since turbo technology and boost are engrained in his family history.

For those who aren't aware, Granatelli's father and uncle were acclaimed racers and car builders who, with Mario Andretti at the wheel, won the Indy 500 in 1969 with a turbocharged combination, promptly leading to a ban on turbos for many years. While Granatelli was only an infant at the time, the technology and racing heritage was a way of life.

He worked at Paxton Superchargers in high school and college. Granatelli even owned an Indy Car team from '87-'92. He sold off his portion of the team and purchased Paxton Products, where his pet-project was the first 50-state-legal, 1,000hp-capable supercharger, dubbed the Novi 2000.

The owner of GMS provided insight into the boosted mod motor realm: "Mod motors absolutely love forced induction. Most overhead valve (OHV) motors do. The mechanical advantages of twin-cam and four-cam engines are endless. More importantly, and to the point, a mod motor gains more power per psi of boost than a conventional pushrod motor.

"For comparison sake, a '93 Cobra is rated at 230 hp from the factory. It makes 370 hp at 10 psi of boost. A '96 Two-Valve GT engine is rated at 215 hp from Ford. Adding the same 10 psi of boost brings output to 378 hp. The number seems insignificant, but consider that number is an increase across the board. If you started out with a good performance motor that made 300 hp in naturally aspirated trim, then added a blower (regardless of type-Roots, twin-screw, or centrifugal), the difference between the conventional pushrod and OHV motor would be 24 hp more.

"At the dragstrip, where 15-20 psi of boost is common, the numbers get more lopsided. A pushrod engine with 300 hp at 15 psi would make 591 hp and a mod motor would produce 615 hp. These numbers are based on an SAE standard, but when factoring in various camshaft profiles, the percentages can become even greater in favor of the mod motor."

The GMS crate engine boasts features that lend themselves nicely to big horsepower. GMS can tailor the engines to fit your requirements, but the one we covered is designed for a boosted application.

The team started with a stock aluminum Three-Valve block. Despite their aluminum nature, these blocks are extremely durable and up to the task of handling well-over 800 rwhp. GMS had the bore cleaned up and the final measurement is 3.552 inches.

In this application, the GMS test vehicle produced an impressive 585 rwhp on the in-house Mustang Dyno. Producing that amount of horsepower requires extra fuel, so GMS added the typical monster-sized fuel injectors (72-lb/hr), as well as larger GMS fuel rails and a custom fuel system designed by GMS.

The fuel system utilizes a bladder tank mounted under the hood. It keeps the returnless setup and stock tank, but the bladder acts as a separate return-style system. It is a trick setup that Granatelli brought over from his LS-engine based projects. Naturally, the ignition system is enhanced with GMS Pro Series X coil packs.

This engine ultimately found its way under the hood of an S197 Mustang belonging to Troy Coughlin of Jeg's Mail Order. It is an '07 Saleen Mustang that has received the complete GMS makeover with a full 1g-capable suspension system and the turbocharged stroker engine shown on these pages. These days, you just can't beat boost on top of a modular motor and companies like GMS make it easy.

Filling the eight holes are Manley Platinum Series forged pistons-built to GMS specs, including coated skirts. The pistons are connected to Manley forged-steel connecting rods. The army of eight slugs and rods are connected to a 3.750-inch-stroke, forged-steel crankshaft. Together the bore and stroke sizes combine for an LS-slaying 297 ci. The company also offers a larger bore size of 3.572, to bring total cubes to 302.

The short-block is complemented with a top-end that consists of GMS Three-Valve heads, which are OEM pieces that have been run through the CNC machine. The heads also feature larger valves than stock.

GMS offers four stages of heads that begin with flowing 178.5 cfm on the intake and 136.2 cfm through the exhaust ports (at 0.500-inch lift) to a race-ported set that flows 227.3 cfm on the intake and 196.4 for the exhaust, at 0.500-inch lift. The Stage 4 race numbers increase to 251.6 cfm (intake) and 207 cfm (exhaust) at a racy 0.700-inch lift. In comparison, the stock ports and valve configuration flow 170.2 cfm through the intake ports and 129.5 cfm on the exhaust side, at 0.500-inch cam lift.

The Three-Valve engine features overhead camshafts, and while the stock sticks work well in most turbo applications, GMS felt this particular engine could make more power with an aftermarket pair. The team turned to Comp Cams and its SPR line of cams. Since the Three-Valve engines feature variable valve timing, Comp offers its more aggressive cams with a Cam Phaser Limiter kit. This limiter prevents too much movement of the cam timing, allowing for larger-than-normal camshafts. We used PN 127600 camshafts, which offer an aggressive idle and help the engine scream to 7,000 rpm. These sticks measure 0.547 on the intake and 0.560 for the exhaust lift. Duration at 0.050-inch is 237/244 degrees.

The stock intake manifold is utilized, but the boost first passes through a GMS twin-bore (62mm each) throttle body. The turbo system is, of course, the single Three-Valve setup from GMS. It boasts a Turbonetics 76mm dual ball-bearing turbocharger and twin-core intercooler. The turbo system is capable of 800 rwhp in its 3-inch exhaust configuration.