There is an age-old decision that high performance engine builders must make: hydraulic lifters, or solid lifters? Typically, (or more accurately, traditionally) the school of thought has been that hydraulic lifters were a better choice for street engines that accumulated a lot of miles at varying RPM, and solids were a better call for race engines that spent more time at high-RPM and were rebuilt regularly.
These opinions were created way back in the flat-tappet era, and followed the respective designs into the modern roller generation. Since hydraulic lifters did not require adjustment once they were set, they were a lower-maintenance item that street enthusiasts would appreciate. Setting hot lash was an art form reserved for the more hardcore race crowd. Certainly, the stability of a solid lifter offered consistency and strength to survive extended periods of high-RPM use, and by setting the lash at the minimum, every thousandth of an inch of precious lift and every degree of duration would be delivered to every valve.
At idle, the reduction in oil pressure would allow a bit more civilized idle in the hydraulic designs, while the solids would demand perfect adjustment to deliver a signature “crisp” lopey idle sound, and the requisite mechanical performance advantage to match.
Well race fans, it’s deep into the twenty-teen years now, and most of those ancient myths are busted. Modern technology and advanced engineering are blurring the line between hydraulics and solids. While both designs have seen durability increase over the years (mostly due to improved materials, tighter tolerances, and wider roller bearing surfaces), the real advances have been on the hydraulic side of the fence.
Modern engineering has led to more precise plunger, spring, and retainer systems. These have resulted in more consistent fluid control, both in and out of the lifters. Combined with the rest of the aforementioned advances, and with the benefit of decades of research on every part of the lifter design, the modern hydraulic roller lifter gives up little, if anything, to its solid counterpart. The benefits of the hydraulic design, especially that lack of a need to set lash or adjust anything once it’s set properly and locked down, brings plenty of benefit to enthusiasts whose valve covers aren’t easy to access.
The current trend toward turbocharging brings with it a commitment to relatively exotic plumbing. The deep engine setback of modern performance cars makes pulling valve covers a real challenge. Not having to do so between races (or, in extreme cases, between rounds) is a real gift. Certainly, improved poly lock designs have really helped minimize the need to set valve lash on a regular basis. Compared to the early parts racers had back ten or twenty years ago, things are much improved.
The question is, can a hydraulic lifter be pumped up past it’s adjustment point, overcome all of its preload, and subsequently hold the valve open? This phenomenon is called “pump-up” and it’s the kind of thing people claim to have seen or experienced, but very few have genuine evidence.
Many of us have experienced the well-documented phenomenon of valve float, where the valve springs are too weak to keep up with the actions of the valve and the valve isn’t able to close all the way. Can people be confusing valve float with lifter pump-up?
We spoke directly with a couple of the industry’s most experienced race lifter experts and got their opinions. We learned a lot, and we think you will too.
We asked Ben Herheim of Howards Cams, who is familiar with the concept of hydraulic lifter pump-up, if he could explain how pump-up could happen, and what we could do to prevent it. “Pump up can be the result of several problems in the hydraulic valvetrain. The most common is dynamic instability of the system. This occurs when the spring is unable to retain contact between the system components, due to insufficient spring load,” Herheim explains. “The rare ‘pump-up’ phenomenon is not constant throughout the RPM range. It can only occur when the spring margin or system stiffness become inadequate.”
“Higher-load springs can sometimes be used to remedy this issue, or a change in cam profile is needed. Other times, pump-up can be caused by system deflection, where one or more of the components in the system actually bends enough to unload the check ball and the lifter reacts by filling with oil,” says Herheim. “Unfortunately, it is filled to a higher level than needed and can hold the valve off the seat. Oil temperature could cause this on a cold start if the oil pressure was great enough to overcome the load from a seated valvespring. It would have to be quite high, though.”
Billy Godbold, Valvetrain Design Engineering Chief at Comp Cams feels that enthusiasts are seeing something that might be mistaken for pump-up, and is still a problem that needs to be addressed.
“What we are talking about here is the bleed down rate and effective lash (clearance) that reduces the dynamic duration and stability of a hydraulic roller lifter system at high-RPM,” explains Godbold. “While valve bounce can lead to the hydraulic system holding a valve open, there is not an actual mechanism that can accurately be described as ‘pump-up.’ The valve bounces up, and the dumb hydraulic system just adjusts to hold it up for quite a while.”
“While the bleed down rates definitely change dynamic duration, and it changes based on RPM and all kinds of other influences, we have not seen anything that can accurately be described as ‘pump-up.’ The closest thing we have seen on the Spintron is when you go into severe valve bounce,” Godbold reveals. “Unlike with a solid lifter bounce, which has a natural, symmetric parabolic shape, when you have significant bounce on a hydraulic system the inner piston can move up and hold the valve open for as much as an extra 50 degrees of crank rotation. I believe the guys running engine dynos in the ‘70s through ‘90s would see fuel standoff above the carburetors when this happened, and they knew the intake valve was being held open.”
“While that part of their hypothesis was correct, the mechanism was initiated by valve bounce, and then the lifter auto-adjusting, and not any ‘pumping-up’ of the hydraulic lifter,” explains Godbold. “Engine builder [and multi-time Engine Masters Challenge Champion] Jon Kaase told me a story once about his experience with hydraulic lifter pump-up. Neither he nor I can fully explain it…”
“They had an internal check valve that got stuck in an oil pump, and the oil pressure went ballistic at higher RPM. This hydraulic lifter engine acted exactly like textbook ‘pump-up.’ The high-pressure piston has a little under half a square inch of surface area, so you would assume that nearly 400 psi of oil pressure would be required to overcome a 150 lb/in spring,” says Godbold. “That type of calculation does not even mention the crazy 1,500-plus-pound inertial forces from opening and closing the valves, but once Jon replaced that messed up oil pump, the engine ran just fine!”
|Characteristic||Value||Unit of Measure||Notes|
|Hydraulic Lifter Piston Diameter||.625||inch||Typical for most hydraulic lifters|
|Piston Area||.307||square inch||Area=Pi(R) squared|
|Oil Pressure||100||psi||pounds of force per square inch|
|Force on Pushrod||30.7||pounds of force||F= pressure x area|
|Rocker Ratio||1.7||:1||The force to the tip is reduced by the rocker ratio|
|Total Force Acting Against the Spring Seat Load||18.0||pounds of force||For typical seat street loads, these values could be enough to offset more than 10% of the total spring seat load, but not nearly enough to overcome the total seat load.|
Here is a quick calculation table with real numbers of the force acting to open the valve. You would have to approach 1,000 psi of oil pressure to actually overcome the valve seat load, but even 100 PSI could offset ten-percent or more of the seat load.
While interesting, that is second-hand experience, which Godbold hasn’t been able to duplicate. “I have never seen anything similar on the Spintron, but we have never gone ballistic with oil pressure either. We could probably make a lifter [overpower the valvespring], but the math looks skewed against it being possible until you get well past 150 psi of oil pressure,” says Godbold. “At that point, you might effectively take away almost 50 lbs of seat load from the spring and thereby make your system unstable, resulting in the bounce up, and then holding open the intake valve 30-plus degrees like I described originally.”
Racers are an ingenious breed, and have tried many things in the past in order to run more aggressive camshafts while being limited by hydraulic lifters. “There are some tricks that have been tried with running tight lash solid profiles on very high bleed-down lifters, but this is not extremely effective as you are typically setting the lash off the collapsed height of the lifter and would be better served with a solid lifter,” Godbold relates. “A Comp Cams ‘Short Travel’ lifter has a smaller high pressure chamber, and can run either a more aggressive profile or higher RPM, and both paths have been used quite successfully. The only factor to consider if using that style lifter is that the preload needs to be set accurately.”
There are also other factors which affect the behavior of the lifter, such as the engine oil itself. “Both oil temperature and aeration play major factors in the effective stiffness of the lifter. As the oil generally becomes more aerated at higher RPM, and the inertial pushrod loads increase dramatically, we do see the hydraulic lifters ‘act’ like they have more lash with RPM,” reveals Godbold.
As oil temperature changes, so does it’s actual viscosity. “The effective duration decreases with temperature. People would be shocked to see just how much the effective lash on a hydraulic lifter changes with these conditions. The short travel lifters reduce this effect, but the reason solid adjustment is so much more common in racing applications is the consistency of valve motion under different temperature and oil aeration conditions,” says Godbold. “I hate to be overly critical, but discussing the effect on pump-up is like asking who would win in a fight between Bigfoot and the Loch Ness Monster. It’s that rare.”
Flat or Roller – Is Anyone Safe?
“The internal adjustment system is very similar in both flat-tappet and roller-tappet designs. Both systems have very similar bleed-down rates. There are minor dynamic differences due to the typical mass, acceleration and velocity characteristics, but in general, these two types of hydraulic lifter behave in very similar ways,” Godbold explains.
Herheim concurs, saying, “Both hydraulic roller and hydraulic flat tappet cams are technically susceptible to pump up. We have seen this problem in hydraulic rollers more often than hydraulic flat tappet cams. This is due to the significant weight of the lifter and the aggressive cam profiles that are being used.”
Godbold continued to dive deeper. “The real differences in bleed-down rates, effective lash, and the dynamic stability are quite dependent on oil viscosity,” he explains. “Like we mentioned earlier, it is the actual running viscosity that matters, hence the dependence on temperature and aeration in the oil as well as the rated viscosity.”
The lifter bleed down rate (which is directly related to effective stiffness and dynamic stability of the lifter assembly, as well as the speed at which the lifter can adjust itself) is probably the most important factor in hydraulic lifter design. “The tolerances between the inner lifter piston and the inner walls of the lifter body are the most tightly controlled dimensions in a modern engine. In other words, trying to make the hydraulic system work accurately and consistently at high-RPM is definitely the place you start on any hydraulic roller or flat tappet lifter.”
We asked Godbold if there are any innovations on the horizon that enthusiasts can look forward to. He told us there were plenty already on the shelves that folks might not be aware of, and even more exciting technology coming in the near future.
“There are some really awesome new ideas coming out in new profile designs, lighter components (to lower the loads on the hydraulic system), and new valvesprings which are all rapidly evolving. The improvements in measuring the bleed down and dynamic characteristics of hydraulics are also improving current designs.”
“Additionally, guys like Lake Speed at Driven are working on oil formulations that more effectively shed the micro-bubbles in oil-reducing aeration. Together, all of these are greatly increasing the safe RPM range of hydraulic systems. We have a 6.0-liter LS engine that has been above 9,000 rpm over 200 times on the dyno at Comp Cams!”
“The components people choose for their build are often not all from one manufacturer, and that is part of the fun of racing and building hot rods,” says Herheim. “Unfortunately, it can also become a problem if the components are not suited to work well with each other. The key to getting the valvetrain to be faithful is having well-matched components for the required purpose.”
After talking it over with some great guys who work on high-end valvetrain components for a living, it sure seems like hydraulic lifter pump-up is a rare occurrence, although it remains a remote possibility. Like most concerns in the realm of higher-end engine building, a little bit of research, and careful selection and matching of components should be all it takes to ensure it never happens to you.
Consequently, your choice of valvesprings is just as critical, and may be responsible for more of the blame being tossed at the hydraulic lifters when an engine creeps into high-RPM territory and suddenly stops making power or gaining any more speed.
Finally, we can safely conclude that today’s modern hydraulic lifters are entirely capable of high-RPM use with more aggressive cam profiles than ever before. If you consult directly with your manufacturer of choice, it’s entirely possible to source a hydraulic lifter-based valvetrain package capable of touching 9,000 rpm reliably. That means plenty of big time fun without the constant need to check or reset lash, and know that with the right combination of parts, pump up is kept in check at the same time.