could also be a cracked head. i no its non turbo but iv struck it before. had the usual crack across the bridge of the valves but if you to the inlet out there was a huge crack up the side. it had done high mileage though. have you tried a cylinder leakage test? this blows air down spark plug hole allowing you to listen to the exhaust, intake, radiator, dipstick etc to see were it is loosing compression. this is different to a compression check where you just crank engine. you have to have the suspect cylinder at tdc on compression to test it