I was just thinking about the problem of intake boost pressure and exhaust system pressure as it relates to spring seat pressures. I was thinking that we're not taking into consideration the pressure that is on the chamber side of the valve during the engine's four cycles in boosted operation. Just looking at the intake side. Sure, intake port pressure is higher, but cylinder pressures are also higher and at some points in the cycles comparatively much higher. When the intake valve is closing, intake port and cylinder pressures are close to equalized if the proper camming and porting is being used. During compression and power stroke, cylinder pressure is much higher than intake port pressure and during exhaust stroke, I believe cylinder pressure isn't going to drop below intake port pressure until the intake is about ready to open anyway. Valve spring pressure is most important for maintaining cam follower contact with the cam lobe, not allowing the cam follower to lose contact with the cam, particularly as it approaches the nose of the cam lobe. Another important function of proper spring pressure is to control possible valve bounce as the valve comes into contact with it's seat. I haven't traced the exhaust out yet, but I ask, just where is the problem?
There are important limits to how fast a pushrod valve train can be accelerated and slowed down. I would recommend finding out what those limits are (I can supply that to you) and carefully degree out your cam and make sure your cam maker followed those limits. If the cam lobe is profiled incorrectly, heavy spring pressure becomes a band aid solution.
When I recieved my cam, I went through the pain staking procedure of degreeing out the cam, measuring lobe lift every 2 degrees, without fancy cam doctor equipment. Plan a lot of time to do it. I was pleasantly surprised when the acceleration and deacceleration rates were right at the limit of the max acceptable rates I had read about. Spring operation can become unstable when max limits are exceeded and can explain why your pressures dropped so drastically.
I also found one exhaust valve that the bottom coil had broken off so seat pressure was below 200# on that cylinder. I spun all valves on the seat with my hand. It's a crude test but I could not feel any kind of warpage so I'm hoping valve seal is still decent and I'm not concerned enough to pull the head now. I was able to close the installed height up to 1.950 and get back to the original 275# on the seat without getting into coil bind.
