Shimming Hydraulic Flat Tappet Lifters

[Greg Reimer 7376]

Some of you guys are experimenting with things in an attempt to try things, and asking good questions. Some of the results reported in this thread rise up some other good questions as well. If some of you report large HP and MPH gains just by changing lifters to a non-hydraulic type, something must be going on here. Topping out the travel on a hydraulic lifter plus a little for lash(.002-.004), prevent the lifter from pumping up, thereby extending the theoretical RPM range of the engine, but what if the trend to stiff valve springs, low pressure oil pumps, and lightweight oil is resulting in a hydraulic lifter steadily collapsing as engine RPM increases going down the track? That would result in cam lift and duration steadily decreasing while on the back half of a run.The change to a solid lifter would prevent this, thereby maintaining valve train activity during the whole run,and resulting in ET reduction and MPH gain, in effect, more peak horsepower. That kind of lifter collapse would probably not result in audible valve noise because the lifters would quickly pump back up while slowing down and driving back on the return road. You would'nt probably hear it over the sound of uncorked headers either.If you switch from hydraulic to solid lifters,you would have to recheck your total valve lift as well, since the more accurate valve train activity might result in a change. Also, if the solid lifter was somehow shorter that the hydraulic, it would result in a change in the effective height of the valve train. Shorter or longer push rods might be necessary to restore proper lift. It would be easier to check lift because you wouldn't have hydraulic lifters trying to squash down under load at max lift anyway. One of those variables that's impossible to measure while an engine is running and the car is going down the track.