Mopar Muscle Association UK

I would love to see how they prevent the valve and spring from rotating.

is it just because of the vibration that the valve turns???

Could be any number of reasons from one engine to the next. Some rotation is generated by friction force of gas flowing across the back of the valve, if the port design encourages swirl or there is a higher velocity on one side of the valve from the other then the valve might start to rotate. However this generally only happens if the friction force of the spring is overcome, i.e. during float.

Some engine designs actively encourage valve rotation by slightly canting the rocker face, this introduces a side force that induces a rotation. A similar thing is used on lifters to encourage them to rotate.

I think in this case the rotation of the valve is being pushed by the spring as it rotates during float. I have to say that this isn't really float, this is bounce. Bounce is when the valve seats and rebounds off the seat, this is a velocity driven event. Float is a frequency driven event where either the spring is unable to return the valve to anywhere near the seat on the back of the lobe and the valve meets the front side of the lobe on it's way back round. Or it is a situation created when the frequency of movement is the same as the natural frequency of the spring and the spring effectively becomes useless. Either way the valve never really shuts in these situations.

Float is also caused by hydraulic lifters at high rpm because they pump up due to the lifters not having time to bleed down. One reason to use lifters with a real circlip holding them together rather than a wire clip and then use as little pre-load as possible.