Call Johnson and ask for a slow bleed down lifter part number and buy them. Turbo motors have high cylinder pressures and that tends to make the lifters collapse or " bleed down" under boost, effectively shortening the effective duration and lift. If the lifter bleeds down about .050 that shortens the lift by that amount multiplied by the rocker ratio, and that is right at .0775". Thats a LOT! Now look at the duration of your cam at advertised and at .050".
A Comp 270 cam is 270 degrees advertised, and 224 degrees at .050".
Yup, you will lose 46 degrees of exhaust duration!
Turbo motors need more spring pressure due to the boost trying to push the intake valve open, and back pressure in the exhaust pushing that valve open too. This causes premature valve float.
Some cam companies compensate for this in their blower Hydraulic cams by lengthining the exhaust duration.
It used to be said that turbo cams should have shorter exhaust durations to "keep the exhaust velocity higher" to help spool the turbo quicker. That may have been fine 30 or more years ago when turbo tech was not advanced as it is now, and boosts were MUCH lower.
The more preload used means less bleed down can occur before the lifter bottoms out, and that means more HP.
On our boosted and big hydraulic cammed cars and Harleys (Harleys use Chevy lifters, but with restricted upper oiling rates) we lash the lifters up a half or 3/4 turn UP from the bottom to minimize collapse.
On my 136 H.P. Harley, it loses 4 to 6 H.P. if lashed a half turn down from the top, depending on oil temp! And it is naturally aspirated! AND H.D.s all run 10w-50 oil too!
I have done a lot of lifter testing on hydraulics in H.D.s because of the Chevy lifters, and ease of changing lash EXTERNALLY with quick adjust pushrods, and cover tubes.
I first discovered the bleed down phenominon when I bought one of the first aftermarket Hydraulic roller cams for my Vette in 1985 or 6. It was supplied with dual springs of about 175 seat pressure. And going from the stock F.T. cam to the bigger Hydraulic roller cam SLOWED the car down by about 7 tenths, and 6 M.P.H. on the dragstrip. After rechecking the lash and making another bad pass, I noticed the valvetrain clattering on the return road, and on a hunch, I bought a can of STP from the fuel vendor and made another pass. It ran almost as fast as the stock cam. The next day I bought a set of solid roller lifters, and installed them at .005 lash, and ran the car again next week. The car then picked up 8 tenths over stock and over 7 M.P.H.
The only Hydraulic roller lifters available then were the stock Chevy ones, so I tried removing the inner springs and changing to fresh oil. The car again slowed down a few tenths and a couple of M.P.H. so I put the solids back in and ran it for another 4 year till I sold it.
As I remember, Hylift Johnson now makes over a dozen different bleed rate Chevy Hydraulic lifters.
Performance Hydraulic cams were lashed "back in the day" at a half turn down from the top. That was because ramp rates exceeded spring tech at that time, and if over revved, the lifters pumped up, and valve/piston contact was possible. Spring tech is much better now and "pump up" only happens with old or incorrect spring application.
Don't worry about lashing "up from the bottom", as the parts in the engine: block and heads, run hotter and "grow" more than the cooler pushrods, especially aluminum heads.
Sorry about the long post, but this is good info!
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