I am a beginner mechanic (still in high school) and I was just wondering if someone could tell me what makes these two types of cams different. Everything I have ever learned about cars is from this site so bear with me! Thanks

 

Based on my understanding of the concept hydraulic lifters use oil pressure to act as a hydraulic piston in order to move the pushrod up and down.

Solid roller camshafts use lifters that are actually solid, and have no hydraulic piston and do not use oil pressure to do the job. These are typically able to run higher spring pressures and therefore steeper lobe ramps and higher durations than could be run with an equivalent hydraulic roller.

 

Solid lifters whether flat tappet or roller are also lighter than a comparable hydraulic lifter and less valvetrain weight equals more rpm. And after all hp = tq x rpm/5252 so more rpm = more hp thats why solids usually make more power than comparable hydraulics.

 

While hydraulic lifters use the oil pressure to maintain "0" clearance on the rocker arms, they don't actually use the hydraulic pressure to push the push rod, the oil pressure simply keeps the lifter constantly in contact with the push rod, and the push rod constantly in contact with the rocker arm, and the rocker arm constantly in contact with the valve tip. In effect, they maintain "0" lash, and eliminate the necessity of frequent valve lash adjustments. Sort of "semantics", because without the oil pressure the lifter would collapse and there would be no lift. The hydraulic feature allows the lifter to keep the lash at "0" under a wide range of operating temperatures.

The disadavantage of the hydraulic lifter the the limits on spring pressure that they can handle. Too much spring pressure and the lifter collapses. That limits the upper RPM on a hydraulic. Solid lifters don't have that limit, because they can handle the extreme spring pressures required to keep the valve train stable at high RPM. But, there must be a clearance between the rocker and the valve tip, to insure the valve fully closes at max operating temperature, when the push rod and the valve stem have thermally expanded. I would estimate 7,000RPM as the upper limit for most hydraulics, although there are add-on springs that will allow some additional RPM. Over 7,000RPM, its probably wise to use a solid roller.

Many years ago, before hydraulic lifters, an annual valve adjustment was required for simple daily drivers, and a high performance engine might require valve adjustment after a day's racing. With upgrades in components, its possible to go several months on a high performance engine in race applications before a lash check is required, and maybe every 6,000 miles for high performance street applications.

 

Injuneer,
Would there be any advantage in a small solid lifter cam on the street if you were'nt going to spin it past 6300 rpm. This would be on a car that is only driven a thousand miles or less a year. I'm just looking for the biggest bang for the buck.
Sorry for jumping on this post.