Carl Fausett

I wish I had a video running that I could show you. The sound is unmistakable, the torque flattens, there is a little more acceleration of the motor, but at a much slower rate. The sound (intake valve float) is similar to the sound of blowing a spark plug out of a head, if you've ever heard that. Only more muffled (because of the intake manifold) and more of them.

ptuomov

I've never heard a spark plug blowing out of a head. That like an interesting experience, please tell us more!

Rob Edwards

Google Ford Triton blown spark plug.

mark kibort

I understand you are hearing something, but the fact that the torque curve is flattening, means HP is still rising... not indicative of valve float, that kills power. remember falling torque is flat HP and really falling torque is what you need to see if there is a problem (greater than 45 degree angle of the torque falling)

Now, you also see that boost can not be the cause, as i said, as soon as that valve is opened, the flow enters the cylinder, and the pressure differential will be highest at the valve back, and progressively less as you get to the inner cylinder. this means the differential between the back of the valve and cylinder side of the valve will be so small as to never be able to provide any measurable force . most of the force will be in the form of aero drag as the fuel aix mixture passes passed the valve. think about it..... as that piston stops at the bottom of the cylinder, there is no more flow , the valve is just in the intake flow path, and there is no pressure forces on it. SO, the boost can not be a part of the valve float problem. your calculations are not correct, as there is no differential pressure from top to face of the valve of what you say, amounting to the boost pressure (boost psi / area of valve top surface).

if you are encountering valve float, i would think power would be going down, (huge fall in torque) . what you could be hearing is valve floating off the cam lobes due to their inertia, weak springs and high lift (and ramp profile) where the valve has no problem closing , but is flung off the cam lobe and then recontacts it like a valve tappet noise. that noise is what you might be hearing, but its not going to be a function of boost levels .... the force potential is just not there.

I also think it might be how you define "valve float"... valves losing contact with the cam momentarily, or never closing all the way.

Carl Fausett

Old School. First-timers working on aluminum heads, striping out the spark plug threads... not as common as it used to be. Sure makes a noise tho!

mark kibort

so Carl, you never responded..... how does the boost effect the valve closing as it occurs after cylinder fill when the piston is near BDC, on the way up there would not be a differential pressure, so how does the icreased pressure around the valve , on BOTH sides, keep it from closing. (or cause it to float)
valve float as most of us have seen, happens when the valve is not controled by the spring and you get some bouncing off the valve seat and lose some compression until it seals. actually, the compression part of the stroke will be 10x the boost pressure, so actually, it will help the valve seat.

the only thing that boost will effect , is on the intake stroke as for a moment, the load on the spring will be less due to the boost pressure, but that will go progressively down as it opens and flow enters the cylinder.

what am i missing?

Carl Fausett

Yep, no point in responding. I have a business and a shop to run and my time actually at a keyboard is limited. This very same topic has already been talked to death on another thread.

Try this:
PN 928 105 905 03 is the valve spring set for a 1985 944 NA.
PN 951 105 905 10 is the valve spring set for a 944 Turbo, the 951

Porsche discovered the boosted engine needed stronger valve springs to help control the valve. Which is also consistent with my experiencel.

Perhaps where I lost you was if you think I thought the beryllium-copper seats controlled valve bounce off the seat completely on their own. I do not. I think they are a help, but it would appear that once up in the rpm's, where the centrifugal supercharger is producing highest volumes and pressures, the valve may be more apt to bounce of the seat as it slams shut.

To me, its somewhat of a bar-stool argument. Is it valve toss off the nose of the lobe? Or bounce off the seat? No matter. We up'd the spring pressure a little bit, and it went away. Lets go racing.

mark kibort

I agree.... it is a bar stool conversation, but you had put some numbers down to justify the occurrence of the valve float, which could be the bounce off the seat , or the lag in the valve closing due to the cam lift /valve toss.
Yes, it has been discussed to death on other threads, but think about it... your numbers of valve pressure is ONLY when the valve is close as it is opened, the pressure on the back of the valve will go away very quickly, and seemingly, in no way could it effect it on the ability of the valve to close. i understand, boosted engines have issues , but many of them have cam changes that could be causing the valve bounce off the seat. It could be as it is bouncing, the pressure builds on the back of the valve which keeps it bouncing longer. hard to say, as the pressure on the combustion stroke is orders of magnitude higher than the boost pressure... but its there with boosted engines and not there without the boost, so maybe that's what is happening. the volume of air is not changing, just the density.
so, it would be interesting to put your theory of how the boost pressure could have any effect on the back of the valve as it closes on way to the peak of the compression stroke. (where pressure is now building in the cylinder and actually being forces out the leak , if there was a floating valve.

Carl Fausett

Please see post #48 in this thread.

Not hypothetical. This is what happened in the real world, what we did, and what happened after we did it.

mark kibort

I think its the definition of anicdotal and hypotetical.

correct me if im wrong... on post #48, you show 3 instances where starting out, at 10psi , and stock cams , all is fine, each of the next two changes were cam changes as well as small boost changes (i.e. 10pis going to 14psi going to 20psi).

How do you know the "float" is due to boost and not the cam change?

also, again, how is it every remotely possible for the boost to be a factor, when the piston coming up as the valve face has probably as much or greater pressure on it, than the back up until ignition?

remember, at BDC, the piston is no longer making more volume. any boost pressure is in and around the cylinder and both top and bottom of the valve now. so , how in its closing can boost be a factor? im very curious!!


Im not doubing what you saw, im doubting your Hypothetical cause.

did you notice in your example of the 944 NA and the 951 turbo, where they both have different valve springs, did you notice they have differnet lift cams too?

mark kibort

your on the right track here, but why is there any force at all at the point this would be an issue. lots of boost could push the open, when it is shut, as Carl mentioned , the spring pressure is 80lbs and the boost pressure might be 40lbs, so that's a lot force fighting against the seal. however, like you said, once the valve opens, there is only a 1-2psi differential until its wide open, and then closing, there would be no differential pressure so, i dont see how the boost at that point would cause any forces at all until the exhaust stroke where the cylinder is emptied.

So, based on what you say here, and i just replied above about half the pressure on the springs (statically) coming from boost, I agree and dont know what those effects would be. However, once the valve starts to open, the pressure differential would not be anywhere near that value, as Ptuomov said. so, i still contend that any issues of "float" are probably due to the cam profiles, as as the valve is starting to close the piston is at BDC . if its bouncing around at that point and not sealing, its not the boost that is doing it as there would be equal pressure on both sides, right?
edit: Ive also read that "mistimed" intake pulses under extreme boost can open valves, as your valve size and boost pressure being 20psi might be half the stock spring pressures for the seating of the valve. this means that those intake pulses have doubling effect of the boost in the intake acting on the valves. maybe that could be hapening as well.

Carl Fausett

True. Each time we changed two variables, not one. The cams were changed to higher lift, and the boost was also increased.

Was the valve float the result of the change in cam lobe profile? Or the increase in boost? I cannot be sure.
What I can say that an increase in valve spring pressure made it stop.

mark kibort

well, what am i missing then? there is a TON of confusion about what causes float, and many agree, that boost has NO part of it. Again, what am i missing, except the chance that there is a pressure pulse i the intake that could be pushing valves open at anything they are closed, but i would imagine that the compression stroke that there is A LOT more pressure in the cylinder than outside the valve 10 to 11x , right? (not to mention what happens at ignition too). so, since you changed two major variables in your test, it's pointless to try and assume the boost was the problem or others will need to look at valve springs for boost alone goals.
again, you produced numbers of pressure on the valve, but those would only be valid before the valve opened, not during, and certainly not at the bottom of the piston stroke where valve bounce could play a part in cam timing/valve closing timing, issues, but again, at near BDC, there would be no differential pressure that would be present on the valve surfaces.

Hey, glad you solved the valve bounce or float with the higher tension springs though!

now, lets get that beast back out there for a full race weekend!!

Carl Fausett

Mid-Ohio, June 6 and 7

Have you considered taking some of your time and searching the internet for a good (vetted) article on the subject of valve float?

mark kibort

Ive looked at a few ... again many say that "float" is the valve leaving the cam and coming down then, later than planned. (due to ramp rates and RPM). valve bounce is different. either way, it happens when mass flow to the cylinder is minimal, as pressure differentials are minimal as well at near BDC of the piston.

cool about mid ohio being the next race...... fast and technical track!