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Valve spring setting for turbo boosted engines ?

General engine tech -- Drag Racing to Circle Track

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Levon Pentecost
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Valve spring setting for turbo boosted engines ?

Post by Levon Pentecost » Mon Feb 16, 2015 1:33 pm

There are many opinions, but I have found no precise formulas. Lobe profile should have some input, rate of boost increase, active mass, and probably a few others. Can anyone help. Thanks ](*,)

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Re: Valve spring setting for turbo boosted engines ?

Post by roadrunner » Mon Feb 16, 2015 3:31 pm

Here's some info on the intake side, but nothing about the exhaust valve as this site is pertinent to crank driven superchargers.
Valve springs
The valve spring tension, both when the valves are seated and open, is normally a function of maximum engine speed, valve train component weight, and the rate at which the cam opens and closes the valves. To this must be added some compensation for the fact that, under boost, intake charge pressure will oppose the intake valve’s closing. Vizard suggests that a useful figure is 75% of the intake valve area (OD × .7854) multiplied by the maximum boost pressure. E.g., a 1.50” intake valve has an area of 1.767”. With 10 psi boost, the figure becomes 17.7 lbs.; 75% of this is roughly 13 lbs. Add this amount to the seated pressure.
I don't know if you can extrapolate this math to the exhaust side, maybe if you knew what the exhaust manifold backpressure would peak at you could get an idea?
Taken from http://victorylibrary.com/supercharger/ ... ngine5.htm
Probably whoever provides your camshaft should advise required valve spring specs?
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Re: Valve spring setting for turbo boosted engines ?

Post by ptuomov » Mon Feb 16, 2015 4:03 pm

This is one area where I need some education as well.

I've never understood any need for increased spring loads on the intake side with turbos. I also have never in my very limited experience seen earlier valve float in terms of rpm with boost.

The reason why I don't understand why boost would lead to higher required spring loads is the following. Intake spring is needed when the intake valve is decelerating, to keep the valvetrain on the cam lobe. Right? By my understanding the higher intake manifold pressure at that point is largely offset by higher cylinder pressure. When intake valve is closed, the cylinder pressure should be higher than the intake manifold pressure in any sort of sensible engine, no?

I can kind of see how a small exhaust side might require some more spring in some limited circumstances. If the exhaust manifold absolute pressure is something like 2.5x the intake manifold absolute pressure, the exhaust valve may not want to close at the cam EVC event. Although some might argue that such a turbo setup might have bigger problems than the valve springs...

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Re: Valve spring setting for turbo boosted engines ?

Post by roadrunner » Mon Feb 16, 2015 4:36 pm

Vizard has pointed out that the kinetic energy of the intake air is much higher than often realised when high velocity ports are used, and that the air tends to "pile up" at the intake valve as it closes, creating a high pressure area. I don't recall the actual figures, but 7 psi for a well tuned N.A. engine does come to mind. Obviously boost + cool, dense air is going to multiply the port pressure at intake closing, as will the Helmholtz effect when it's doing its thing.
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Re: Valve spring setting for turbo boosted engines ?

Post by Orr89rocz » Mon Feb 16, 2015 5:06 pm

Not sure i buy the requirement of more pressure. There may be more pressure on intake side but there is alot more pressure on exhaust side in most turbo combos. Motor should still behave somewhat like a na motor with more restrictive exhaust except much denser air is being used

I turn 7200 rpm under 25 psi with a hyd roller over .640" and didnt do much more pressure than normally needed given the valve weight and spring/retainer mass. Vizard spec would never work on hyd roller lifters

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Re: Valve spring setting for turbo boosted engines ?

Post by Warpspeed » Mon Feb 16, 2015 6:30 pm

roadrunner wrote:Vizard has pointed out that the kinetic energy of the intake air is much higher than often realised when high velocity ports are used, and that the air tends to "pile up" at the intake valve as it closes, creating a high pressure area..
That is nonsense.
Think about it.
You close the intake valve optimally when the incoming flow falls to zero due to the meeting the rising piston.
If air is still roaring into the cylinder when you close the intake valve, you are closing the intake far too early.
Cheers, Tony.

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Re: Valve spring setting for turbo boosted engines ?

Post by roadrunner » Mon Feb 16, 2015 7:50 pm

Warpspeed wrote:
roadrunner wrote:Vizard has pointed out that the kinetic energy of the intake air is much higher than often realised when high velocity ports are used, and that the air tends to "pile up" at the intake valve as it closes, creating a high pressure area..
That is nonsense.
Think about it.
You close the intake valve optimally when the incoming flow falls to zero due to the meeting the rising piston.
If air is still roaring into the cylinder when you close the intake valve, you are closing the intake far too early.
How does one close the intake optimally if there is no variable valve timing? The way I understand it, the pressure difference on either side of the intake valve on valve closure is going to vary from idle right up to the redline, and then there is the question of dynamic compression ratio /reversion to consider at lower engines speeds if you require decent low end torque!
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Re: Valve spring setting for turbo boosted engines ?

Post by ptuomov » Mon Feb 16, 2015 7:58 pm

roadrunner wrote:
Warpspeed wrote:
roadrunner wrote:Vizard has pointed out that the kinetic energy of the intake air is much higher than often realised when high velocity ports are used, and that the air tends to "pile up" at the intake valve as it closes, creating a high pressure area..
That is nonsense. Think about it. You close the intake valve optimally when the incoming flow falls to zero due to the meeting the rising piston. If air is still roaring into the cylinder when you close the intake valve, you are closing the intake far too early.
How does one close the intake optimally if there is no variable valve timing? The way I understand it, the pressure difference on either side of the intake valve on valve closure is going to vary from idle right up to the redline, and then there is the question of dynamic compression ratio /reversion to consider at lower engines speeds if you require decent low end torque!
Valve springs are only at the limit of their ability to control the intake valve during the deceleration part of the cam profile at near the redline rpm, so only the high rpm behavior is likely to matter. I can't imagine a situation in which turbo engine could force the intake valve open at low rpms. There's not going to be much pressure differential between cylinder and the intake port at low rpms, and the ABDC-IVC reversion goes in the direction of trying to shut the intake valve, not open it.

I've never understood the need for more intake valve spring load with boost at the given rpm. Doesn't mean that you don't need it, all it means that I don't understand any reasons why you would need it.

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Re: Valve spring setting for turbo boosted engines ?

Post by roadrunner » Mon Feb 16, 2015 8:17 pm

ptuomov wrote:
Valve springs are only at the limit of their ability to control the intake valve during the deceleration part of the cam profile at near the redline rpm, so only the high rpm behavior is likely to matter.
I can't imagine a situation in which turbo engine could force the intake valve open at low rpms. There's not going to be much pressure differential between cylinder and the intake port at low rpms, and the ABDC-IVC reversion goes in the direction of trying to shut the intake valve, not open it.
Neither can I, high revs is where the energy of the moving air apparently contains enough energy to influence the valves ability to seat correctly. V.E.'s of over 100% on an N.A. engine point to inlet flow having more kinetic energy than the air/fuel mix being pushed upwards by the piston at the beginning of the compression stroke me thinks!
I've never understood the need for more intake valve spring load with boost at the given rpm. Doesn't mean that you don't need it, all it means that I don't understand any reasons why you would need it.
The way I understand it is that the incoming high speed air/fuel charge can put more pressure on the back of the intake valve than what the air/fuel being pushed up by the piston can on the front of the inlet valve at certain engine speeds.
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Re: Valve spring setting for turbo boosted engines ?

Post by Warpspeed » Mon Feb 16, 2015 8:35 pm

The air pressure acting on the intake valve is only the direct differential pressure across the valve head.

Whenever the valve is open, there is not going to be much pressure difference, except maybe right at the very initial opening point.
The opening half of the cycle, is driven directly by the cam lobe anyway.

During the much more critical closing phase, governed greatly by the valve spring, the pressure drop across the valve head will be very small indeed.
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Re: Valve spring setting for turbo boosted engines ?

Post by Kevin Johnson » Mon Feb 16, 2015 8:49 pm

Some LS7 Spintron data from Katech to ponder.
903785_10151713809854354_214041069_o.jpg
1077789_10151713809849354_1484341770_o.jpg

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Re: Valve spring setting for turbo boosted engines ?

Post by user-23911 » Mon Feb 16, 2015 8:52 pm

Misinformation always comes in handy for selling parts that aren't needed.

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Re: Valve spring setting for turbo boosted engines ?

Post by Kevin Johnson » Mon Feb 16, 2015 8:59 pm

Levon Pentecost wrote:There are many opinions, but I have found no precise formulas. Lobe profile should have some input, rate of boost increase, active mass, and probably a few others. Can anyone help. Thanks ](*,)
My guess would be to get in contact with experts on the 2.2 and 2.5 NA and turbo Dodge engines and see if cam profiles are similar and then what the stock springs in each case are rated at. Factor in the ability of an OEM to test a design for hundreds of hours.

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Re: Valve spring setting for turbo boosted engines ?

Post by MadBill » Mon Feb 16, 2015 9:16 pm

If a cam was ideally specced for an RPM band of say 5500 to 7500, the IVC event would be a little early from perhaps 6500 on up. Theoretically, particularily if due to wide gear splits it was being shifted at 8500, there could be some inertia-induced pressure build up that would work against the spring force. I can't see it myself...
(Besides, especially with solid rollers, most guys would rather be 75# over the calculated minimum for their application rather than 10# under, so these small corrections would probably be lost in the shuffle.)
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Re: Valve spring setting for turbo boosted engines ?

Post by ptuomov » Sun Nov 24, 2019 9:52 am

Came across this old thread while surfing the web in a swim meet...

This from a twin turbo crossplane V8 with log-style compact exhaust manifolds. This is a simulation at high boost pressure. Vannik’s software.

Here's the simulated exhaust pressure trace of cylinder #3:
7CC06A98-7B06-4826-ACED-564ED4681CB1.png
9169FDCA-F9CC-4257-A19C-73C74459727D.png
See how whether the pressure wave peak hits the exhaust valve at or right after the EVC depends on the rpm?

I'm only graphing the exhaust port pressure here at all rpms, not the pressure differential between the exhaust port and the cylinder because the graph would get cluttered. However, in reality it's the pressure differential that matters. The other image plots both the exhaust port and cylinder pressure at 6000 rpm, you can see how that's a big force on the, the scale on the left is atmospheric pressure ratios.

The culprit is the 180-degree exhaust blowdown interference, so the behavior is different for every cylinder and little things like bore spacing and exhaust manifold geometry matter a ton.

This is the one reason that I’ve come up with that clearly justifies higher seated load and lower spring rate for turbo engines.
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