One for the RV8 engine buffs.

[stevieturbo]

So how much pressure, rpm, force, whatever does it take to start pushing the oil back out of a lifter ? creating issues with its operation ?

Must admit, whilst I know hydraulic lifters work, a little paranoia about them was one of the reasons I switched to a solid roller cam/lifter setup.

[Boosted LS1]

KiaDiseasel said:

Boosted LS1 said:

And as the cam wears etc the spring takes up the slack.

Actually the oil pressure takes up the slack as it does in all situations. The helper spring is only there to keep the lifter fully extended at fitting time.

Read the rest of what I said :-)

[Boosted LS1]

stevieturbo said:

So how much pressure, rpm, force, whatever does it take to start pushing the oil back out of a lifter ? creating issues with its operation ?

Must admit, whilst I know hydraulic lifters work, a little paranoia about them was one of the reasons I switched to a solid roller cam/lifter setup.

The entry hole is tiny compared to the volume in the chamber and there is also huge per square inch back up oil pressure in the gallery. I can't imagine how pushrod pressure will overcome the pressure in the lifter. Once it's full, it's staying filled if you get what I mean :-)

Even with massive valve seat pressures it still come down to can a fluid be compressed.

[KiaDiseasel]

stevieturbo said:

So how much pressure, rpm, force, whatever does it take to start pushing the oil back out of a lifter ? creating issues with its operation ?

Must admit, whilst I know hydraulic lifters work, a little paranoia about them was one of the reasons I switched to a solid roller cam/lifter setup.

That's an excellent question. The necessity of having the internal piston slide in its bore means there will always be a certain amount of oil leakage. Normal modern manufacturing tolerances mean that this can be contained to limits that enable the lifter to work to about 7000 rpm. The problem comes primarily with rpm rather than spring forces. As rpm increases there is less time for the lifter to pump up each cycle and of course there will also be leakage between the o/d of the lifter and its bore. This means that the full oil pressure will not be transmitted to the lifter. I believe the highest rpm that stock hydraulic lifters have been designed to run at is in a Ferrari engine at circa 8000 rpm or a bit more. Similar rpms are claimed for Chevy V8 performance lifters.

However at high rpms there will always be some loss of valve lift and duration as the lifter bleeds out. Up to 7000 rpm this is not a major factor but you'll always get a bit more power from a solid lifter on a similar cam profile and this will become exponentially a bigger factor at rpms much above 7000.

[stevieturbo]

Boosted LS1 said:

The entry hole is tiny compared to the volume in the chamber and there is also huge per square inch back up oil pressure in the gallery. I can't imagine how pushrod pressure will overcome the pressure in the lifter. Once it's full, it's staying filled if you get what I mean :-)

Even with massive valve seat pressures it still come down to can a fluid be compressed.

Using an LS as an example, it would be very easy to see spring pressures in the 400lb+ range when the valve is fully open, and that's just static. That's a lot more force that there is oil pressure trying to keep it all in.

So at some point there has to be a crossover from lifter holding, to lifter starting to lose oil. Running a low pre-load this could happen much sooner ?

Of course...I never really understood why some lifters would pump up as you hear in Rover circles ? Clearly the oil pressure in these must overcome seat pressure and literally hold the valve open ? That surely can never be a desirable condition either.

[Boosted LS1]

True, it's a lot of seat pressure but it's still hard to compress oil or squeeze it out of the chamber. Some leakage is built in depending on the oiling system. I was also wondering about the position of the oil entry/exit bore when the lifter has been raised. The oil is exiting into the lifter bore which should be reasonably snug top and bottom. There's usually a recess around the lifter which presumably the oil fills. I don't know what effect this has on things.

Ls7 engines redline at 7k iirc and even at that speed I'm assuming the lifters aren't bleeding down. The rover is interesting, they like thicker oils but I don't know how you can keep forcing oil into the lifter unless there's slack ocurring in the valvetrain? Maybe it's not been installed correctly? interesting.

ETA. I'm being a plank. The waistline of the lifter allows oil to flow along the gallery, doh :-)

Edited by Boosted LS1 on Wednesday 14th September 15:40

[KiaDiseasel]

The pre-load doesn't affect anything as I said before. It's just some random point within the piston's travel. The ONLY thing to be concerned about is that as the engine heats up and clearances change you don't want the piston to be so close to the top of its travel that it tops out or so close to the bottom of its travel that it bottoms out. End of. 20 thou from either end should more than suffice but 40 thou gives you some margin for wear.

As for spring poundages, they aren't such a big factor as might be thought. The lifter is doing its real work when the valve is opening and the forces here are mainly those generated by accelerating the mass of the valve train at several hundred G force. Obviously the lifter is also having to overcome the valve spring but that's only part of the load. Of course big heavy valves and strong springs increase oil leakage but if the lifter is sized proportionately to the loads it's facing then it should work fine. It's always rpm that is the problem. It increases the forces, hence oil leakage, and decreases the time available to replenish that. Eventually it's a losing battle - at about 8000 rpm.

[KiaDiseasel]

Boosted LS1 said:

I was also wondering about the position of the oil entry/exit bore when the lifter has been raised. The oil is exiting into the lifter bore which should be reasonably snug top and bottom. There's usually a recess around the lifter which presumably the oil fills. I don't know what effect this has on things.

Ok you've got to be careful here not to confuse separate things. Where the lifter body sits in its bore, and hence where it is relative to the oil feed that supplies it, is determined by the base circle size of the cam lobe. That should never change. Cutting valve seats etc doesn't make any difference to this. What that affects is where the piston needs to sit inside the lifter body. Pre-load is a very bad term for this. It doesn't affect any loads. It's just a random spot somewhere in the piston travel.

[KiaDiseasel]

stevieturbo said:

Of course...I never really understood why some lifters would pump up as you hear in Rover circles ? Clearly the oil pressure in these must overcome seat pressure and literally hold the valve open ? That surely can never be a desirable condition either.

Lifters can only "pump up" in two situations.

1. When the oil pressure is so high that it generates forces inside the lifter that overcome the valve spring closed pressure.

2. When the engine is revved to valve bounce and the valve loses contact with its follower. The lifter then tries to do what it's supposed to and takes up the clearance and ends up holding the valve off its seat until it bleeds down again. That tells you a very important thing. There has to be some bleed down (leakage path) in a lifter or it would never accomodate to valve bounce or even work properly when first fitted to an engine as it would never squeeze down to let the valve close. The trick is balancing the amount of bleed down to enable both first fitment and also high rpm operation.

[stevieturbo]

Very informative....but I'm still glad I moved to solid.

[Boosted LS1]

stevieturbo said:

Very informative....but I'm still glad I moved to solid.

Masochist ;-)