slt200mph

iTrader: (42)

You took the words right out of my mouth sir..thanks..I sure didn't want to type that much..

Grimes

iTrader: (3)

If things happend relatively slowly in an engine, I would agree with you. But it's much more dynamic than the explanation you give. I'm sure there is quite a force from the air rushing in a 2" diameter hole when the engine is ripping along at 5000-6000 rpm. Add to that 10psi of boost at the same time.... People look to shave 5-10 grams off a valve - I don't see why it's hard to believe that 10 lbs extra air pressure could amount to AT LEAST that.

Oh well, I guess it's just two guys with different views debating on the internet, lol. I would like to see some FI guy use stock ls6 springs @ 10 lbs, then change nothing else except to a dual spring package. Dyno each, and look for changes. Anyone have an engine they wanna risk? hehe.

96 Comp T/A

iTrader: (8)

The same laws of physics apply at 600 RPM that do at 6000 RPM. The only two effective forces that change vaules are inertia which would lead to MECHANICALLY induced valve float and Combustion Chamber pressure which increases up to the point of peak torque and then begins a downward slide just after when inefficiencies in the induction system prevent cylinder filling from keeping up with the cyclic rate.

Also your above statement about back to back dynoing wouldn't nescesarily be a valid test for valve float caused by boost (although it will tell you if you create more power). The only way to test it would be with a high speed camera (spintron).

Grimes

iTrader: (3)

Well, at higher speeds air will be rushing in faster, air has mass, and therefore inertia = more forces on the back of the valve when compared to slower speeds (to be exact you would have to account for volumetric efficiency as you said). Same principal goes for running positive pressure (boost). You are forcing more air into the cylinders than normally aspirated. More mass rushing in = more forces on the back of the valve.

Think of it this way - blow with your mouth onto a straw, and put your finger over it. You feel some resistance. Now blow harder - takes more force to close off the straw right? A valve spring will have an harder time closing a valve under boost as opposed to normally aspirated.

96 Comp T/A

iTrader: (8)

You are totally missing the point, By the time the valve has travelled far enough up the opening ramp of the cam to where valve float would begin to occur, the cylinder has already been filled with the same air that is acting on the other side of the valve. If you think it's holding it open as the valve is closing the pressure in the cylinder has already begun to rise as the piston has begun it's movement back up the cylinder. The pressure acting on the valve face (combustion chamber side) is the same or higher than the intake tract that is working on the other side of the valve and that's with 1, 10, or 100 lbs of pressure. Net at minuimum is zero or to the positive side of the combustion chamber.

Grimes

iTrader: (3)

I'm sorry, i still don't agree - there has to be more pressure outside the combustion chamber for almost the entire time the intake valve is open. Otherwise the air would not go inside the cylinder. Air will only move from high to low pressure. Even at moderate RPM, air is still in fact rushing in even as the piston has started moving up on the compression stroke.

96 Comp T/A

iTrader: (8)

I am not disputing that the higher pressure in the intake tract is what causes air to flow in to the cylinder. I am trying to convey two things. One is that when there is positive pressure on the backside of the valve there is no mechanical motion of the valve to create valve float. If the valve is on the seat it's on the seat unless you create enough boost to overcome the seat pressure of the spring, which is generally in the 120 psi + range. If the cam doesn't move it and pressure in the intake isn't enough to overcome the spring pressure, it doesn't move, period. So I am guessing you must mean that the pressure in the intake is holding the valve open longer than it should which brings me to number two. So now we have 10 pounds of pressure in the intake tract. With the valve open you have a transient period where the intake tract equalizes the pressure between the two (high pressure to low pressure as described by you above) and the two chambers effectively become one as long as the valve is open a signifigant amount. Now you now have equal forces acting on both sides of the valve. When the valve does shut (probably somewhere around 50-60 degrees ABDC) the pressure in the cylinder goes way up.
What you are describing is inertia tuning and is not relevant in a forced induction application which we are specifically discussing here.
I guess the overall point of this whole conversation would be to run the proper springs for the cam you are running. Granted you could take a forced induction engine change the springs and pick up power. There is however no gurantee that valve float would not occur with the same cam profile in a non forced induction application. You would have to watch it with a high speed camera. A forced induction car would be extremely sensitive to floating valves where you might not notice it as much on a naturally aspirated car.

I am hunting around the internet right now for a site I stumbled across a few years ago. It was a site for an engineering college at a major university, that had some great java apps for doing volume calculations for every degree of crankshaft rotation. It may do a better job of explaining it than I have, thus far.

Grimes

iTrader: (3)

Oh - I'm not saying that the valve is hanging open due to the boost. I'm saying that the boost will add additional forces to the valve (opposite to the forces that the spring imposes) that reduce the effective spring pressure the valve sees. This may cause valve float.

I can assure you that in regular production engines, the charge is still rushing in (due to momentum) as the piston is already moving up on the compression stroke. This is regardless of whether the intake on that particular engine is tuned or not. This is why the intake valve is shut after BDC not before.

I'm not trying to be a flamer or anything, Comp, I'll try to find some actual research to back up my theory too

SteveDoten@ARH

Cylinder Heads - #12564825
Same as the 2002-04 LS6 cylinder head, no changes on the bare head. However the LS2 engine uses the stock LS1 valves rather than the lightweight Z06 valves.
this is cut from scoggin's post on ls2 parts, so are they 01's or 02's, who's right/wrong?