Unless lifters are specifically reduced or short travel, Hydraulic roller or Hydraulic flat tappet lifters typically have .120" to .130" of travel. The deeper the preload, the more power is produced.
As oil temps rise, thinner viscosity oil is used, synthetic oil is used, rpms go up, boost rises, more spring pressure is used, cam ramp rate is quicker, rocker ratio is increased, running less oil pressure, then lifter bleed down is increased. That's bad.
This means less lift at the valve, and less duration is had. The less duration happens as sooner closing of the valve.
In effect the cam seems smaller to the engine, especially as related to intake closing which determines RPM peak power point, lowering it, sometimes substantially.
On the exhaust side, this phenomenon also reduces the duration which means less efficient evacuation of exhaust in the cylinder.
This is much worse on a turbo motor which has a much higher cylinder pressure than a N/A engine.
A 1.5" exhaust valve has a head area of 1.77 square inches. Multiply that times the cylinder pressure as the valve is just opening, (1000 psi and more,) and you easily have more than 1700 pounds pushing against that poor lifter!
This is so why you need thick wall chrome moly pushrods too!
On the intake side, the manifold pressure is trying to open the valve, easing the load on that lifters bleed down.
Another thing to think about is anything you do on the exhaust side to help "breathing" on a blown engine will drop the apparent boost, but raise the horsepower. This reduces the residual exhaust in the cylinder and allows morefresh air/fuel in.With belt driven blown motors this is easily seen on the boost gauge, but the turbos wastegate being "fixed" masks this symptom.
With a belt driven blower, anything done to raise the intake flow drops the boost because it allows the air into the cylinder to make HP, rather than stack up against the backside of the intake valve.
Improvements on the exhaust side reduce residual exhaust taking up space where fresh mixture belongs.
BOOST IS SIMPLY HORSEPOWER THAT HASN'T MADE IT INTO THE CYLINDER!
Boost (backpressure) is bad. Flow is good!
Once the RPM, BOOST, or spring pressure is higher, the more gains are possible with solid or reduced travel lifters.
HP boats react the same way. They can run miles at high RPM and the oil temp keeps getting hotter, allowing more bleed down. They benefit from water to oil coolers.
Bottom line back to the original point of the post, more lifter preload makes more power! Less lifter plunger collapse.
NHRA stock eliminator records are set with lifters set to about .010" from solid. The tech guys figured that out, so we filled them with 140wt gear oil!
Ya tow to the line with the engine off, do the burnout, set the record, then keep the RPMs up out of gear while slowing after the pass, and on the return road on the way to the tech barn to "wash out" the lifters so they pass tech.
The less or reduced travel lifter or more preload use is made better by BHJ decking the block square and measruring/correcting the rocker shaft to head surface distance. That allows running the preload/ravel tighter.
End of lesson.
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