What happened
The aircraft experienced a mechanical failure involving the exhaust manifold. Hot exhaust gas escaped from the broken component and reached the firewall. This heat melted the sealant in several areas where hoses and wires pass through to the cockpit area.
Heavy black smoke, produced by smoldering wire insulation and fire shield material, entered the cockpit and cabin through these openings when the landing gear was retracted. The pilot declared an emergency with the airport tower controller. He opened the pilot's vent window, extended the landing gear, and reduced engine power. The smoke diminished somewhat but became more intense each time engine power was reapplied.
Adequate power for speed and runway alignment could not be obtained due to the increased smoke. Therefore, a hard, off-runway landing was accomplished.
The investigation
The investigation determined that escaping hot exhaust gas from the broken exhaust manifold melted the sealant from several areas of the firewall where hoses, wires, etc., pass through to the cockpit area.