Valve spring seat pressure - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 25 Old 08-08-2016, 07:45 PM Thread Starter
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Valve spring seat pressure

I know this question has been asked several times but I couldn't find anything similar to my combo.
Stock bottom end 306, f303, trickflow tw's with track heat upgraded springs. 70mm journal bearing turbo.
According to trickflows data sheet on the heads, the springs can take up to .600 lift and have a seat pressure of 125lbs.
I want to know if that's enough pressure for my combo? I don't plan on pushing more than 10-14psi. I know even at that level I'm well beyond the breaking point of the stock block.
If it's not enough pressure, how much should I be at? Just intake side or exhaust too? Just shim the springs or buy a better set? If a better set, what's a good spring?

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post #2 of 25 Old 08-08-2016, 07:49 PM
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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.


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post #3 of 25 Old 08-08-2016, 08:06 PM
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Quote:
Originally Posted by sbfnut View Post
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
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post #4 of 25 Old 08-08-2016, 08:12 PM
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Quote:
Originally Posted by KEVIN$ View Post
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
"Effectively", not in reality...when the lifter is on the back of the lobe, is #14's really gonna push around the valve? The split, fraction, millisecond that the valve remotely begins to open with the pressurized air behind the valve is so negligible that it does not matter.

Cam specs determine spring pressure, not boost.
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post #5 of 25 Old 08-08-2016, 08:45 PM
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Exhaust back pressure can open the exhaust valve.

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post #6 of 25 Old 08-08-2016, 09:00 PM
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Originally Posted by sbfnut View Post
"Effectively", not in reality...when the lifter is on the back of the lobe, is #14's really gonna push around the valve? The split, fraction, millisecond that the valve remotely begins to open with the pressurized air behind the valve is so negligible that it does not matter.

Cam specs determine spring pressure, not boost.
It's 14PSI (pounds per square inch). A 2" diameter valve will have an area of 3.14 sq-inch. Multiply that by 14psi of boost and you have approx. 44 Pounds of pressure trying to push the valve open or trying to keep the valve open. So if the seat pressure is 120lbs with the valve closed then there is a net spring pressure of 80lbs at 14psi of boost.

When running a lot of boost like I am spring pressure is highly considered.

ks
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post #7 of 25 Old 08-09-2016, 05:19 PM
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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.

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post #8 of 25 Old 08-09-2016, 05:25 PM
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Quote:
Originally Posted by y2k02 View Post
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.

MFT
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post #9 of 25 Old 08-09-2016, 07:05 PM
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Quote:
Originally Posted by DreadFox View Post
I looked into this myself awhile back when choosing a spring for my setup.
Just curious how much boost you're running?

ks


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post #10 of 25 Old 08-09-2016, 09:34 PM
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Quote:
Originally Posted by DreadFox View Post
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.
Valve float issues with weak ass valve springs used in a modular 2V
head while running a turbo setup is a known issue!
For sure not confused about wastegates vs valve springs.


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post #11 of 25 Old 08-09-2016, 11:05 PM
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Originally Posted by KEVIN$ View Post
Just curious how much boost you're running?

ks
Rings are gapped for 23 psi on a 10.5:1 SCR, cam is in the 250ish range @.050" lift. Just over .600 lift, and my heads are set up at 160lbs on the seat. It's a hydraulic roller setup.

MFT
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post #12 of 25 Old 08-09-2016, 11:13 PM
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Quote:
Originally Posted by y2k02 View Post
Valve float issues with weak ass valve springs used in a modular 2V
head while running a turbo setup is a known issue!
For sure not confused about wastegates vs valve springs.
Like I said, you've got other problems than valve spring selection.

If the springs are set up for the cam, and you're not exceeding the redline RPM of the camshaft then the addition of boost will not result in valve float. The physics just doesn't work out. Sorry. But to each is own, you can spend your money on whatever spring you want. It'll still work as long as it isn't too small! The point is if you're opening exhaust valves with backpressure then that is not anywhere near an optimized package.

MFT
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post #13 of 25 Old 08-10-2016, 08:44 AM Thread Starter
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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
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post #14 of 25 Old 08-10-2016, 10:40 AM
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Quote:
Originally Posted by DreadFox View Post
Rings are gapped for 23 psi on a 10.5:1 SCR, cam is in the 250ish range @.050" lift. Just over .600 lift, and my heads are set up at 160lbs on the seat. It's a hydraulic roller setup.
I'm no mod-motor expert but from the research that I did for my 2V motor the recommended spring pressures are not as high as Windsor type motors. This is probably why we differ in our statements.

ks


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post #15 of 25 Old 08-10-2016, 10:55 AM
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Originally Posted by DevaRoh2.0 View Post
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!

MFT
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post #16 of 25 Old 08-10-2016, 11:02 AM
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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.
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post #17 of 25 Old 08-10-2016, 12:18 PM
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Quote:
Originally Posted by KEVIN$ View Post
I'm no mod-motor expert but from the research that I did for my 2V motor the recommended spring pressures are not as high as Windsor type motors. This is probably why we differ in our statements.

ks
I don't know much about the Ford Mod Motor. It's a relatively new platform, and it doesn't have the aftermarket research and track/dyno testing behind it like the Windsor platform. I just know the spring up for boost theory doesn't hold up from a engine dynamics standpoint. My guess is there still might be some things about the valvetrain that aftermarket cam grinders and builders were still trying to work out. If the spring is marginal to begin with then throwing boost at that setup will expose that weakness thereby reducing RPM capability of a setup which results in valve float. The Spintron has made some old ideas about valvetrain obsolete. Also, the pressure differentials acting on the valve aren't static at any point during the combustion process and this is where that theory falls apart.

Stab in the dark at it, it's overhead chain driven camshafts with a bit more mass than a traditional OHV single camshaft engine. You've got a much larger and heavier chain, two camshafts on opposite sides of the block and about as far away from the crank as you can get. And these shafts are on an angle in reference to the crank. So the chain is flexing, the camshafts have rotational flex as well, then you have the harmonics from the crank as well being transmitted through the chain, plus movement from the heads and block being distorted under load. So yeah, when you put a power adder on a setup where probably the quantity of valvetrain inertia is relatively unknown you're going to run into valve float issues as you increase cylinder pressure while trying to reach the same redline RPM. What's happening is the Max RPM at which the spring can control the valve is reduced due to the increased cylinder pressure(power). The easiest fix for this is to of course go with a stiffer spring. I can totally see that. A Windsor making 600hp at 6000RPM probably doesn't require the same seat pressure as a 2V mod motor making the same power at the same RPM because the harmonic and valvetrain inertia quantities are different. My guess is it would require more spring pressure in the modular V engine? Was this taken into account early on in the aftermarket for the Ford modular engine for camshaft and valve spring selection? My guess would be not effectively, which is why valve float was seen as power output increased. Then you add aftermarket turbocharging into the mix! Yeah I could see issues arising.

Many things done in aftermarket builds are fixes for things not quite understood just yet or that have been looked over. We don't have the resources to measure much of what's going on so we rely on trial and error, and what others have done. Early on in the aftermarket, this in turn led to rules of thumb that are taken as legit theory and applied across the board today. But OEM technology innovation along with new technology available to aftermarket companies are changing the rules by giving us a better understanding of what's actually going on.

MFT
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post #18 of 25 Old 08-10-2016, 12:35 PM
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Originally Posted by TomR View Post
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.

MFT
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post #19 of 25 Old 08-10-2016, 12:54 PM
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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
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post #20 of 25 Old 08-10-2016, 01:05 PM
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Originally Posted by y2k02 View Post
Valve float issues with weak ass valve springs used in a modular 2V
head while running a turbo setup is a known issue!
For sure not confused about wastegates vs valve springs.
Another known issue are the weak valve springs on junk yard LS builds. That is why you see them changed even for a slightly aggressive off the shelf cam.

However it is not the choice in forced induction preference; rather the choice in cam shafts.
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post #21 of 25 Old 08-10-2016, 01:29 PM
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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


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post #22 of 25 Old 08-10-2016, 02:49 PM
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Originally Posted by KEVIN$ View Post
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.

MFT
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post #23 of 25 Old 08-10-2016, 03:28 PM
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Originally Posted by DreadFox View Post
I don't know much about the Ford Mod Motor. It's a relatively new platform, and it doesn't have the aftermarket research

Since 1994...22 years

Stab in the dark at it, it's overhead chain driven camshafts with a bit more mass than a traditional OHV single camshaft engine.

Umm, No. no pushrods, smaller valves, smaller lighter springs/retainers/keepers/rockers/hollow camshafts ect.

You've got a much larger and heavier chain,

single non roller chain, larger? Nope, longer? yes



So the chain is flexing, the camshafts have rotational flex as well, then you have the harmonics from the crank as well being transmitted through the chain, plus movement from the heads and block being distorted under load.

This all happens on a pushrod engine, even more so


My guess is it would require more spring pressure in the modular V engine?

Nope, smaller valves/springs/retainers/keepers ect as mentioned above

.
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post #24 of 25 Old 08-10-2016, 04:15 PM
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.
If you're going to quote me then please do so using the full and complete sentences I wrote, and also please don't assume that statements apply to one platform but not in another....

But I'll bite,
Finish the sentence, and I said "It's a relatively new platform, and it doesn't have the aftermarket research and track/dyno testing behind it like the Windsor platform." So the 22 years is still a bit young compared to specifically the Ford Windsor aftermarket. Also, so with the introduction of the modular engine in 1994, the aftermarket for this platform was already well established and churning out go fast parts in 1994? I don't think that was the case. So yeah, relatively new platform when compared to the Ford Windsor. The relatively should have given that one away but meh.

Never once did I claim to be an expert on the modular engine valvetrain or even its components. That is evident by my statements, "I don't know much about the Ford Mod Motor", "Stab in the dark at it", and " 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 was merely stating what I thought were possibilities that could contribute to valve float early on with aftermarket camshafts and valvetrain in the modular engine given the lack of widespread knowledge that is yet to be learned in nearly every new platform aftermarket.

By larger chain I mean the typical dimensions used when determining size, how isn't length included in its size? Longer then would equal larger. And I just googled the chains, and there are two of them on the mod engine, correct? The chains in the mod engine have to have more mass than a double roller Windsor chain. I've got a windsor double roller chain on my bench right now, I'll weigh it when I get home...

You said "This all happens on a pushrod engine, even more so"
When did I state that the flexing, harmonics and distortion doesn't exist in the Windsor platform? Clearly if I've demonstrated the knowledge that these forces are at play in one platform then surely I have enough sense to know that they exist in every other ICE design as well. You're doing that internet forum thing again aren't you...

You said, "Nope, smaller valves/springs/retainers/keepers ect as mentioned above" in response to my "My guess is it would require more spring pressure in the modular V engine? " I formed it in the form of a question, thank you for answering it. My question to you then is why are there issues with valve float and forced induction on the Ford Modular Engine?

MFT
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post #25 of 25 Old 08-10-2016, 08:35 PM
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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.

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