Action of hydraulic lifters

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RDY4WAR
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Action of hydraulic lifters

Post by RDY4WAR »

I have a couple questions for you guys regarding the action of hydraulic lifters in a running engine.

I've always heard that hydraulic lifters tend to absorb approximately .006" lift as hydraulic lash. (correct me if I'm wrong about that) Let's say if that's correct, what I'm wondering is if that .006" is absorbed all at once when lobe starts meaning the lifter won't begin to lift the pushrod until that .006" has been absorbed? Or does it gradually absorb that .006" as it rides up the lobe?

My second question is what happens at peak lift. If the lifter does absorb that .006" lift, would it be thrown back into the lift profile once the lifter is on the closing side of the lobe? Or does high spring force around peak lift keep the lash absorbed?

I'm also curious if the amount of lash absorbed by the hydraulic lifter is a static figure or does the amount increase as RPM, and thus valvetrain inertia, increases?

Thanks in advance.
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Re: Action of hydraulic lifters

Post by F-BIRD'88 »

Good question.. To analyse it you have to track and trace the lift motion curve of the lifter body and the pushrod sepatately. And dynamicaly thru thr rpm range.
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Re: Action of hydraulic lifters

Post by engineguyBill »

Quality hydraulic lifter should have very little, if any, bleed-down as the valve is being opened. The clearance between the plunger and the body of these lifters is extremely close, as I recall it is in the order of .000180" or so - which the lifter manufacturers like to identify as "one hundred eighty millionths of an inch". With clearances this tight there should be only minuscule amount of oil escaping the reservoir as the lifter opens the valve. If the lifters are not quality products, or they are worn out, that is a different situation.
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Re: Action of hydraulic lifters

Post by F-BIRD'88 »

To get the whole picture you'd want to trace and compare the motion of the hyd lifter VS the actual motion of the valve in action. Then you'd see the effect of deflection of the rocker and pushrod etc at speed. It will be dynamic. No to mention what the valve spring does.....

What (and when) actually goes on at the valve is what matters.
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Re: Action of hydraulic lifters

Post by MadBill »

Aeration is a variable but never absent factor in the actual vs. theoretical lift ratio.
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Re: Action of hydraulic lifters

Post by PackardV8 »

Oil is not compressible, but then neither is steel and a solid lifter valve train with competition springs will always net less lift at the valve than the lobe lift times rocker ratio minus clearance.
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Re: Action of hydraulic lifters

Post by MadBill »

As a liquid, 100% pure oil is not compressible by definition, however what's circulating in our engines is a solution of air dissolved in the oil plus entrapped air bubbles and especially for full travel lifters is more than compressible enough to allow significant lifter collapse.

Here's a great example: Back in the day, a pal of mine raced a Dodge Omni Turbo GLH with some factory help. One race the car suddenly lost over a second. After tearing out a few clumps of hair, they discovered someone had 'changed' the oil without remembering to first empty the pan. They drained it back to normal, hit the track and... they were still 1/2 sec. off. After more fruitless checks, they hunted down the factory rep and explained their problem. He said: "change the oil."
That did it; back to running on the numbers. He explained that the overfill had thrashed a lot of air into the oil, making it so compressible that the engine was losing a bunch of lift and that it could take hours for the micro bubbles to dissipate...
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Re: Action of hydraulic lifters

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Re: Action of hydraulic lifters

Post by MadBill »

The chart there shows mineral oil's bulk modulus at 260,000 psi/inch, so if the 'button' of a hydraulic lifter is 0.5" diameter, its area is ~ 0.2"² and if the spring plus inertia load on the pushrod is 500# and the lifter oil column 0.125" tall, the collapse would calculate as:
500/.2 = 2500 psi.
2500/260,000 = 0.009"
0.009 x 0.125 = ~ 0.0012"

Obviously, absent any compressible component, oil alone is not a problem.

There are oils designed to minimize aeration. David Vizard found (AIR) ~ 10 HP in a crate SBC via Ultra Pro's hydraulic lifter formulation.
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Re: Action of hydraulic lifters

Post by RDY4WAR »

Good discussion. I guess that debunks that myth. I first heard that from my father about 20 years ago when I was a youngster trying to learn everything I could. He was showing me how to check lift at the valve, and told me that hydraulic lifters can lose up to .006" lift due to bleed down. That was a 5.0L SBF from an 80s F150 which was a roller block with a factory flat tappet cam in it.

So could it be that this may have been true back in his era of mechanics (70s and 80s) but not necessarily today?

I noted this in Engine Analyzer Pro as well. With a hydraulic roller cam with .580" lift, it shows net lift as .574". The user manual doesn't give much info as to why as it only references net lift with solid lifter cams.
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Re: Action of hydraulic lifters

Post by midnightbluS10 »

RDY4WAR wrote: Thu Mar 22, 2018 6:25 pm Good discussion. I guess that debunks that myth. I first heard that from my father about 20 years ago when I was a youngster trying to learn everything I could. He was showing me how to check lift at the valve, and told me that hydraulic lifters can lose up to .006" lift due to bleed down. That was a 5.0L SBF from an 80s F150 which was a roller block with a factory flat tappet cam in it.

So could it be that this may have been true back in his era of mechanics (70s and 80s) but not necessarily today?

I noted this in Engine Analyzer Pro as well. With a hydraulic roller cam with .580" lift, it shows net lift as .574". The user manual doesn't give much info as to why as it only references net lift with solid lifter cams.

Could the .006 be accounting for flex in the valvetrain?
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Re: Action of hydraulic lifters

Post by cjperformance »

Get a hydro cammed car with consistent et's - Shim the hydro lifters so thier plunger is in the same posi as unshimmed so that its oiling all works as it should and only allow the plunger .015" movement till it would be bottomed. Now go run the car at the track again. It will run faster, why? Because the hydro lifters bleed off more than .006". If they only lost .006" then shimming till they have .015" till bottomed would have made no difference to net valve lift and HP.
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Re: Action of hydraulic lifters

Post by jsgarage »

How old are the lifters? There are (or were) a couple of different types of hydraulic valves used in lifters- the edge-orifice and the ball-check. As I remember, one of those designs was found to not work as well at higher rpms.
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