Cracked exhaust manifolds can cause a lean condition which can ruin an engine.
Make sure the exhaust manifold gaskets aren't leaking.... you should be able to hear when the cylinder nearest the leak (if any) fires.
Also look for big cracks in the flex section of the downpipe -mine is VERY obvious..... feels like about 10* KR and that'* with very light throttle. At least my transmission feels awesome!!!
Mine is probably cracked from when I had my motor swapped -I suspect there was not much care put into installing the motor, hense the cracked DP, leaking trans, and damaged passenger side CV axle (boot clamp was completely gone, no damage to the boot, but the axle was SHOT).