12-29-2015, 12:27 AM
Join Date: Jun 2015
Location: TAMPA, FL
Total Cats: 9
Originally Posted by BogusSVO
18psi...A stock engine is NA, so it operates in a vacuume, springtions do not have to be as much as a Turbo engine.
When you add boost, the spring now has to hold the valve closed against the boost pressure.
With a stock 1.8 intake valve, there is appox 1 sq/in.
So the first thing you do is subtract boost pressure from spring seat pressure.
So with 18psi of boost, and 70lbs on the seat ( ST single if dropped in) leaves 52lbs of spring pressure to keep control of valve motion with the cam.
Factory spring is 43, so you have 9lbs more than stock.
Efini, Once I am happy with the set up, and get back with BC on them. I will let you know.
That's the intake side. On the exhaust side, pressures might be twice that (according to the internet).
A while back, I found that above 5500 rpm, the pressure at my turbine inlet was pushing open the wastegate valve and preventing me from maintaining my target boost, even with all the air bled off. It got worse the higher my RPM climbed.
It would not surprise me to find that I was getting 30+ psi of pressure between the exhaust valves and the turbine inlet at 18 psi of boost, especially at higher RPM.
Even with the double heavies, 30 psi of pre-turbo exhaust pressure would take you back down to nearly stock valve seat pressures.
Of course, me being a log manifold kind of guy, that might be contributing to the exhaust pressure.
edit- I went with supertech double heavies on my rebuild, should have dyno results in a few months