Boost does not effect valve spring pressures. With the F303's ramps, 125# is fine as long as you don't rev the piss out of it.

Neighbor of mine had a 306", stock rods and crank, Speed Pro hyper pistons, TW170's, B303, Vortech V2--14#'s, ran 10's shifting at 6,500 through a C4, then cracked the block.

 

Boost does not effect valve spring pressures.

Yes it does.
The boost is on the back side of the valve trying to push it open so you effectively loose spring pressure.

ks

 

Exhaust back pressure can open the exhaust valve.

 

If you've got enough backpressure to push open the exahust valve then you've got a whole heap of problems unrelated to valve spring selection. If backpressure is pushing the exhaust valve open then that means you've got the wrong camshaft timing, the wrong size turbine, etc.

You might be thinking of the wastegate valve being pushed open by exhaust when using a light spring and a controller.

 

Time for my 2 cents...Old school way of thinking is that seat pressures do matter with boost because of the theory stated above about valve area and pressures acting on said valve.

But the latest technology available confirms what sbfnut has stated. Choose a spring for the cam and not for boost. I looked into this myself awhile back when choosing a spring for my setup. I was worried about what spring I needed to run, and after a bit of research, I found that in nearly every scenario builders are running springs for the camshaft selection without accounting for positive manifold pressure. The only substantiated examples I could find were roots type applications on BAE Hemis. Like 14-71 and up roots type blowers. This was due to the pressure waves generated by that type and size of blower as well as the additional harmonics transmitted through the valvetrain from huge crank driven blowers.

Like sbfnut said, the time that the valve is open is too small and the pressure increase too small to actually make enough of a difference to warrant an increase in seat or open pressure at the valve spring. The biggest obstacle that the valve springs have to overcome is cylinder pressure, and that is at an order of magnitude above manifold pressure (and backpressure) at its peak. If a boosted engine responds favorably to a higher spring pressure then that means the wrong spring was selected to begin with for the camshaft and desired operating RPM.

So basically, I just went with the spring pressure recommended for the camshaft I bought, and ignored the jazz about springing up for boost. Running more spring pressure than required to control the valve translates into unnecessary wear on the lifter which then can lead to lifter failures. If you go too light on the spring then you see broken springs. For me, the valve area equation didn't make sense to me when I first read it which is why I looked into it more. But it's all in what you want to do ultimately...you can spring up if you want, but the physics behind says it isn't warranted in most cases.

OP I'd leave those springs in there, especially for only 10psi. That isn't worth going through the heads for that. 125 on the seat is plenty for that F cam.

 

I looked into this myself awhile back when choosing a spring for my setup.

Just curious how much boost you're running?

ks

 

Thank you guys. I really appreciate all your responses. As my car sits it's getting a bigger wastegate installed and placed in a more efficient location. Should have it back in a couple days. I'll post back up with how things work out

 

Good Luck!

 

This was interesting to think about!!

The differential pressure across a valve from back side to front face matters. Differential pressure is the pressure gradient or difference in pressure from the high pressure side to the low pressure side.

We would have to know the cylinder side pressure or vacuum compared to the port side to calculate a difference, and that would only apply while the valve was not flowing air. Once it starts to flow air, the pressure rapidly equalizes on both sides of the valve.

Boost would try to lift an intake off the seat, but at that point the valve is not moving anyway. 100% of seated spring pressure is available to counter the boost. I can't see how or why boost levels applying dead head pressure far below seat pressures could have a measurable effect.

It looks to me like it would help as much as hurt, since boost could not subtract from intake spring pressure unless the valve was almost totally closed.

Exhaust would be the same. If the cylinder is less than the exhaust manifold pressure, it would try to lift the valve. If the valve lifts even a little, the pressure differential between the two sides quickly moves toward zero.

Also the stem surface area would subtract from pressure available on the stem side of the valve, since the stem is not a wall between the high and low pressure sides. The stem occupies part of the barrier wall's area and has zero opening force pressure from air, since the tip extends to a zero pressure area (the valve cover). That's really minor though, but the stem does represent a very small area of zero pressure.

I never thought about this much, but now I've decided the rumor about adding pressure is probably not accurate. Initially I just accepted it.

 

Yes! I spent the better part of 2 weeks looking into this theory and after some heavy reading came to the conclusion that it didn't have the physical science to support it. The largest(but still small) pressure differential across valve occurs at points in the combustion process where it wouldn't make a difference anyway because the valve is either opening or closing depending on which valve is in question.

 

As an engineer myself, along with many years building "hot rods" I see another instance to where Paper & Physical don't equal each other.

From my professional career, I want to believe spring pressure should be a factor with boost.
But I've had boosted cars for over 20 years and it's never been a problem. Back when I boosted an 6.0L lq9 LSx motor in 2005 it was the new cam that punished the springs, not the boost pressure.

Fast forward to my new fox and I'm running 17.2psi on springs only spec'd per the cam. If I can hit full boost in 0.5 seconds, I don't see the valves leaking much. Nor do I have any signs of significant blow by. So where did the proposed air pressure go??

Sometimes engineers need to do real world R&D and challenge the hypothesis against as-found results. For me my 20 years of R&D has me myth busting monkeying with spring pressure.

Myth: DENIED

 

So I'm curious about ones thoughts on the below combo:

Cam card spec's .500 lift, recommended valve seat pressure of 90lbs-100lbs, running 35psi of boost to 7000rpm. None of you would increase the valve seat pressure due to the additional 35psi of expected boost?

ks

 

Who did the cam? And what's the duration and ramp rate? If it's a reputable cam grinder/builder for your platform who ground you a profile that they said will turn 7000RPM with that spring spec and camshaft then that's what I would run. I wouldn't change the spring for boost. This is a modular engine? I've done zero research on modular engine valvetrain so I wouldn't know what the spring rate should look like for a given duration/RPM/lift profile.

 

I just hit the high points is all. A lot of inaccurate info that was assumed. A set of hardened pushrods for a Windsor weighs in at over 2 lbs. A pushrod engine has lifters that move up and down with the cam lobe, a mod motor has lifters that do not move with the cam, plus they are less than half the size. The overhead cam engine is inherently more efficient than a pushrod engine. Less limited by rpm due to the significant reduction in mass. It takes less energy to spin a OHC engine compared to the pushrod engine.
As to why they have valve float issues when boosted, much smaller springs and a lot less seat pressure. Plus as deliveted, boost was not a considerstion, but efficiency and fuel economy was. That is the short answer.