Quote:
Originally Posted by Dimman
First off there IS a point where lifting the valve higher off the seat offers no gain in flow. Not necessarily that the valve is not obstructing flow (stem is still in the way no matter what).
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That's pretty much what I was saying, wasn't it lol? That there is no reason lifting the valve to flow any more than the ports can?
Quote:
Originally Posted by Dimman
1) Bingo. Maximum valve lift (and any flow associated with it) is instantaneous, only at the apex. So extra lift beyond the flow limit, allows the valve to be at maximum flow for longer.
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Woohoo!
Quote:
Originally Posted by Dimman
2) I've heard from a flow point of view the angle affects whether the combustion charge 'swirls' or 'tumbles' (and tumble is favoured by the pent-roof multivalve combustion chamber) but that isn't the critical issue. As the valve angle increases any lift of the valves is more towards each other (intake to exhaust). So a problem that bikes ran into because of their need for both high lift AND overlap is that the interference was the valves smacking into each other during the overlap period, and not into pistons. So to maximize the cam use, valve angles need to be narrower (sport bikes are around 20ish degrees).
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This was the other point I thought of, though for some reason forgot about when I got around to writing the post haha.
Obviously different combustion chamber shapes prefer different angles for the valves to produce the best airflow for the cylinder, but is there any generalisation that can be applied to that? I would assume that having the valves at very small or very large angles to the perpendicular of the piston travel would be detrimental to performance, but to hear that sports bike's use ~20 degrees kinda throws that out a little...
Quote:
Originally Posted by Dimman
3) How higher velocities are used is that they can take advantage of even longer durations. (What is the force formula again? 1/2 mass x velocity^2?) What they are finding is that at high velocity the intake charge has enough inertia to continue stuffing itself into the chamber even as the piston begins compression (as long as the cams duration take advantage of this). The trade off (where the 'this is for power, but you lose torque' comes in) is a low intake velocity (lower rpm) is that the longer duration will push out the low energy, slow intake charge back into the port, losing power. This is where profile switching systems like VTEC allow for such aggressive high rpm profiles without losing too much on the bottom (VTEC kickin' in yo!).
Hope that gives you some things to think about...
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So really, to have a more perfect engine you would want a continously variable duration? I assume a similar effect could be achieved with the use of variable lift? I can't think of any, but I'm going to have a search on variable duration systems and see if anything turns up...