seat/open pressures

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?

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

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.

Wow, those are some high seat pressures. How high can you go without collapsing the lifter?

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.

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).

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

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.

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.

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.


Ok, but that would only be signifigant if there was an extremely large pressure drop over the valve. Lets say at peak valve lift their is 20psi in the intake port, pressure probably isn't much lower in the cylinder at that point and a few psi differential isn't going to be enough to start lofting the valve unless the springs were marginal to begin with.

There are 2 types of valve float: loft and bounce. Loft occurs at peak lift or after peak lift and is generally not detrimental to power, and can actually increase power in some cases (ever hear of the "launcher" series of cams?). Bounce is when the valve closes and bounces off the seat a few times. This is detrimental to power and can be fixed by using more seat pressure.

I agree with INTMD8, that the only time FI needs more spring pressure is in a turbo motor on the exhaust side. As he said, that's the only time when you're trying to close the exhaust valve and you actually have alot of dP trying to hold the valve open, which can contribute to bounce. When you are closing the intake valve, cylinder pressure should be at or above boost, so the valvespring will not be affected.

Mike

There are 2 types of valve float: loft and bounce. Loft occurs at peak lift or after peak lift and is generally not detrimental to power, and can actually increase power in some cases (ever hear of the "launcher" series of cams?). Bounce is when the valve closes and bounces off the seat a few times. This is detrimental to power and can be fixed by using more seat pressure.

I agree with INTMD8, that the only time FI needs more spring pressure is in a turbo motor on the exhaust side. As he said, that's the only time when you're trying to close the exhaust valve and you actually have alot of dP trying to hold the valve open, which can contribute to bounce. When you are closing the intake valve, cylinder pressure should be at or above boost, so the valvespring will not be affected.

Mike

So we don't need a valvespring to close the intake?

Rich

No! The Brad Anderson blown alcohol hemi I was looking at yesterday runs over 1000 pounds open simply because of the RPM and cool factor. :|

So we don't need a valvespring to close the intake?

Rich

Not any more than a naturally aspirated motor.

I do disagree, that was a tongue in cheek question ;)

I'm with Rich on this one... you can easily calculate the additional required seated pressure on the intake valve if you know the valve size and boost amount.

Bret