Preload for Performance - RV8
Discussion
Ok so in theory a hydraulic lifter can lift whatever preload is put into it (what with liquid being virtually incompressible)
And the RV8 calls for between 20 and 60 thou of preload in the lifter before oil enters.
Thus when the lifters are full they are pushing the valves open whatever lift is on the cam + whatever preload put into lifters then subject to rocker ratio (1.6 in RV8 case)
Therefore there must be a point over 60 thou preload where there is a possibility of the valve staying open when on the heel of the cam and the piston striking the valve when on full lift if too much is applied?
Am I correct in my assumptions so far?
If so it follows that for a performance application it is vital that maximum safe preload is attained - that is to say one that satisfies both the above.
I have been setting my preload to 40 thou but in theory if it could take another 40 or so then should I do it? I would need to check that valves are closing (compression test) and check piston crown clearance (plasticine)
After all, whats the point in getting stage 3 heads, big valves and enlarged manifolds if the valves aren't even opening to their full potential!
And the RV8 calls for between 20 and 60 thou of preload in the lifter before oil enters.
Thus when the lifters are full they are pushing the valves open whatever lift is on the cam + whatever preload put into lifters then subject to rocker ratio (1.6 in RV8 case)
Therefore there must be a point over 60 thou preload where there is a possibility of the valve staying open when on the heel of the cam and the piston striking the valve when on full lift if too much is applied?
Am I correct in my assumptions so far?
If so it follows that for a performance application it is vital that maximum safe preload is attained - that is to say one that satisfies both the above.
I have been setting my preload to 40 thou but in theory if it could take another 40 or so then should I do it? I would need to check that valves are closing (compression test) and check piston crown clearance (plasticine)
After all, whats the point in getting stage 3 heads, big valves and enlarged manifolds if the valves aren't even opening to their full potential!
The hydraulics inside the tappet are not sealed, otherwise how could they bleed out any air bubbles in the oil in order to maintain the correct clearance?
There is a very small bleed hole/ valve which allows the pressure to decay when the engine is not running, this can be tested with a rig which enables measuring the time taken for a thin liquid, usually kerosene, to be forced out through the orifice,known as the "leak down" time, manufacturers will batch test tappets to ensure that problems don't arise in service and aving once had the pleasure of testing 2 whole stillages full of them I can reliably inform you that it is a very tedious pastime!
I suggest you read all of Crane Cams web site thoroughly, they will explain the basics, either that or Google hydraulic tappets, the knowledge is out there if you can be bothered to look for it .
There is a very small bleed hole/ valve which allows the pressure to decay when the engine is not running, this can be tested with a rig which enables measuring the time taken for a thin liquid, usually kerosene, to be forced out through the orifice,known as the "leak down" time, manufacturers will batch test tappets to ensure that problems don't arise in service and aving once had the pleasure of testing 2 whole stillages full of them I can reliably inform you that it is a very tedious pastime!
I suggest you read all of Crane Cams web site thoroughly, they will explain the basics, either that or Google hydraulic tappets, the knowledge is out there if you can be bothered to look for it .
Auntieroll said:
The hydraulics inside the tappet are not sealed, otherwise how could they bleed out any air bubbles in the oil in order to maintain the correct clearance?
There is a very small bleed hole/ valve which allows the pressure to decay when the engine is not running, this can be tested with a rig which enables measuring the time taken for a thin liquid, usually kerosene, to be forced out through the orifice,known as the "leak down" time, manufacturers will batch test tappets to ensure that problems don't arise in service and aving once had the pleasure of testing 2 whole stillages full of them I can reliably inform you that it is a very tedious pastime!
I suggest you read all of Crane Cams web site thoroughly, they will explain the basics, either that or Google hydraulic tappets, the knowledge is out there if you can be bothered to look for it .
I agree they aren't sealed but the tappets aren't designed to bleed down during operation (not the normal ones anyway)There is a very small bleed hole/ valve which allows the pressure to decay when the engine is not running, this can be tested with a rig which enables measuring the time taken for a thin liquid, usually kerosene, to be forced out through the orifice,known as the "leak down" time, manufacturers will batch test tappets to ensure that problems don't arise in service and aving once had the pleasure of testing 2 whole stillages full of them I can reliably inform you that it is a very tedious pastime!
I suggest you read all of Crane Cams web site thoroughly, they will explain the basics, either that or Google hydraulic tappets, the knowledge is out there if you can be bothered to look for it .
I'm aware of how the lifters work in theory - just wondered if anyone has been down this road in practice and if my assumptions are correct.
Bobby Shaftoe said:
The bleed down lifters like the Rhoads are a different kettle of fish again, when i disassmbled one it had a large slot machined down the side of the internal piston, which i guess allows the lifter to be in a permenant state of bleed downedness and the idea was that at higher revs the oil would have less of a time window to bleed down and therefore you would have almost the full lift and almost the full duration of the installed cam. Seemed a bit of a haphazard solution to me.
And it is 
some report no problems on Yank V8'S (i.e no noise) some say they are noisy but I am yet to meet an RV8 owner that as used them and was happy with the noise at low RPM (bleed down state) 
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