What seat and open pressures am I looking for with a 224/236 xe high lift and 20+ psi boost?
Thanks, Steve

 

Anyone done this?

 

Something wrong with the spring that Comp Cams specs for that cam?
Scott

 

Run it at 150 and 330 with good quality lifters.

 

I think I'm going to try Comp's 914 spring.
Steve

 

I'm using that same cam and boost level using the Crane 99893 springs. I have mine set up at 155-160 lb on the seat, for about 400 lb open (.605"). Coil bind is at .693 lift. No problems here!

 

I picked the Comp springs (914) because of the installed height I have available (1.780").
I ordered everything today, I just hope this is my problem.
Thanks, Steve

 

I would put a bunch more seated and rate to it than that.

 

The issue here is the intake side. With 20psi there is about 60lbs of air pressure trying to keep the valve from seating. You need good seat pressure on the intake side, ~60lbs more than you would otherwise need to compensate for the pressurized intake tract.

Rich

 

The 914 spring is rated 165lb on the seat at 1.80", I will be at 1.780" installed. I also got titanium retainers for some weight savings. I am hoping this will help my situation.
Steve

 

Wow, those are some high seat pressures. How high can you go without collapsing the lifter? Can you run Schubecks at 200 on the seat?

 

Yes but the compression cycle starts at IVC and the pressure in the cylinder immediately eclipses intake manifold pressure.

I think the only real area of concern as far as spring pressure and forced induction is the exhaust side of a turbo motor as the valve is usually trying to close against double the pressure in the intake manifold, during overlap.

 

You have to look at open pressure, not seat pressure. You can run 200 lb of seat pressure on a hydraulic lifter as long as you keep open pressure down around 400 lb (maybe more). That's why I like the Crane 99893's - they have a low spring rate so the open pressure stays reasonable with alot of seat pressure.

 

I agree, though my preference in spring is the PSI CT1225ML.

 

Correct, but the force seen on the valve (due to intake air) as it is coming over the nose reduces the spring pressure. I think we all know that the spring isn't there to simply hold the valve closed.

Just for fun: the very early four-stroke engines relied on the engine's negative and positive pressures for valve actuation.