2003 Mustang Cobra - Under The Hood

Horse Sense: Word that two-time NMRA Factory Stock champion Shawn Johnson has added a Paxton blower to his combination and moved to Real Street isn't much of a surprise. What is a surprise, however, is the news that Mike Washington (Shawn's friendly rival/pushrod nemesis in Factory Stock) also plans to compete in the class with a Paxton. It has us wondering whether the two might consider becoming teammates. That's highly unlikely, but could you imagine?

Shawn Johnson is a winner, plain and simple. In three seasons, he has captured points titles in two separate NMRA classes (Open Comp and Factory Stock) with performance from a naturally aspirated 4.6-liter that many people didn't think was possible. Three titles. An amazing feat, you say? You're right-it is.

What's more amazing is the fact that Shawn's Factory Stock championships were achieved with a Four-Valve engine combo that completely out-performed its competition in 2005. It made an even bigger mockery of its pushrod rivals the next season, despite rule changes (higher gear ratio and increased vehicle weight) imposed in an effort to level the playing field between the two engine platforms in the ultralimited class.

With Shawn moving on and trying forced induction in Real Street, we got the inside scoop from his engine builders, John and Mike Tymensky of Modular Performance. They gave us details about the Four-Valve bullets they prepared for Shawn's reign in Factory Stock.

The engines, as a collective, were basically just the sums of their parts. While the Tymenskys didn't use cutting-edge parts to make 375 rwhp (370 lb-ft of torque), emphasis was put on preparation of the championship-winning Four-Valves' internal pieces.

"We use aluminum blocks for the Factory Stock engines, and our build process begins with a 0.040-inch overbore, wire-brushing the block's oil passages, and opening the returns," John says. "After that, the block is decked and line-bored. Each cylinder is honed with a three-step process."

Modular Performance likes Diamond Pistons' lightweight, flat-top slugs for engines of this caliber. According to John, Shawn's pistons were off-the-shelf pieces that underwent special machining and were matched at one weight with a zero range of variance. A dry-film coating was added to the piston skirts to minimize friction.

After inspecting the eight-bolt Cobra crankshaft and Manley lightweight rods, John bead blasts, measures, and weighs the rods. They, along with the pistons, are then honed to 0.0009-inch clearance.

The gang at Modular Performance believes oil pressure and bearing clearances are interrelated, and their union is the true key to making horsepower. Federal Mogul's HP-Series rod and main bearings, a '99 Mustang GT oil pump with custom billet gears, and the factory windage tray are utilized with a larger oil pan from a Lincoln Mark VIII.

Transitioning topside, John and Mike used '03 Cobra cylinder heads on Shawn's bullets. "Not all heads flow the same," John says. "For Shawn's engines, we test flowed a few pieces to get the best and most consistent set of heads. Next, we performed a custom valve job (standard FR 500 valves and springs) and shimmed the springs appropriately to achieve proper seat pressure."

Fel-Pro's Multilayered Steel head gaskets and ARP studs were used to maintain a head-to-block seal and ensure there wouldn't be any loss of all-important compression.

Apparently 1996 was a good year for Cobra camshafts, as that's the vintage John used atop Shawn's '03 heads. "Cam timing is critical for making horsepower and torque where you need it," John says. "Each year we'd change it to offset rule changes and still make more horsepower and torque. Mike spent hours on the computer doing simulation studies to determine where the centerlines needed to be, and the research paid off with Factory Stock championships.

"The Mach 1 intake manifolds we used were tested to determine which one flowed the best and was the most consistent. We did the same for Shawn's '99 Cobra throttle body. Once we established the killer combination of induction parts, the entire works was bolted together and flow tested.

"On the exhaust side, we mounted short-tube headers to the heads and test flowed the ports. The differences between each port were unbelievable. We ultimately took the data of each runner and determined a way to cut, shape, and weld the headers so they produced a consistent exhaust flow.

"I'm sure I sound like a broken record, but the real secret behind the success of Shawn's engines is that we did extensive research testing to get the most out of each part we used."

There you have it. If you want to go for the gusto in a limited class like Factory Stock, it's more about the hard work than the cubic dollars. We can't wait to see what this team brings to our Real Street class.