A valvetrain is designed so that in normal operation, the lifters remain in constant contact with the cam lobes.It's the job of the valve springs to maintain this contact, lifting the valve to its seated position as the trailing egde of the cam lobe rolls by. As the cam lobes begin spinning faster, there is less time for the springs to pull the valves back up and maintain contact with that trailing edge. At really high speeds (redline), the cam lobes move so fast that the spring rate is insufficient to raise the valve back up quickly enough to keep the lifter in contact with the cam lobe. This loss of contact, when the valve is not being directly controlled by the cam, is what constitutes "float." If the cam is spinning so fast that the lifter is still floating when the leading edge comes back around, it will smack the hell out of the returning valve lifter. For an analogy of force, image yourself throwing a baseball as hard as you can (normal engine), then contrast that with the force of someone smacking a 60+mph pitch with a bat (floating valve). Not only can that put incredible impact strain on your valvetrain components, it can actually launch a valve open further than the cam lobe was intended...and when you have an interference engine like ours, that can mean valve-piston collisions. So, you can raise the float threshold with higher-rate springs, BUT this means the cam lobes will have to work harder to move them at all speeds and can cause all sorts of fun problems in the lower RPMs. You could also lighten the valves themselves by using different materials, like titanium, but then you face cost considerations when using "exotic" metals...like retooling your production line to accomodate. So, it's an engineering tradeoff. As someone mentioned earlier, they basically decide what performance characteristics they're looking for in the engine, and what their cost of production limits should be, and design the valvetrain accordingly. The laws of physics pretty much determine where the redline is drawn based on their design choices.
"I used to play professional bone..." -z32ttvert (mwsmith) on June 23, 2003 at 11:14 PM |
