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Old 10-15-2017, 10:57 PM
Jim Hand Jim Hand is offline
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Join Date: Dec 1999
Location: Lees Summit, MO, USA
Posts: 933
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There are distinct differences between the true variable lifters made by Rhoads, and the stock type lifters that Crane and others sell under their brand. Rhoads have a patented groove or flat down the outside of the inner part to allow oil to leak. The Crane lifters are merely stock style lifters with slightly more clearance between the inner body and the case (we have read that the components are quality fall outs from regular production that are reassembled with looser clearances internally). Independent tests show that the Crane units have about 20 percent of the leak down of the Rhoads. That is why they are quiet and function pretty much as standard lifters.

Oil thickness/viscosity does have a large effect on the leak down of any lifter. When the lifter is on the base circle, the fill hole is exposed to the oil pressure channels in the engine, and the lifter fills with oil to take up any clearance between the cam, lifter, push rod, rocker arm, and valve, exactly as do any hydraulic lifters. As quickly as the cam lobe begins to lift, the check system in the Rhoads closes which traps the oil within the lifter body, exactly as in all other hydraulic lifters. As the valve spring exerts pressure on the lifter, it begins to leak, just as do all lifters, but the Rhoads leaks more due to the machined-in groove or flat. This reduces the peak lift, and because the lifter leaks continuously from opening point through the entire lobe profile, the duration is shortened, but only on the valve closing side of the open/close cycle. The result is less lift, shorter duration, and an advanced lobe center, all helpful to low rpm operation. The advanced intake lobe center effectively increases real compression, as the earlier closing intake valve allows compression to build earlier on the compression stroke. The Rhoads never “stop” leaking, but as the leakage is a function of time, they leak less at higher rpm, and at around 3000-3500, the leakage effect is minimal (and the clicking stops). The noise made by the Rhoads is caused by the valve setting down on the seat earlier, and not tracking the closing ramp. I have run Rhoads for at least 35 years, and have never experienced any seat problems or recession. Again, oil pressure has no direct effect because the lifter sees actual oil pressure only when the lifter is on the base circle with minimum spring seat pressure. Of course, the thinner the oil, the more they will leak, and click, but that will also shorten the cam lift/duration even more. Several major cam companies sell Rhoads under their brand of leak down lifters, but not Crane. Will Rhoads lifter help you? It depends totally on where you apply a load to your engine. If your converter does not hook up until 3000 or higher, you will see little or no difference. However, if you run a tighter converter and a cam larger then about 220 on a 400, or 230 on a 455 Pontiac, you will likely feel and measure distinct differences in operation, providing you have traction.

STANDARD STOCK TYPE LIFTER OPERATION
(And comparisons to Rhoads operation.)

The easiest way to explain hydraulic lifter operation is to quote from a '65
Pontiac shop manual.

"When the lifter is riding on the low point of the cam, the plunger spring keeps the plunger and push rod seat in contact with the push rod. When the lifter body begins to ride up the cam lobe, the ball check valve cuts off
the transfer of oil from the reservoir below the plunger. The plunger and
lifter body then rise as a unit, pushing up the push rod and opening the
valve. As the lifter body rides down the other side of the cam, the plunger
follows with it until the valve closes. The lifter body continues to follow
the cam to its low point, but the plunger spring keeps the plunger in
contact with the push rod. The ball check valve will then move off its seat
and the lifter reservoir will remain full. During operation a small amount
of oil leaks out of the lifter between the plunger and body. A controlled
amount of leakage is important to provide continuous adjustment of the
plunger position within the lifter. This leakage is called "leak down" and
must be within certain limits to provide correct operation.”

The manual provides more information about push rod/rocker arm oiling, but the above defines the operation we are interested in. Note the description of "oil leaks out of the lifter between the plunger and body." Rhoads lifters have a groove machined into the side of the plunger in order to allow greater leakage, but it is still leakage in the exact same manner as
standard hydraulic lifters. The groove (earlier Rhoads lifters –pre 1980 - had a
machined flat in the plunger) allows a certain increased amount of oil to leak past. The leakage amount is dependent on the following factors: time for a complete cycle, which in turn is dependent on engine rpm, thickness of the oil, which is dependent on temperature of the oil and the oil viscosity, and resistance seen by the lifter, which is dependent on the valve spring pressure and lobe configuration.

It becomes obvious that at low rpm, any lifter, and especially the Rhoads,
will leak more, due to time available in each lobe cycle, whereas, at higher
rpm, there is not as much time in each lobe cycle, so the leakage is
decreased. They never stop leaking, but the leakage amount at higher rpm is almost insignificant in regards to the lifter function.

SEQUENCE OF OPERATION OF A TRUE VARIABLE LIFTER, ie. Rhoads:

1. When the lobe begins to rise, the check circuit in the lifter seals and
the lifter becomes isolated from oil pressure, and the valve opens at the
SAME time as with standard lifters.
2. As quickly as the lifter experiences lobe/spring pressure, it begins to
leak, and the lift profile of the valve begins to deviate from the cam lobe
profile. At the time the lobe reaches peak lift, the lifter plunger has
sunken down into the lifter body, and has not lifted the valve to full rated height.
3. As the lobe begins to fall, the lifter follows, but the plunger
continues to sink even further down into the body (due to the spring
pressure), and the valve contacts the seat before the lifter reaches the
closing ramp.

Here are the results of the above sequence of events:

A. Rated peak valve lift was not reached.
B. As the valve seated early, the duration of the valve open was
reduced.
C. As the duration was reduced while the valve opening remained the same, the valve center was changed to an advanced position.
D. As the valve did not follow the closing ramp, but rather the leaking
lifter/plunger, it was not allowed to seat as smoothly as the lobe is
designed, and as a result, set down harder, causing a noticeable click, or
clack.
E. The end and final result of less lift, less duration, and earlier intake
lobe center is increased torque and vacuum at the lower rpm. The additional result of extra noise also occurred.

This sequence continues through the entire rpm range, but as rpm increases, time for lifter leakage decreases, and the effects described become minimal and insignificant at about 3000-3500. And above that RPM, the cam/lifters begin to operate just as do standard hydraulic lifters. The 3000-3500 number is only a general estimate, and will vary, depending on oil characteristics, valve spring pressure, engine temperature, lobe design, and even the rate at which the engine accelerates. My standard Rhoads lifters quit clicking at between 2500 and 3000 when the oil is warm.

Another relatively unknown advantage of the Rhoads lifters is that they are a true anti-pumpup lifter, and will run to much higher RPM then standard hydraulic lifters. And they have the best quality circlip retainer which assures they will stay together at high RPM.

The lifters that Crane sells as variable have slightly more clearance
between the plunger and the lifter body, and do leak slightly more then
Crane's standard lifters, but according to independent tests, are not even
close to the rate of the Rhoads. That is why the Crane lifters don't click.

Note that the groove on the internal unit of Rhoads lifters is patented. Grooves on the exterior of the outer unit are not patented, and are used by various lifter companies (including Rhoads) for improved oiling between the lifter and cam lobe.

Jim Hand