Quote Originally Posted by miked View Post
I've been thinking what will happen if I replace a 136.5 mm valve with a 130 mm valve (intake and exhaust valves):
Compression ratios will surely go down but what will happen with the following?

1. hp & torque band (will it adjust dramatically? will it widen or shorten?)
2. starting characteristics (cold)
3. operating temperature (this will go down for sure but how drastic?)
4. what else?
5. do i need to change the valve springs too?

Theoretically higher CRs will result to higher efficiencies but the pressure will surely go up too. same goes with the operating temperatures.

waiting for your replies everybody. thanks
As far as I know, using a shorter valve is an engine rebuilder fix if the valve seat has been refaced. (Same thing if the cylinder bore was honed, the engine rebuilder will use an over-sized or bigger diameter piston)

If the valve seat has been refaced or the hole for the valve seat has been re-bored, the valve will sink deeper into the cylinder head and the valve stem protrusion will become higher.

Especially for overhead cam type, possible na tumukod ang valve stem sa cam if the height of protruding stem is way above the specs. Thus the valve could not be shut-close completely.

For rocker arm type, the force exerted by the partially depressed valve spring might not be enough to produce a tight fit if the height of protruding stem is way above the specs. Thus leak might happen.