Understanding basic principles of cam timing.

Someone  posted on one of the facebook groups asking why cams needed to close so far after bottom dead center.
This was a bit of typing so I had to post it here too. Like all my quickly blurted out articles I need to try to improve on it and edit it when I have time.
This article also closely ties into my article on why DCR is such a load of BS.

If one thinks in static terms you would assume that the cam should close at BDC as that is where the cylinder would be most full of air.
You have to remember that at 6k RPM the motor is spinning 100 times a second. That's hard to fathom.
The compression stroke is happening 50 times a second.
At 12k RPM on a Formula Atlantic or sport bike engine it's happening twice as fast.

You have to try to imagine things taking place hundreds of times a second. Then you have to remember that even air has mass and takes time to change direction.
At close to 0 RPM you are right, the piston will travel down and at BDC the cylinder will get as full as it's going to get.
As RPM increases it takes longer in relation of crank timing for everything to happen.
The intake valve opens while the piston is still traveling upwards so the overlap and exhaust momentum can help start pulling the fresh air in. After TDC the exhaust cam closes and the piston travels downwards. The air can't accelerate as fast as the piston so the piston creates a low pressure zone as it acceleraes downward at a mind boggling rate. The air starts to rush into the intake valve as it tries to fill that low pressure zone. When the piston hits BDC the pressure in the cylinder is still below the pressure in the port so that air is still rushing in through the valve.
The piston starts accelerating upward at a mind boggling rate but that air will take time to change direction. The piston will hit the air at the bottom of the cylinder and create a high pressure zone but there is still lower pressure at the top of the cylinder so air is still rushing in.
Perfect valve timing is closing the valve right as that pressure wave traveling upward hits the valve and pressure equalizes on each side of the valve. If the valve closes right then the cylinder will have as much air in it as it can get and the most power potential it will have.
The faster the motor spins the more duration you need for this to happen so a static cam can only hit that perfect point at one RPM. The rest of the time it will be a little off. If the valve closes too early it will stop air still flowing in. If it closes a little too late it will allow a little air to be pushed back out.


courtesy of webmatter.de