i was wondering if you are running around 20 psi would that lower the revving ability of the engine because of the actual air pressure pushing down on the valves? I know there are tons of setups with way over 20 psi but i was just thinking. If you have 150 lb springs with the valve surface area around 1.75 inches^2 wouldnt that decrease the valvesprings ability to seat by 35 lbs? I know this might all be false and wrong i just thought i would throw it out there.

 

hmm interesting view...subscribing

 

I figured it would work the opposite way and let you run higher. Not sure though..

 

Opens Physics Book + Statics Book + Looks for dynamics book.....

On the same concept once the air is in the cylinder and the piston pushes back up to TDC wouldnt it assist in the pushing back of the valve? IDK, random thought.

 

Valve float is entirely a product of the valve inertia and spring rate. The cylinder pressure during all strokes where the intake valve is closed will always be greater than manifold pressure.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beepy &raquo;</TD></TR><TR><TD CLASS="quote">Valve float is entirely a product of the valve inertia and spring rate. The cylinder pressure during all strokes where the intake valve is closed will always be greater than manifold pressure.</TD></TR></TABLE>

What if you boost to 10,000psi?

 

but lets say you were revving you motor (n/a) right before it started to have the valves float, if you then boosted the motor and ran those same revs wouldnt the valves float then because of the extra force (air from the turbo) pushing them open?

 

^^ I agree on this theory... Interesting tread..!!

 

i see your point.. when the valves are closed in the compression stroke is not were i see it being a problem.. its when the vales are being opened i would think the boost would force them open more at a certain point

 

good so i'm not insane

 

anyone else?

 

Take when the valves float NA (both RPM and lift) - it would be obviously high RPM, with high lift. As the rocker slides down the slope, the spring can't pull the spring fast enough to follow the cam's profile.

For the intake, it'd happen after the cylinder reaches full pressure. You then have nearly static pressure on both sides of the valve before it closes. Worst-case [or so I'd imagine] you'd see 20% less in the cyl, but that's if it volumetric was dropping off severely (way past max HP). For the exhaust, you have the turbine restricting the flow, putting more pressure on the flow. As it closes, you would still have air being held in by that back pressure (well, preventing scavenging I believe). You'd only see more pressure in the cyl than the exhaust as the valve is very far closed, but then you have overlap opening the intake, trying to pressurize the cyl again. There's also the venturi effect, which also introduces lower pressure past the valves (outside of the cyl) since the air is moving very fast past the head's port walls.

In short, I'd think that you'd only see marginal extra float on the intake side once the engine starts rapidly loosing volumetric efficiency.


I don't claim to know any of this concretely, but it should be able to stick up there with some of the best 'hypothosising'

 

since during valve-float both side of the valve is exposed to approximately the same amount of pressure, the effect is minimal.

that's also just a hypothesis

 

I have dealt with this numerous times. There are two things to remember:

Intake side : Lets say that you have 20 PSI of turbo boost. There is a mathematical equation to figure out the amount of pressure being applied onto the back face of the valve. This pressure number ( rated in lbs. ) would have to be subtracted from your valve spring rate and would equate the ACTUAL effectiveness of the spring.

Exhaust side : Same as the intake, but now you must account for the back-pressure. Lets say that you are building between 40 PSI of back pressure( pressure built up between the exhaust valve and the turbo ), that pressure will effectively lessen the effect of the calculated spring rate by it's mathematical factor.

Think of it this way, if you have a 30 pound spring and also have 30 psi of pressure behind the valve trying to push it open, then you will effectively have a "0" PSI rate on your valve. If you had a 50 pound spring and 30 PSI behind the valve, then you are only keeping the valve closed with 20 PSI. This is a HUGE problem with high RPM, high boost motors. I have experienced it first hand. The only way that I could get my motors to handle 11,000 RPM and about 50 PSI was to run my spring pressures up around 150 pounds on the seat!!! That is almost DOUBLE what the normal Honda performance spring can yield!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RyanCivic2000 &raquo;</TD></TR><TR><TD CLASS="quote">What if you boost to 10,000psi? </TD></TR></TABLE>

Then you'd have 10,000 psi being compressed in the cylinder by your compression ratio, and then ignited, so yeah you'd still be ok.

 

Yes, the additional force of the boost would affect the valves ability to close...but very insignificantly. As mentioned above it is the rotational inertia of the valvetrain that gets them to float (exceeding the springs ability to close them), but moreso at the very beginning of the "float" situation...high boost could OF COURSE affect there closing, and help keep them open too....But before i get BBQ'd, remember...I said VERY insignificantly.

Dont ask me for a mathematically proven theorem, I dont have time!!!

XBOX 360 out this month!!!
Gotta get back to work!!!


Piece and grease bro. Piece and grease

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bigdaddyvtec &raquo;</TD></TR><TR><TD CLASS="quote">Yes, the additional force of the boost would affect the valves ability to close...but very insignificantly. As mentioned above it is the rotational inertia of the valvetrain that gets them to float (exceeding the springs ability to close them), but moreso at the very beginning of the "float" situation...high boost could OF COURSE affect there closing, and help keep them open too....But before i get BBQ'd, remember...I said VERY insignificantly.

Dont ask me for a mathematically proven theorem, I dont have time!!!

XBOX 360 out this month!!!
Gotta get back to work!!!


Piece and grease bro. Piece and grease </TD></TR></TABLE>

What about the pressure inside the cylinder pushing back up on the valve, which has more surface area perpendicular to the direction of travel?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GoldenEagleMfg.com &raquo;</TD></TR><TR><TD CLASS="quote">Think of it this way, if you have a 30 pound spring and also have 30 psi of pressure behind the valve trying to push it open, then you will effectively have a "0" PSI rate on your valve. If you had a 50 pound spring and 30 PSI behind the valve, then you are only keeping the valve closed with 20 PSI. This is a HUGE problem with high RPM, high boost motors. I have experienced it first hand. The only way that I could get my motors to handle 11,000 RPM and about 50 PSI was to run my spring pressures up around 150 pounds on the seat!!! That is almost DOUBLE what the normal Honda performance spring can yield!</TD></TR></TABLE>

I don't think thats true at all for the compression/combustion portions of the cycle. You may have 20 psi on the intake side, but think about the mixture inside the cylinder, which as beepy stated above is at a much higher pressure. Once the exhaust valve opens, however, I could understand some scenarios where the valve opens a bit and there is effectively more overlap, but only during that portion.

 

It will NEVER affect the valve. Let's explore why.

1) Intake stroke:
Intake valve: Already open
Exhaust valve: Backpressure from spinning the turbo will cause one force on the tulip side of the valve. Boost pressure must be lower because the turbo is not more than 100% efficient. This would seem to give a net force on the tulip side of the valve. However, this force is less than at partial throttle when the cylinder is put at a large partial vacuum.

2) Compression stroke:
Intake valve: Compression forces.
Exhaust valve: Compression forces.

3) Power stroke:
Intake valve: Huge pressure spike.
Exhaust valve: Huge pressure spike.

4) Exhaust stroke:
Intake valve: The pressure on the face of the valve must be equal to or greater than exhaust backpressure. Since the turbocharger is not more than 100% efficient, the backpressure must be greater than the boost pressure. Therefore there is a net force on the face of the intake valve.
Exhaust valve: Already open.

So, for example, what if you had 10,000 pounds of boost? Since energy is conserved, you would have to have more than 10,000 pounds of backpressure when the exhaust valve opens, the compressive pressure would be about 100,000 pounds, and the combustive pressure would be like 1,000,000 pounds. The valves will not float. If anything, they won't open.

 

Beepy,

I could not disagree with you more. Although your points have some validity, it DOES affect the valve rate immensely. Even if the turbo wasn't effective 100%, then how could you ever build ANY boost at high RPM. The boost is reserved pressure inside the intake between the compressor and the valve. The opening of the valve does NOT void that pressure, it merely allows some of it to escape into the combustion chamber which is exactly what the turbo was designed to do, squeeze more air into the cylinder.

Also, there are certains times of cycle where obviously the combustion pressure will GREATLY overshadow any sortb of pressure from behind the valve, but think about this. During valve overlap, when the exhaust valve is closing and the intake is opening, there are hardly ANY cumbustive pressures to aid in the exhaust valve closing, except for the pressure coming in directly from the intake side. Once the exhast valve closes, the pressure bhind that valve will grow. I see your point that in a perfect worl you will have very low backpressure, but being realistic, and seeing this first had, the exhaust backpressure will almost always out-weigh the intake pressure of any turbo system. I have seen as high as 80 psi of backpressure. This is due to poor ehaust manifold design, incorrect turbo AR sizing, etc. but it does happen, even on our car utilizing a Garrett GT45RX with a massive AR sizing, and this is still only a 2.3L motor. So when you do all the math, and actually see what 11,000 RPM and tons of backpressure can do to a valvetrain assembly, then you will understand why it DOES affect the valve control, more than most people would like to think. Ask any engine builder that runs a forced inducted type set-up. They will all agree.

 

so if someone running 15-16psi, is that something to worry about? would adjusting valve clearance help?

 

Boost pressure absolutely does have an effect on valves and springs. Case in point, a buddys 10.5 tire car. On shakedown runs with 10-15 pounds of boost it was all good. With 35-40 pounds of boost the car would nose over on the top end and we ended up busting a lot of valve springs. With a 2.18" intake valve that extra 30 pounds of boost pressure worked out to be about 112 pounds of force working against the springs. Naturally aspirated or at low boost this motor would free rev to 9000 no problems, but at 40 psi it was nosing off at just under 8, and it wasn't spark blow out or anything like that.

 

would additional valve cover vents help in this situation? and does this make a difference if runnin 15-16psi?

 

[QUOTE=vtec.dc2]would additional valve cover vents help in this situation?/QUOTE]

No. The pressure is in the ports and combustion chamber(hopefully).

I have always heard this theory,but usually in a race engine I'm already running all the spring I can.Its not like I can all of a sudden run 20% more because its forced induction.I would think that the problem would occur as the valve is closing.The pressure in the intake runner would keep the valve from completely closing as the closed pressure is the lowest pressure the spring exerts.
Glenn