Slant 6 Turbo 68Dart Project

Took a while to read through all of this so maybe I missed it, but I don't remember anyone mentioning anything about valve springs.

I know Shaker223 says he shimmed his valve springs and at some point he went to 340 valve springs. Looked for some links, but didn't find what I wanted in the allowable time I had, so I can't point to anything other than what I remember. What I remember is he put 340 valve springs in at the beginning, and then shimmed them later, but I don't have any documented proof, just a faulty hard drive (brain).

My understanding is, with boost, you need stiffer springs than when the motor is NA to avoid problems. Pretty sure the boost pushes the valves open, but it's been awhile since I ready anything about that so my memory might be off. Just sticks in my head that anytime you boost a motor, valve springs should be looked at to make sure they aren't too weak.

Just a guess, but I wonder if your issue with boost never climbing above 8 psi is related to this. I remember reading that the head was rebuilt, but that doesn't mean it had turbo quality valve springs installed. If they were just rebuilder quality NA slant 6 springs, they might be way too light and the boost is just pushing the valves off the seat during the exhaust stroke (assuming the compression and expansion stroke would push the valve shut).

Another thing I always remember hearing about turbo motors is a higher quality exhaust valve. Might be that during your exploration of turboing this motor, you fried an exhaust valve due to the quality of the valve used during the head rebuild leading to mystery problems.

Not questioning the head work done, just saying that if the head was rebuilt prior to the idea of turboing the motor, it might be that the components used don't fit the application any longer.

Just some thoughts.