[Land-speed] Finally an LSR Question
saltfever at comcast.net
Tue Dec 11 01:28:10 MST 2007
Mayf . . . always thought provoking :-) Here is my $.02.
The intake valve must be seated and SEALED during the entire period of the
exhaust flow (overlap accepted). During this time the MAP is at its highest
value which lowers seat pressure accordingly. If the intake valve leaks, and
exhaust flow can get past it, you have IGNITION! Nowadays, with port EFI it
may not be a big deal. But with all the volume in a ROOTS blower, manifold,
and intake ports, you have a huge volume of explosive air-fuel! In your
turbo set-up the exhaust back pressure is a very small percentage of boost.
The delta P lowers your intake seat pressure. Assuming you had 5 psi exhaust
back pressure, 15 psi boost and 2.02 valves you probably lowered intake seat
pressure 30+ lbs. Maybe a 25% reduction, or more, in seat pressure.
The volume efficiency (VE) with any blower varies due to many things but it
will be greater than 100%. You are assuming that as the cylinder fills the
delta p across the valve will equilibrate with MAP. Then compression takes
over and further closes the valve. But such is not the case. As the valves
closes to within .030-.060 of its seat, intake flow slows or effectively
stops. MAP starts to increase greater than cylinder pressure. Since the
valve is still a few thousandths off of its seat, even more crank degrees
are needed before compression actually becomes meaningful enough to help
seat the valve. At this point the spring is near its stack height and force
is at its LOWEST value. You have a condition where MAP is high, there is no
compression, and LOW seat pressure. As Dave mentioned, you get valve bounce.
With bounce you get loss of effective compression and pounding of everything
in the valve train. -Elon
I am sort of ambivalent about this. Here is why. On the intake stroke
the vlave is open so no added spring pressure is needed. On the
compression stroke, no added spring is required because the cylinder
pressure overcomes that. (snip . . . ) Mayf
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