Does LSA really matter?
Originally Posted by Pro Stock John
My point was that they are not looking at LSA when they first think about the cam specs... I know that they do that, since I heard that from Comp and Cam Motion... I guess Bret is smarter than them. 
I would sure like to know who you talk to at Comp, that "only looks at LSA in the end". The people I know and deal with there certainly don't. That would be one dumb-assed way to choose a cam.
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With respect to scavenging, I don't believe the overlap bias has much
to do with piston position at all.
At the point when overlap occurs, the pressure changes in the chambers are
affected mostly by the exhaust port and intake port; the piston is fairly stationary
from a few degrees BTDC to a few degrees ATDC. Therefore, pressure changes
from the piston at this time are slight to none.
The timing of the negative exhaust pulse at the port initiates the act of
scavenging, the delay/inertia required to move the intake charge toward the chamber needs a period of time.
Opening the intake valve sooner may cause the exhaust gasses to revert back
into the intake port depending on RPM. Keeping the exhaust valve open too
long after TDC may cause the exhaust gasses to push back into the chamber
as the piston lowers the pressure in the cylinder.
As Bret mentions, it's dependent on RPM as everything is tuned for specific
wavelengths. There is only one resonant RPM, then the efficiency rolls off
above and below that point.
As for LSA, I never understood why that value was/is so important. I think it's
just something that caught on within the industry to sum up lobe properties in general.
to do with piston position at all.
At the point when overlap occurs, the pressure changes in the chambers are
affected mostly by the exhaust port and intake port; the piston is fairly stationary
from a few degrees BTDC to a few degrees ATDC. Therefore, pressure changes
from the piston at this time are slight to none.
The timing of the negative exhaust pulse at the port initiates the act of
scavenging, the delay/inertia required to move the intake charge toward the chamber needs a period of time.
Opening the intake valve sooner may cause the exhaust gasses to revert back
into the intake port depending on RPM. Keeping the exhaust valve open too
long after TDC may cause the exhaust gasses to push back into the chamber
as the piston lowers the pressure in the cylinder.
As Bret mentions, it's dependent on RPM as everything is tuned for specific
wavelengths. There is only one resonant RPM, then the efficiency rolls off
above and below that point.
As for LSA, I never understood why that value was/is so important. I think it's
just something that caught on within the industry to sum up lobe properties in general.
Last edited by Adrenaline_Z; Oct 2, 2006 at 12:45 PM.
