Hi Davison,

Well there are a load of different scenarios and you've not mentioned which engine. Generally speaking, the pistons and valves are likely to clash, bending and possibly even breaking bits off the latter and doing huge damage to the pistons, cylinder head and maybe the bore. It depends what speed and load the engine is under at the time as to how bad this will be and whether the engine will actually seize up. If it does, it could mean damage to the transmission as well and a very sudden and dangerous deceleration. If the engine doesn't seize, the car will still come to a pretty abrupt stop. In short, it's a dangerous situation.