What affects the smog capability running a larger cam?
04-30-2008, 09:43 PM
#5
Overlap is when the intake and exhaust valve are both open at the same time. The reasons why this happens are numerous, but it is mostly because you can only make the cam so big before both valves are just always open. In a race motor, having the exhaust hang open while the intake is open can help draw in more intake charge as the exhaust exits. At low rpm, however, a lot of unwanted side effects can happen. The intake can stay open so long that part of the formerly metered air gets pushed back into the intake. This is called reversion. It completely screws up the low rpm fueling if not properly tuned (emissions get screwed). At other points in the rpm range, the exhaust opening can allow a bit of the unburned fuel charge into the exhaust. This completely screws emissions. This long-*** post is only a TINY window into how cams affect everything the motor does. Most modern stock cams don't just have zero overlap, they are usually very negative (wide spread) in order to make power without screwing emissions tests.