super charged valve spring pressure

[machoneman]

On shutdown, I was told long ago the worst thing to do is to snap the throttle closed very quickly as that contributed to rod stretch. Instead, door slammer or injected dragster, we always motored down a tad (say 1/2 throttle closing) before closing the throttle all the way.

[cjshaker]

It's not a guess, it's physics...

Maybe someone just forgot to tell the engine about this? Sometimes they just don't listen very well

Just a guess, but we've all seen the Kaase 'finger' video, where reversion has such a dramatic effect. Then there is the port velocity slamming into the valve as it closes. It may have a more pronounced effect when boost is added. Just because we don't understand why something happens, doesn't mean it isn't happening.

[plovett]

How do you know the differential pressure is the same?  I'm not saying you're wrong, and I get that there's more pressure in the cylinder to counteract the higher pressure in the intake.  But is it exactly the same as naturally aspirated?

paulie

If your intake port is keeping your intake valve open because of pressure, are you not closing the valve too soon? Hopefully when you close tine intake valve, you've put as much air into the cylinder as you can, whether it's @ atmospheric or 45# of boost. Pressure delta should be zero or very close.
I think a lot of people "assume" the pressure behind the valve thing because it's somewhat intuitive but they fall short of thinking about the other side of the valve. If adding spring pressure to a boosted deal "makes more power", it's somewhere else.

That is a good explanation and seems reasonable.  Especially when you consider that the piston is on the way up at intake valve closing,  creating more pressure to keep the valve closed  Like Doug, I wonder if there is more going on though?  An intake manifold is a chaotic place with all those valves opening and closing.

paulie

[scott foxwell]

It's not a guess, it's physics...

Maybe someone just forgot to tell the engine about this? Sometimes they just don't listen very well

Just a guess, but we've all seen the Kaase 'finger' video, where reversion has such a dramatic effect. Then there is the port velocity slamming into the valve as it closes. It may have a more pronounced effect when boost is added. Just because we don't understand why something happens, doesn't mean it isn't happening.

People mistake boost with an increase in airflow. While, yes, there is that, the more significant part of boost is increased air/fuel density. Remember...boost is a measurement of restriction, not an indicator of airflow. Airflow, as in CFM, does not increase like some may think it does.
I always thought the finger video wass showing the return pulse wave from the moving column of air hitting the closing valve, not reversion.

[My427stang]

It's not a guess, it's physics...

Maybe someone just forgot to tell the engine about this? Sometimes they just don't listen very well

Just a guess, but we've all seen the Kaase 'finger' video, where reversion has such a dramatic effect. Then there is the port velocity slamming into the valve as it closes. It may have a more pronounced effect when boost is added. Just because we don't understand why something happens, doesn't mean it isn't happening.

People mistake boost with an increase in airflow. While, yes, there is that, the more significant part of boost is increased air/fuel density. Remember...boost is a measurement of restriction, not an indicator of airflow. Airflow, as in CFM, does not increase like some may think it does.
I always thought the finger video wass showing the return pulse wave from the moving column of air hitting the closing valve, not reversion.

I agree with you Scott, I thought it was the valve movement too

I haven't wrapped my head around your comment that pressure is equal though on each side of the valve.

- Pressure differential has charge moving into cylinder as piston drops creating a depression.  If pressure was equal, there would be no cylinder fill. 
- Valve closes at somewhere after BDC where piston is starting upward, if pressure was rising near valve seat because of that change in the cylinder, there would be reversion, but boost still overcoming it, so still, pressure differential favoring manifold pressure (and in a NA motor, atmospheric pressure plus charge momentum
- Intake valve closes, pressure at valve should spike, although I'd say it's sort of like an damper as it will rise, the next one in firing order should reduce, so I would expect the valve to see some, but not all

Now, that is looking at one or two cylinders, what I have a hard time wrapping my head around is each cylinder doing that in order.  However, physics (and atmospherics as an aviator) tells me, there is always a lower pressure on one side or the air would not move, regardless of it being atmospheric pressure or un-naturally aspirated

[Heo]

To move air you got to have a wavemotion of high and low
pressure areas i was thought by my old physic teacher.
Wavelength dependent of airspeed, tubelenght, etc.
Sound is nothing more than air moving at a specific wavelength
So if you get a low pressure area behind the valve and a high
pressure area i front of the valve maybe it forces the valve open
A blower i think would create high amplitude on the waves
If some one understand what I'm try to say in my bad English

[scott foxwell]

The goal is to fill the cylinder. The only way that can happen is to create a pressure differential (delta). As soon as that pressure delta is equalized, cylinder filling is done. We need to close the valve, now. Any earlier and we cheat ourself out of some cylinder filling and any later, the pressure starts rising in the cylinder (because of piston movement) and we're giving up cylinder filling and going backwards. That's reversion. Intake valve closing is critical.

[WConley]

It's not a guess, it's physics...

Maybe someone just forgot to tell the engine about this? Sometimes they just don't listen very well

Just a guess, but we've all seen the Kaase 'finger' video, where reversion has such a dramatic effect. Then there is the port velocity slamming into the valve as it closes. It may have a more pronounced effect when boost is added. Just because we don't understand why something happens, doesn't mean it isn't happening.

There are additional physics at work here besides just airflow.  I've done work on shock waves and their effect on induction and exhaust systems.  In a supercharged engine, the induction air is more dense.  This will speed up and often amplify the shock waves. 

The reason tuned intake runners work is that the opening of the intake valve generates a low-pressure (rarifaction) shock wave.  This shock travels up to the plenum, where it reflected back towards the valve.  The reflection changes the shock into a pressure wave.  If everything is perfect, this shock hits the intake valve just as it's closing, forcing a bit more charge into the cylinder.

Long runners work at lower rpm because the time between opening and closing the valve is longer.  You need a longer pipe for the wave to travel up and back at the speed of sound.  As rpm goes up, you need less and less time so the ideal pipe gets shorter and shorter.

Now back to a supercharged engine.  I haven't spent much time with them, but a more dense charge will tend to create more energetic and faster shock waves.  It seems likely that at certain rpm the stronger shocks are hitting the backs of the intakes at the wrong time, potentially bouncing them.  That would jibe with test results I've seen like Jay's.

Heo's got the right idea!

[scott foxwell]

Wave motion does not move the air. Wave motion is a result of the valves opening and closing, and as you stated, return waves from the ends of the runners in the plenum. Air movement is from pressure differential.
A closed intake valve isn't going to get opened prematurely by a shock wave or boost pressure. Even mild engines have more seat pressure than that. An open valve isn't going to be held open longer than it's supposed to be or be kept off it's seat by a shock wave or boost pressure. Like I said, if adding spring pressure seems to solve a "boost" issue, it's something else, not boost and not shock waves or energy pulses.
Look at spin tron testing. Look at the valve train on a 11k rpm, 1400+hp Pro Stock engine. Imagine the wave energy in the runners in that engine with a 2.5+ intake valve (thinking area)!! It's tested with zero induction dynamics and they make zero compensation with spring pressure for that combination. I worked at (top fuel) Alan Johnsons for almost 5 years. Learned a little about boost and cylinder heads. Don't remember ever adding spring pressure for more boost.
Not trying to say I'm right, everyone else isn't. Just sharing thoughts and experience.

[Barry_R]

Am I allowed to agree and disagree at the same time?

A boosted engine certainly has a significant increase in cfm/airflow.  Thats because all of the common cfm measurements are assuming a given amount of test pressure to generate comparative data.  The chosen test pressure may well have absolutely nothing to do with the actual dynamics in a running engine - they just needed some sort of a "number" to commonize the data.  Its always "cfm at xx.x".

That spinning thingamajig in the inlet tract is gonna increase the test pressure in the real engine by a bunch going from base atmospheric to some increment of that - or above.  The cfm number gain might be modest on one package versus another, but the xx.x" will change.  I will definitely agree on the boost comment - it is a measurement of restriction - not of airflow.  Push harder against a given orifice and the boost measurement will go up even though the gain in airflow might be much less  on a percentage basis.

Might want to approach Mr. Conley with a bit of caution.  He has had access to some rather spendy test equipment over the years, and often screws up these threads by providing facts backed up by test data.  Makes him really hard to argue with even when you want to....

[cammerfe]

+1

KS

[WConley]

Not trying to stir anything up, and an engineer with a lot of test equipment does not become a Kaase.  That said, to a Top Fuel engine with 450 lbs seat pressure, the effects of 60 psi boost is crumbs (shock or no shock).  All of that spring pressure is needed to keep that valvetrain stable at 8,200 rpm.

For our "normal" engine builds, there's plenty of anecdotal evidence supporting the need for stiffer valve springs on supercharged engines.  Here's one good example:

http://magnumpowers.com/valve-spring-data/

flame on!

[CaptCobrajet]

Yes, to further what Barry stated.....we should treat Mr. Conley like E.F. Hutton.  We he talked.....people listened.  Mr. C opines with much knowledge and experience.  When really smart people don't know, they are not afraid to admit it, and when they make a statement, it is often hard to debunk. 

With all that said, we gotta get back to Leny's original question.  He wants to know how much spring on a stock cammer with those 1.3 ratio rockers and stock cams, with a blower.  I don't know that answer......I just threw out two things to consider......one, that it IS a cammer with low ratio rockers and no pushrods......and two, that it is boosted.  Somebody with experience should give Leny an idea.  I am not the right guy, but  I'm gonna say 175# seat and 450# open if you turn it 7000+.  I'd say 125/375 at 7000 or less.  It is a mildly educated WAG based on what I have seen over 40 years of this, and knowledge of what was available when those cams were new and men with big balls put blowers on them.  Somebody agree or disagree and tell us why.

Back to the discussion of it......with everything I do regarding the intake charge, I think of that whole "thing" as a freight train, that we have to convince to start rolling down a hill.  We can pull it a little (maybe with a good header and some overlap, or we can push it a little (maybe with a big plenum and steep runner), or we can really load those freight cars, and more of them, and then make the wind blow to get the train moving.  The last example being the blower.... wind and a heavy bunch of freight cars (Mr. C's comment on more dense).  So the train gets moving....it is a heavy train with lots of cars....then we drive another  bigger train into it, head on(intake valve closing).  You see my analogy..... it takes a bigger train coming the other way to stop the first one in it's tracks.  I'm just a Hillbilly (capital H), but I live at the foot of a mountain, and there is a train track out by the river here.  It keeps me thinking about the fact that the charge has mass and we have to control it.  We gotta do stuff to make it move, and then control it when we want it to stop.

[blykins]

It's hard to argue against data and experience, even when the physics dictate otherwise.  It's not that the physics are wrong, it's just that the wrong formula/method/hypothesis is being used to correlate it.

In situations where the cam has not changed, the peak hp rpm has not changed, but boost has been added and the valve springs are giving up, it's hard to pull anything else away from that.  There has to be *something* causing the loss of valve control and it could be that the guys who are not seeing it are possibly over-springing to begin with. 

[blykins]

Don't remember ever adding spring pressure for more boost.

My guess is that the spring requirements necessary to turn 9000-10000 rpm would take care of whatever spring requirements needed by a boost increase.

The situations provided by Joe Craine and Jay Brown probably offer up a scenario where the spring requirements were marginal at best (certainly that's the case with factory 5.0 stuff, where the engine was done at 4500 rpm) and the addition of forced induction found the limits.