# Generation IV Internal Engine - Lifter preload VS tight PTV clearance

i420tom
05-18-2012, 10:16 AM
I am putting a cam in a ly6. I know the cam will have TIGHT ptv in the range of .075-.090 on the intake side. I will of course be checking it to the best of my abilities.
I have prc .650 duals pat g cam and stock ly6 lifters stock ly6 bottom end

Should i be concerned with the lifters ability to become fully pumped up?
What i am concerned with is the lifter preload versus the piston to valve clearance.

So here it goes.
This will all be hypothetical
In theory
If my ptv is .080 and i set my preload at .060 then if the lifter gets fully pumped up hydraulically when preload is .060 multiplied by rocker ratio 1.7 would equal .102 more lift and the piston would hit the valve.
If ptv is .080 and preload is .040 then if lifter got fully pumped x 1.7 = .068 more lift. .012 PTV clearance IF this happens

Am i over thinking this? Do the lifters never get fully pumped to the max travel?

-TheBandit-
05-18-2012, 11:45 AM
How did you check you P-V clearance? The methods I'm aware of are:

Method 1:
Hydraulic lifters, light checker springs, zero lash (best done with adjustable pushrods), dial indicator on valve tip, rotate every couple of degrees and press down on the valve to meausure clearance.

Method 2:
Solid lifters, zero lash (best done with adjustable pushrods), clay on piston top, rotate engine min 2x, remove head and slice/measure clay

Either of those methods would not include preload in the equation.

i420tom
05-18-2012, 11:53 AM
I thought P-V clearance was checked with a solid lifter at zero lash having no "preload", which would give you worst case clearance. How did you check you P-V clearance? Whether you used checker springs or solid lifters, you shouldnt' have preloaded the lifters.

I have checker springs and adjustable pushrod to check PTV correctly with no preload, and a degree wheel. as well as a dial indiactor. However i am asking if the preload comes into play BECAUSE of the chance that the lifter can in theory be fully pumped up, in certain situations.

However WORST case scenario is when it is all assembled with the preload. you have a longer pushrod in order to achieve the preload. and IF the lifter gets 100% pumped up(this is what i need to know, if how and when a lifter can do this), your preload specs would then be added to total lift thereby not letting the valves close completley and lessening your PTV. Does this happen only when a lifter is faulty? Does this happen only at very high RPM? at too high oil pressures?

GC99TA
05-18-2012, 11:55 AM
I am putting a cam in a ly6. I know the cam will have TIGHT ptv in the range of .075-.090 on the intake side. I will of course be checking it to the best of my abilities.
I have prc .650 duals pat g cam and stock ly6 lifters stock ly6 bottom end

Should i be concerned with the lifters ability to become fully pumped up?
What i am concerned with is the lifter preload versus the piston to valve clearance.

So here it goes.
This will all be hypothetical
In theory
If my ptv is .080 and i set my preload at .060 then if the lifter gets fully pumped up hydraulically when preload is .060 multiplied by rocker ratio 1.7 would equal .102 more lift and the piston would hit the valve.
If ptv is .080 and preload is .040 then if lifter got fully pumped x 1.7 = .068 more lift. .012 PTV clearance IF this happens

Am i over thinking this? Do the lifters never get fully pumped to the max travel?

I'll preface my answers by first saying that I am not an expert and this is all just my opinion.

Having said that.....I don't think you have too much to worry about with lifter pump-up affecting your PTV clearance. I think the preload stays pretty consistent or even gets slightly deeper during high RPM operation, actually decreasing lift slightly. Way back in the day, some guys would run almost zero preload or "on the clip" to fight this condition.

I believe you must first float the valves (thereby losing contact between the pushrod and lifter pluger and leaving an "air gap" to be filled) for the lifter to have a chance to pump up. This makes weak valve springs the bigger issue and cause of why you'd have valve contact and it makes lifter pump-up a secondary result of the valve float.

The way I see it valve float occurs at high rpm. Everything happens so fast at high rpm that I honestly don't even know if the lifter spends enough time collecting the additional oil required to pump up.......but it certainly could. Even so.......you still may get PTV contact due to the valve float itself even if the lifter doesn't pump up.

To me, the key to avoiding all of this is to run adequate valve springs that won't allow you to float the valves to begin with. But again.....I could be wrong. I will say though (and somewhat contradict myself) that I prefer lesser lifter preload numbers for basically the same reasons you outlined above.

Again.....just my opinions.

-TheBandit-
05-18-2012, 12:01 PM
I agree with GC99TA. Lifters shouldn't pump up unless you have valve float.

This article has a pretty decent explanation of how hydraulic lifters work. I have always been fascinated by them. They're actually very simple. Very little oil pressure is supplied to keep the pushrod, rocker, and valve in contact when the cam is on its base circle. As soon as the cam ramps up, a check valve type mechanism (for explanatory purposes) traps the oil in the lifter, allowing the cam to translate motion to the pushrod through the trapped oil. Unless a lifter is faulty or you're in an extreme valve float situation, you shouldn't see any pump up.

http://www.ratwell.com/technical/HydraulicLifters.html

Patrick G
05-18-2012, 12:25 PM
The hydraulic lifters on the LS engine is one of the most precisely machined components in the automotive world. The lifter has a plunger that has around .160" of travel. The depth you place the pushrod is typically called preload. It is generally accepted in performance circles to set the preload towards the middle of the plunger depth in order to keep the pushrod in the sweet spot. As temperature changes, setting the pushrod in the middle of the plunger travel will keep it from being too deep or too shallow at any given time.

Normally when you run a stock GM rocker set and lifter, this means the rocker bolt will torque down 1 1/4 to 1 3/4 turns past 0 lash. One turn = .047" so 1 1/4 turns = .059" preload and 1 3/4" turn = .082" preload.

The high lift cam you're running has a smaller base circle than stock making it necessary to run a longer pushrod than stock to maintain preload in the .060-.080" range. The preload, when set properly will have no affect on piston to valve clearance. But proper preload is a key element in getting clean valve control at higher rpm. Clean valve control will ensure that you do not run into P to V clearance issues greater than those designed in the build. There should be no P to V worries on your end as long as you have the cam installed at the recommended centerline.

i420tom
05-18-2012, 12:41 PM
Thanks PAT G, Bandit, and 99TA!!!!!!
Pat what do consider acceptable P to V measurements?

468BIGBLOCK
05-18-2012, 03:35 PM
thats neat.

05-18-2012, 06:52 PM
The hydraulic lifters on the LS engine is one of the most precisely machined components in the automotive world. The lifter has a plunger that has around .160" of travel.

What is the plunger travel on the LS7 lifter?

427 zeo6
05-18-2012, 09:44 PM
Thanks PAT G, Bandit, and 99TA!!!!!!
Pat what do consider acceptable P to V measurements?

I'd love to see this figure :corn:

BriancWS6
05-19-2012, 11:08 AM
The old rule of thumb for P-V clearance is .080 intake, .100 exhaust.

hymey
05-21-2012, 05:02 PM
Always use pushrod length checkers as this will vary do not use.the thread pitch or counting method i did a.swap last night and half a turn was 35 thou and 70 thou full turn ALWAYS the case. Have i bent valves from to much preload. Yes once and it had 100 thou ptv . There isnt enough spring pressure to bleed down a lifter with small diameter springs. Valve float is the issue. They will most definately pump up once u venture past 6800 . I have purposely tested this till i found failure. People run to much preload all my setups are very quiet even with aggressive lobes. I run down to 50 thou clearance and preload 20 thou tops sometimes less. And yes tom u are a smart man 20 x 1.7 in this case keeps me safe. Turn them to 7500 rpm all day with this preload spring surge does not induce pump up. I never run past 50 thou preload . Valve train noise is always commented on how quiet it is plus engines rev so much cleaner. I found going deeper in preload has increased valve train noise. Plenty of spring low preload is the key and imo 75 thou is very safe run 20 to 30 thou and forget .