I am rebuilding my 12.5:1 CR SCCA racecar engine, and I am installing relatively large lift and duration cams. The is too much subjective speculation with regards to valve float.
I am attempting to quantify valve float. The assumed governing equation is:
Resonant frequency = sqrt(k/m)
It is assumed that the driving frequency is the cam speed, which is half of the crank speed. One valve is treated as a spring-mass system, assuming zero deflection of the valve stem and cam follower and no damping. Also, the sprung mass is comprised of the keepers, retainer, and valve, and 90% of the valve spring contributes to the sprung mass. (The rotational polar moment of inertia of the finger followers is treated as a slender rod, pivoting about one end. Non floating valve motion amplitude is governed by undamped forced vibration. Since we all love equations:
mxDD + kx= Fsin(wt)
This is the governing equation of motion, however how to quantifying the conditions at which the follower leaves the cam is still not evident.
I measured a minimum piston to exhaust valve clearance of 0.025" at 8 degrees BTDC on the closing ramp. (which will be machined to allow 0.080" clearance).
Below are measured values of my race engine:
308-05-0300 Ti Retainer mass: 8.4 grams
Keeper mass: 1.0 grams
311-05-5350 Pro Series valve spring mass: 43.6 grams
(Assuming a linear spring rate of 300 lbf/in) or 52538.05 N/m
OEM B16 Exhaust valve mass: 41.8 grams
OEM B16 Intake valve mass: 46.2 grams
(I still need to determin the moment of inertia of the cam followers).
I calculated a natural frequency of 118.4 Hz for an intake valve, and 121.2 Hz for an exhaust valve. This assumes the camshaft speed is half of the crankspeed, thus the driving frequency is 79.16 Hz. Resulting in the intake valvetrain driven at 66.84% of its resonant frequency, and the exhaust valvetrain driven at 65.27% of its resonant frequency. (This is assuming 9.5k rpm crank speed, and 90% of the valvespring mass contributing to sqrt(k/m))
At the peak lift angle, the piston is miles away, and valve clearance is not of much concern.
My question is two-fold:
Is this a reasonable manner to quantify valve motion? Also, should we assume that engine speed, valve spring rate and valve train mass are the only factors to consider? Should the opening and closing ramp rates be explored? (Such that the opening ramp rates can be compared to the spring-mass EOM velocity rates?)