Understanding Short Travel Hydraulic Lifters and Valvetrain Control
03-13-2013, 07:34 AM
#1
Understanding Short Travel Hydraulic Lifters and Valvetrain Control I intend this to be more of a theoretical discussion to try and understand valvetrain control, how short travel lifters work and why they are recommended by so many builders. I'm in the process of building a motor and have been recommended johnson semi limited travel linkbar lifters - it's been shown that they work, I'm just trying to understand how..
From reading other articles / threads / etc., here's what I've been able to find so far..
More generally speaking, several conditions to avoid in a valvetrain: valve float, valve bounce, net lift losses
Factors that contribute to valve float/bounce:
-insufficient spring pressure or improper spring harmonics
-excessive lifter preload
-aggressive cam profiles
-heavy valvetrain components
Factors that contribute to net lift losses:
-weak pushrods
-heavy valvetrain components
-excessive spring pressure
-sloppy lifter tolerances / excessive leak down rates
Related to the above, I'm trying to understand the "pump up" condition of lifters at high rpm. You have the spring pressure transmitted down through the pushrod opposing the hydraulic pressure built up in the lifter, but to "pump up" the lifter to the point where you're floating valves (basically adding your preload * rocker ratio to the valve lift, i.e. 0.060" turns into .100"+ at the valve), it seems to me that one of two things need to happen:
-the hydraulic pressure needs to exceed the transmitted spring pressure
-the lifter cup loses contact with the push rod (or the rocker arm loses contact with the valve tip), allowing oil to flow in and move the cup upwards
It doesn't seem to me that the first case could exist, otherwise you'd be opening valves with oil pressure alone.
Assuming you have optimized your valvetrain to help with control valve motion at high rpm - lightweight components, strong springs, rigid pushrods, proper preload, mild cam lobe shape - do the above conditions still exist? At high speed, does valve float become more of a function of the harmonics of the spring, modeled as a simple spring-mass system? Then when the spring starts to bounce in response to the initial input from the rocker arm, it loses contact allowing the lifter to pump up. I'm wondering regardless of what kind of springs you're using, does this occur to some extent at high rpm?
At any rate, how do limited travel lifters help you - at both high speed as I've mentioned and elsewhere in the rpm range of the engine? Lot of guys always preach about the "smooth curve" these help to produce because your valves start more accurately following what your cam is commanding them to do. Just wondering how they do it / what conditions exist that allow them to do it...
Am I thinking about any of this right? Sorry for the long winded post.
-Chuck
From reading other articles / threads / etc., here's what I've been able to find so far..
More generally speaking, several conditions to avoid in a valvetrain: valve float, valve bounce, net lift losses
Factors that contribute to valve float/bounce:
-insufficient spring pressure or improper spring harmonics
-excessive lifter preload
-aggressive cam profiles
-heavy valvetrain components
Factors that contribute to net lift losses:
-weak pushrods
-heavy valvetrain components
-excessive spring pressure
-sloppy lifter tolerances / excessive leak down rates
Related to the above, I'm trying to understand the "pump up" condition of lifters at high rpm. You have the spring pressure transmitted down through the pushrod opposing the hydraulic pressure built up in the lifter, but to "pump up" the lifter to the point where you're floating valves (basically adding your preload * rocker ratio to the valve lift, i.e. 0.060" turns into .100"+ at the valve), it seems to me that one of two things need to happen:
-the hydraulic pressure needs to exceed the transmitted spring pressure
-the lifter cup loses contact with the push rod (or the rocker arm loses contact with the valve tip), allowing oil to flow in and move the cup upwards
It doesn't seem to me that the first case could exist, otherwise you'd be opening valves with oil pressure alone.
Assuming you have optimized your valvetrain to help with control valve motion at high rpm - lightweight components, strong springs, rigid pushrods, proper preload, mild cam lobe shape - do the above conditions still exist? At high speed, does valve float become more of a function of the harmonics of the spring, modeled as a simple spring-mass system? Then when the spring starts to bounce in response to the initial input from the rocker arm, it loses contact allowing the lifter to pump up. I'm wondering regardless of what kind of springs you're using, does this occur to some extent at high rpm?
At any rate, how do limited travel lifters help you - at both high speed as I've mentioned and elsewhere in the rpm range of the engine? Lot of guys always preach about the "smooth curve" these help to produce because your valves start more accurately following what your cam is commanding them to do. Just wondering how they do it / what conditions exist that allow them to do it...
Am I thinking about any of this right? Sorry for the long winded post.
-Chuck
Last edited by ckpitt55; 03-13-2013 at 08:54 AM.
03-25-2013, 12:07 PM
#2
Look at the limited travel lifters as solid lifters with just a little cushion...that is basically all they are which gives you the best of both worlds (accuracy of a solid lifter with long or no valve adjustments needed after it is first setup and assembled) plus solid lifters tend to be more noisy.
03-27-2013, 09:58 PM
#4
I got this from another site, but it's good:
Basically, pump-up = preload cancellation causing the valve not to close by the same amount the preload is set at x rocker ratio. This happens because of improper oiling, shitty lifter design, crazy lobes, etc.
When properly set, you have some preload that depresses the piston in the lifter a certain amount. When you get into RPM's where the springs can no longer keep the lifter on the cam, you start getting some clearance. With this clearance, the hydralic lifter trys to take up the slack. It can only do this to the amount it was originally depressed. That's the 'pumping up'.
What happens when that happens is that now the lifter is effectively too long that when the valve should be closed, it nolonger is. It's left slightly off the seat by that slack amount times your rocker ration. Hence, power drops off as the valves don't close completely on compression and power stroke.
Better parts (lighter and better matched springs and their rates) and limited preload lifters help this problem by helping keep the contact all the time. As far as damage, the lifter would most likely lose contact as it's coming over the nose. Piston is not near that event. However, once it gets pumped up. You'll have a closer piston to valve clearance the what you measure. If you were close, you'll be that much closer. A lifter with a preload of .040" will cause the valve to stay open .040" x rocker ratio when pump-up occurs.
What happens when that happens is that now the lifter is effectively too long that when the valve should be closed, it nolonger is. It's left slightly off the seat by that slack amount times your rocker ration. Hence, power drops off as the valves don't close completely on compression and power stroke.
Better parts (lighter and better matched springs and their rates) and limited preload lifters help this problem by helping keep the contact all the time. As far as damage, the lifter would most likely lose contact as it's coming over the nose. Piston is not near that event. However, once it gets pumped up. You'll have a closer piston to valve clearance the what you measure. If you were close, you'll be that much closer. A lifter with a preload of .040" will cause the valve to stay open .040" x rocker ratio when pump-up occurs.
