Why do Solid Lifter Roller motors make more power than Hydraulic Roller motors?

[Nasty Wendy]

What allows the solid lifter cams to be more aggressive than a hydraulic? Shouldn't the hydraulic roller lifter be just as stiff once its pumped up with oil? Oil doesn't compress so shouldn't the hydraulic lifter act like its solid? What would happen if you used hydraulic roller lifters with a cam designed for solid lifters? Can solid lifter cams make power before the engine is screaming? The big duration profiles I see for solid cams seem like they won't make any power until 4500+ rpms. Do the solids ramp up faster than the hydraulics? If so why? What limits the hydraulic cams? Thanks in advance for answering these rookie questions.

[Steve Wood]

too damned many questions!!

The hydraulic lifter is a complex mechanism that has to deal with a variety of conditions. Within reason, it does pretty well.

But, it tends to fail in extremes.

The worst case is that at high rpm, it (the hydraulic) tends to pump up. When this happens, the plunger extends, removing the preload, and the valves are held open rather than completely closing. This is usually very obvious as the engine just stops revving and will often backfire as both valves are slightly open at all times so the ignited mixture goes back into the intake as well as the exhaust. On a serious cam with good lift, this leads to bent valves

Valve spring dynamics can also contribute to this when various unwanted harmonics start doing a jig backwards thru the rocker arm against the plunger.

The camshaft profile must also be different with the ramps designed to accomdate the plunger characteristics in order to not upset the hydraulic dynamics as the lifter adjusts to the lift slope and tries to run quietly. Otherwise it might fill with air bubbles as well as oil.

Also, the ramps required to make the hydraulic function correctly take up a few degrees of duration with very little valve lift.

Various cam companies may rate things slightly differently but, duration is typically rated around .006" on a hydraulic cam card while a solid cam shows duration around .020", I believe. This makes it hard to compare hydraulic to solid lifter cams.

Hydraulic lifters are generally heavier as well. This makes them harder to control at high rpms as well. Again, it aggravates the problem of trying to make them translate cam lobe profile into consistent lift.

Solid lifter cam profiles can be much more aggressive with lighter lifters that do not vary in length and will reliably turn much higher rpms before the spring is overcome and the valvetrain ceases to do what it was designed for.

[Nasty Wendy]

Thanks Steve. That makes it clearer. What still kinda has me is that there are other engines out there turning 8k rpm with hydraulic roller cams. And I thought that you wanted the lifter to pump up. Are you saying that the lifter can be over pumped? Thanks for all the info.

[Steve Wood]

You set the preload at 30-40 thou with the valves closed....if you rev it hard enuf, the preload will disappear as the plunger pushes up against the circlip...now you have zero preload and the valves are 30-40 thous off the seat...engine just blubbers and pops. This tends to upset the valvetrain and the springs may lose control and suddenly the valves are hung even further out because they are not following the lobe anymore.

Comp has two sets of hydraulic roller lifters...one is made to be set to almost zero preload....the engine will rev higher because it takes longer for the valvetrain to lose control.

In the end, the weight of the lifter is a big negative as the spring has to be able to control it. There is a limit to how strong the spring can be before it starts doing more damage than good.

I don't know what engine you are referring to that can rev 8000 rpm on hydraulics, but, I bet its something small with very little weight in the valve train components including the lifter??????

[Nasty Wendy]

Originally Posted by Steve Wood

I don't know what engine you are referring to that can rev 8000 rpm on hydraulics, but, I bet its something small with very little weight in the valve train components including the lifter??????

and it had a short stroke.

Thanks again Steve.

Would you happen to know this weeks winning Powerball numbers?

[Steve Wood]

Of course....

[Joseph]

Wendy, to answer your questions, 1: Solid lifter cams, if properly adjusted, have little to no lash and the lash that is present isn't actually lash, it's clearance for thermal expansion of metal and or oil. Therefore the ramp rates can be extremely aggressive due to the ability of the roller follower to actually roll rather than float.
2&3: Steve, I believe actually answered these.
4: If you use Hyd. lifters on a solid cam, be careful. The lifter bores need to be on the tight side of the clearances, if not side loading the lifter in the bore or on the ramp WILL happen. Crunch scrape bang$$$$$$$$
5: Solid lifter cams can make power at low rpm, but the valve timing has to be optimized and the cam choice has to optimized for this use. The reasoning behind a solid follower is that at elevated ramp speed and engine speed, you have fewer variables (oil pressure, flow, preload, etc) so the mechanical control of the actual valve movement is more direct.
6: Most solid cams are designed for racing, and since power is just a theoretical measure of torque at a given rpm, the higher your VE at a higher rpm, the more power your engine will make. Opening the valve faster, longer and higher, along with closing.
The last questions should be answered as well, but the Hyd. limits. Basically the hyd. lifter is used versus solid due to the mechanical noise from solid cams and the use of the oil and oil pressure to control and dictate the valve movement. Hyd. Plunger extension is what Steve was explaining and this is when addtional loss of valve control happens as well as a metal stress on the plunger, retainer and flexural distortion of the push rod.

I hope this helps. If you can, read anything and everything on paper by David Vizard. His principles explained in every one of his books can be applied to any internal combustion engine used, plus his endless time on dynos helps people like use avoid costly mistakes.

[Nasty Wendy]

Great info Joseph. Thanks.