A spark from a 2008 Ford Shelby that veered off the pavement, overturned and rolled down a hillside off Turnbull Canyon Road ignited a brush fire near Hacienda Heights Tuesday, charring 70 acres of rugged terrain and prompting the temporary evacuation of about a dozen homes. Excessive speed of the limited edition performance vehicle appeared to play a role in the accident along the winding road, and the driver was hospitalized with injuries. A firefighter who suffered heat exhaustion while battling the fire also was hospitalized. In all 300 firefighters, assisted by five water-dropping helicopters, were on the scene in a blaze-fighting effort complicated by steep terrain and extensive brush more than 6 feet high.