470HP Vortec 4200 GM Inline-Six
Jeff Wieser, Dublin CA
The hue and cry continue over the preponderance of LS engines in this magazine, and it has not gone unnoticed. So let's travel to a distant land; affect your best English accent and repeat after us, And now for something completely different. Jeff Wieser wanted exactly that for his '69 Pontiac Firebird Sprint. He wasn't happy with the original Pontiac single-overhead-cam (SOHC) six and opted for an '05 Vortec 4200 (4.2L) purchased from Parts Planet in Rancho Cordova, California. With a turbocharger and some tuning, this simple GM SOHC inliner is now making 470 flywheel horsepower and 450 lb-ft of torque with a mild 8 psi of boost while limiting the factory rotating assembly to just 5,800 rpm. Wieser attributes much of the reliable power results to tuning by Mark Romans, who used his HPTuners software to dial in the spark and fuel. Wieser thinks he can squeeze 10 pounds of boost out of the turbo to make 500 hp, but he also has plans for a new, more aggressive 4200 with forged pistons, aftermarket rods, and a later '07 head that will allow him to punch the power up to perhaps 600 hp with 14 to 15 psi. Wieser's current engine is still plenty efficient, as he reports the current 8 psi/42 lb/hr injector package delivers 21 mpg split between highway and in-town driving. The most difficult part is keeping his foot out of the throttle because it's so much fun to drive. Wieser says he did all the work in his two-car garage, with the exception of the fabbed Hogan sheetmetal intake and the upholstery. The rest of the Firebird is as nice as the engine compartment, which explains why it took three and a half years to complete. Who says you need eight cylinders to have a party?
A. Wieser started with a Walbro GSS 340 in-tank fuel pump, stock 26-lb/hr injectors, and 5 psi of boost but quickly learned he needed bigger injectors and a larger pump. He installed a Bosch 044 external pump along with 42-lb/hr injectors cranked up to 70 psi. The latest version is a set of 60-lb/hr injectors (at least he only needs six) that will be enough to keep the air/fuel ratio safe at 12.0:1 air/fuel with 10 psi of boost.
B. The underhood buzz begins with a universal Squires Turbo Systems (STS) kit that includes a 61mm Garrett water-cooled, ball bearing–fitted turbo that is ultimately aimed at 600 hp. The fresh-air inlet employs a K&N filter located under the passenger fender and plumbs boost pressure through aluminum tubing to the intercooler ahead of the radiator. Wieser uses a manually controlled Tial wastegate along with a Turbonetics blow-off valve to manage the boost pressure and protect the turbo. The stainless steel heat shield prevents the paint from melting and minimizes heat migration into the interior.
C. The base engine is an '05 Vortec 4200, but Wieser says '06 to '07 engines offer a better-flowing cylinder head with more cam timing. The engine features a 3.70-inch bore and a 4.00-inch stroke (258ci) and uses a pair of overhead cams to drive the four valves per cylinder. The 4200 also relies on an electronic throttle that Wieser has retained on his turbo version.
D. That's a Hogan sheetmetal- fabricated intake manifold that Wieser felt was necessary for better performance. The original manifold was a composite material he thought wasn't going to withstand boost pressures.
E. The front accessory drive is all stock, including the water and A/C pumps. Wieser says the A/C conversion blows plenty cold even after 5,000 miles of street driving. California Rewire constructed the custom wiring harness that integrates the GM powertrain computer with the vehicle.
F. Wieser also spent additional time plumbing a separate oil cooler and oil pressure reduction valve to limit feed into the turbo at 45 psi and prevent seepage around the oil seal for the shaft bearing. He also runs Mobil 1 synthetic, taking advantage of its superior thermal stability. It is filtered by a combination of Fram HP8A full-flow and Amsoil bypass filters that clean the oil every 45 minutes of engine run time.
G. One reason this installation looks so clean is because the majority of fabrication work is hidden underneath the engine. Wieser says the Vortec 4200 is both longer and taller than the original SOHC six, so he had to take 2 inches off the bottom of the stock cast oil pan and increase the front sump area while notching the engine crossmember. He also built a custom trans crossmember along with fabbed subframe connectors.