I don't understand the problem of the valve face is too wide

[vtr1005]

When i read the manual, it said if the valve face is too wide and narrow, it need to cut the valve seat to meet the specification.

I guess if the valve face too narrow, then the valve performance is not the best.
but i don't understand if the face too wide, what is the problem.

 

[Simon Bridge]

I always understood it was associated with the gas flow around the valve.
Quite small-seeming changes in geometry can have a noticeable effect on performance and wear.

 

[Highspeed]

If the seat is too narrow, it not only affects flow and performance, but it can also lead to overheating of the valve. If it's too wide, flow is also affected, but it also spreads the spring pressure across too much area which can affect sealing and seating. Ditto to Simon.

 

[jack action]

Wide seats cause the following problems:

• Seating pressure drops as seat width increases;
  
• Less force is available to crush carbon particles that stick to the seats;
  
• Valves run cooler, allowing deposits to build up on them.

http://enginesrme.hubpages.com/hub/Engine-Rebuilding-Valve-Grinding