^^ Possible but I've never seen that happen. The tappet 'reservoir' is a pressurized oil gallery that can drain back, not really a reservoir. I understand your thinking that the lifter would draw the oil back in under a vacuum powered by the internal spring but I don't think that's likely with no dynamic seal on the lifter components or pressurized oil available. Pretty much any engine I've seen, the lifters parked on the nose of the cam will be soft after engine disassembly.
Yes, I edited that out of my post as it was an after thought. I don't think the vacuum alone would be sufficient to open the check valve anyway. Dave and I also discussed that but under a different condition. Even a perfect vacuum could only develop a fraction of an oz pressure to open the valve.
Hey Jim - one question on the point of "do not compress"?? Isn't part of the function minor compression such that a valve will properly seal when closed and the lifter will handle the tolerance stack up and expansion and contraction of components under temperature change? I'm thinking we are talking about .001" - .010" (at best) of travel....?
Maybe minutely but on the base circle filled with oil that's your starting point, lifter extended all the way, valve closed/sealed. Cam opens the valve and oil being incompressible (unless aerated but not likely at all with these dry sump systems) the lifter will not leak down/compress as there is not enough time for it to do so. If there is wear to the valve/seat that would cause the lifter to be shorter on the base circle, or wear on the cam that would cause it to be longer on the base circle it will compensate for that but the amount it's compressing while running/normal operation is very very little. (so yes falling into the range you posted I would guess)
Hmmm...I'm thinking (not sure) that the initial point is not full extension of the the lifter piston but a preload of some amount. I'm thinking during cam dwell, contact is maintained but the valve spring will overtake the force from the lifter allowing the valve to fully seat (close)
Dave, I'm not entirely sure what you're describing. I don't know the measurement but lets say on the base circle, valve closed the lifter is 1 inch, it will never exceed 1 inch and need to be squished back down by the valve spring as during valve opening it's under compression load and returning to the base circle it could only maintain it's 1 inch length. It could only exceed that initial length if there was a loss of valve control (lifter lofts over cam nose) and oil pressure was able to fully or further extend the lifter as the constant force of the spring pushing it against the cam lobe was lost and the internal pressure of the lifter allows it to extend. It would then take several revolutions for it to bleed down the excessive oil on base circle to return to it's original height and operate normally/not hang the valve open. Not an issue on these as they maintain perfect valve control to the rev limiter.
Jim, what I'm saying is the piston in the lifter (which makes contact with the tip of the valve) must be able to float to A) adjust for tolerance differences in manufacturing and assembly B) adjust for temperature change which will effect the components size. So, I don't think that piston is fully extended at cam dwell (base circle as you call it). I think by design it has some preload amount. And, when the valve is closed, the valve spring overcomes whatever force the lifter piston exerts to allow the valve to fully close and seal.
Ah yes, I didn't mean it was fully extended to the end of it's ability, I meant that on the base circle it's fully extended within it's operational range when running. (minus temp/wear variations as mentioned)
Well, let's not muddle around with words. The term incompressible is just a label to separate a class of fluids from another fluids, like gases, which obey some type of gas law, and liquids. Petroleum oils typically will have a volume change of about 0.5% for pressures between 1000 psi and 4000 psi, and more at higher pressures. So consider the diameter of the piston in the tappet. It's what, on the order of 1/4 diameter? 1000 psi acting on a 1/4" diameter piston would provide about 50 lbs force on the valve stem. Ferrari specifies the force for static and dynamic lengths of the 355 valve springs at 70 and 160 lbs (sum of inner and outer spring forces). Thus we can rightly assume that the pressure inside the tappet is of the order of over 1k psi before the valve actually start to move. This would, based on my guess of the length of the oil chamber in the tappet, yield a compression of about 0.001" to build the pressure from engine oil pressure to that required to start opening the valve. Engine oil pressure (80 psi) alone would only yield about 4 lbs on the valve stem.
Would the above give you the impression I'm not in agreement? 0.001 falls under the range Dave posted and I agreed with and also the definition "very very little" as I would see it. Otherwise we would be talking about a solid lifter which this obviously isn't.
Jim, pretty sure John agrees with you but was just adding more. I'll be modeling the components to design a tool and when done will post some nice illustration
I don't know exactly what this is supposed to means, Jim. But was simply responding to you statement "Cam opens the valve and oil being incompressible (unless aerated but not likely at all with these dry sump systems) the lifter will not leak down/compress as there is not enough time for it to do so." I was simply pointing out that there is a compression of the tappet from its position on the base circle before it can start moving the valve and noting degree to which engine oil pressure can impart a force on a closed valve. Just as much for my own enlightenment as well as for others.
I am of the opinion that the factory "assembly marks" are very close to dead accurate. In fact when using lesser timing methods like a calculated lobe centerline often the marks are dead nuts. However, when using Ferrari's cryptic timing instructions they seem to have much greater accuracy and small deviations make for big changes seen at the dial guage and I am more likely to be a tick off the assembly marks. This is such a big issue that I also maintain that the factory scribed rear "cam timing" marks that can only be seen with a mirror when motor is "in situ" are not possible to be used as cam timing marks due to visual parallax error and the precision called for by the Ferrari method of cam timing. There are many who disagree. That's OK I will degree my cams anyway. This said I question the true accuracy of the Ferrari method because this method can only be done with cam shafts in place and belts installed and Hydralic lifters in place. The question being discussed here regarding the full extension (or lack there of) of the lifters filled with oil is exactly the reason that on lowly Chevy small block motors the #1 lifter is removed and replaced with a solid lifter for cam timing procedures. We can't do that with Ferrari motors and in my opinion calls into question the whole process. But who am I to question Ferrari engineers? I cam time their way anyway but pay very close attention to the assembly marks and give them equal credence to what I am seeing on the dial guage on hydralic tappet Ferrari motors. Anymore than a little bit off which fortunately I have never seen and I am more likely to err on the side of the assembly marks over the dial guage on hydralic lifter Ferrari motors unless someone can tell me why a Ferrari hydralic lifter has magic powers to maintain its oil volume and why a chevy lifter cannot. So let me have it JohnK and DaveRocks you guys are pretty good at telling me I'm crazy and if there are dissenting views on this I would really like to hear them. If you both agree with me...well it is a very special day on Fchat.
Well that's what I get for deleting what I had originally wrote which was "basically incompressible". In terms of the functionality of this system my description was meant to convey it's pretty much solid, if you want to go with it's not solid and compresses .001 I agree
Yea, I know. Fricking internet molds what we think into what we think we should think. NHNF. I'm going back to not posting.
I understand what you're saying and it's logical. The difference being in my opinion, when measuring on the hydraulic lifter of a Ferrari, you are measuring the part of the lifter being directly actuated by the camshaft so that should fairly well translate to valve movement under dynamic conditions. On a small block chevy with a hydraulic lifter, you are not measuring against the lifter body, or the part directly actuated by the camshaft but the internal cup (or worse at the valve which further amplifies lifter compression and variation) which will be moving/compressing inside the lifter body. If you had a fixture to time the cam on an SBC directly off the part actuated (lifter body) it could stay hydraulic with no variation. But yes, if measured at the valve you would need to replace it with a solid lifter.
Well, it's a special day. Not sure I totally agree with what you are saying (only because I need to study the very detailed function of these lifters more), but I will say your thought process is spot on with this point
Darn! I thought I was missing something and hoped you guys would tell me what it was but intmd8 was right on it. I never thought about it this way. I can totally see you point. It is so obvious. Small block chevy world polluted my thinking or obvious lack there of.
