the Bullitt in the use of the Cobra's Brembo front brake rotors, unique Tokico gas shocks and struts, and lower and stiffer springs, the Mach 1 received a huge performance gain over the base GT and even the Bullitt in the form of a unique variant of the DOHC 32-valve 4.6 L Modular V8. Commonly known by Mach 1 owners as an "R" code DOHC, (for the unique VIN engine R code) this all-aluminum engine features the same high flow heads as the 2003–2004 SVT Cobra, 2003–2004 Mercury Marauder, 2003–2005 Lincoln Aviator, and the 2003–2009 Australian Boss 5.4 L V8s (see Ford of Australia Boss 5.4 L), the engine also has intake camshafts sourced from Lincoln's 5.4 L "InTech" V8 to provide more mid-range torque. The Mach 1 engine had a 10.1:1 compression ratio in contrast to the 1999 and 2001 Cobra's 9.85:1, and the Mach 1 was equipped with a Windsor Aluminum Plant or WAP block unique from the Teksid aluminum blocks used in the 1996–1999 Cobras. The Mach 1 also featured a relatively high redline of 6,800 rpm (5-speed cars) and fuel cut off at 7,050 rpm or 5800 rpm (4-speed automatic). While on paper the 305 hp (228 kW) ratings seem a loss when compared to the 1999 and 2001 SVT Cobras which produced 320 hp (239 kW), in practice the Mach 1 engine produced similar peak horsepower and substantially more torque. Further differences included the use of Ford's solid rear axle with a 3.55 final ratio (As opposed to SVT's Independent Rear Suspension) also the availability of a 4-speed automatic in addition to the Tremec sourced 5-speed manual. Factory steel "Box" cross-section subframe connectors were also added to increase chassis strength for both the added handling and to deal with the prodigious torque over the stock GT. Style-wise, the Mach 1 was distinct from other Mustangs as it drew heavily from the 1970 Mach 1. In addition to the matte black spoiler and hood stripe, flat black chin spoiler, Mach 1 rocker panel stripes, and Mach 1 badging on the rear, there were also faux Magnum 500 polished 17×8-inch alloy wheels. A retro-themed interior was included with well-bolstered dark grey leather seats featuring 1970s style "Comfort Weave" textures, a 1970s style gauge cluster, and a machined aluminum shift ball. An optional 18G interior upgrade package included stainless steel pedals, a 4-way head restraint, aluminum finished shift