The same is true of the 9.5-lb/hp rule. NASA allows any engine modification in American Iron, so you can make all the power you want and play with titanium connecting rods if that's your thing. However, there's no sense in going too crazy with the power mods. For one thing, you'll only have to add weight. Second, another rule allows only 100 pounds of ballast, so you can't spend a million dollars on a carbon-fiber Mustang, then add lead ballast over the right rear corner.
Practically speaking, by the time a Mustang is lightened by simply jettisoning the usual undesirables (spare, jack, sound deadening, all upholstery, seats, air conditioning, heating, sound system, and so on), then adding the required six-point rollcage, fire extinguisher (on-board fire systems are recommended but not mandatory), and whatever chassis bracing and suspension mods are desired, a dry weight of 2,900-3,000 pounds is about as low as you'll easily get. NASA weighs with the driver, so assuming a 3,000-pound car and nearly 200-pound driver, the resulting 3,200-pound car allows 337 hp at the rear tires. This is more than enough to hold your attention on a road course-even big, open venues such as Willow Springs.
Now, you certainly don't need titanium connecting rods to make 325-350 hp. An Edelbrock intake and Holley carburetor will do the job, as will Twisted Wedge heads and EEC IV. Furthermore, you may prefer a simple 5.0 engine, or perhaps you have a Cobra with a Four-Valve 4.6. Any of these combinations are legal, and mixing and matching chassis and engines is allowed. Thus, you could build a Fox-chassis with Four-Valve modular power or vice versa. All engines can be EFI or carbureted, but do expect to run some sort of muffler, as there are sound limits. Catalytic converters are not required, of course.
The tricky spot in the 9.5-lb/hp rule is determining the horsepower. NASA admits it's something of an honor system, but not that much! Chassis dynos are the main lie detectors, with the car owner submitting a dyno report no more than 90 days old at the beginning of each season for the engine configuration he'll be racing. During the season, random dyno testing by NASA keeps things from getting out of hand. Furthermore, it doesn't take long for competitors to spot suspicious power increases, and the racing we've seen is extremely close. There is no claiming rule.
Suspension-wise, the same laissez-faire doctrine continues to reign. Basically anything goes as long as it fits under the stock bodywork. So not only are enhancements to the stock suspension, such as springs, shocks, urethane bushings, and so on legal, but also are the replacement torque-arm suspensions from Griggs Racing and Maximum Motorsports. These two companies have found American Iron an open battleground, with cars from both camps constantly atop the win column in AI results. In fact, so dominant are these suspensions that simple bolt-on cars are really not competitive, although such racers can still come out and have a blast trying.
A special niche in the suspension rules allows IRS Cobras, but all other vehicles must be live axle. In other words, no Corvettes, and no live-axle Mustang can switch to IRS, although IRS Cobras can swap to a live axle, if desired. So far, all the winning has been done by stick-axle cars, and the only late-model Cobra to run has opted for a live axle. At press time, however, the feeling was there may be an advantage to the IRS after all. The ability to experiment with suspension, brake, and powertrain combinations such as these is a great advantage of American Iron.