Stupid question. Can you explain "overlap"?
05-13-2007 | 08:54 PM
#2
OVERLAP
As the engine cycles, there is a period when both the intake and exhaust valves are open at the same time. This valve timing is known as "overlap." Think of this as the exhaust and intake cycles overlapping each other.
The valves are timed so the intake valve opens slightly before the piston reaches top dead center (TDC) on the exhaust stroke. Likewise, the exhaust valve is timed to close just after the piston starts down on the intake stroke.
The objective of overlap is for the exhaust gas which is already running down the exhaust pipe, to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. This valve timing is a product of the cam's duration and separation specs. For more information on these cam specs see the Cam Specs & Effects page.
The science involved with overlap is quite complex. Pressures, runner lengths, temperature, and many other aspects influence how well the overlap effect works. The advantage to using software like Engine Analyzer Pro is that the software does all of the calculations for you. Simply enter a different duration and separation in the computer and the software will show you whether or not to expect more power from your engine.
from http://www.auto-ware.com/combust_bytes/valvetiming.html
google is your friend!
As the engine cycles, there is a period when both the intake and exhaust valves are open at the same time. This valve timing is known as "overlap." Think of this as the exhaust and intake cycles overlapping each other.
The valves are timed so the intake valve opens slightly before the piston reaches top dead center (TDC) on the exhaust stroke. Likewise, the exhaust valve is timed to close just after the piston starts down on the intake stroke.
The objective of overlap is for the exhaust gas which is already running down the exhaust pipe, to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. This valve timing is a product of the cam's duration and separation specs. For more information on these cam specs see the Cam Specs & Effects page.
The science involved with overlap is quite complex. Pressures, runner lengths, temperature, and many other aspects influence how well the overlap effect works. The advantage to using software like Engine Analyzer Pro is that the software does all of the calculations for you. Simply enter a different duration and separation in the computer and the software will show you whether or not to expect more power from your engine.
from http://www.auto-ware.com/combust_bytes/valvetiming.html
google is your friend!
