On some overhead cam engines, valve shrouding occurs
when the edge of a valve is in close proximity to the combustion chamber
wall, making for a tight squeeze for air to travel around the valve in
this area. Some hot rodders will try to increase this gap for better air
flow in this area. They call this unshrouding the valves, or it is also
refered to as relieving the valves
Valve shrouding will usually restrict flow through the valve at some point in the valve lift, but not always. The example below was flow tested on a Valve shrouding will usually restrict flow through the valve at some point in the valve lift, but not always. The example below was flow tested on a Flow Performance
equipped micro bench which showed the biggest gain on the intake valve was at .400" lift with a gain of 8 cfm. While there was about a 2 cfm gain at higher lifts, there was also about a 2 cfm loss at lower lifts.
You should check with your mechanic, engine builder or a knowledgable person first,
to see if unshrouding the valves is a suitable modification for your head
Removing material from the combustion chamber will lower your compression ratio.
The following shows how this fast, easy modification is usually performed
Ink is applied to the deck surface near the valves.
The gasket is positioned with the alignment dowels that align
the gasket and head to the block. I found these sockets work
well in place of the dowel pins.
The gasket is positioned via the dowel pins and the inside
diameter of the bore is scribed on the head deck surface with
a sharp instument.
With the gasket removed, the scribed line shows where the gasket
is located on the head and is a guide for unshrouding the valves.
The gasket cylinder bore, or fire ring, will likely be larger than the cylinder diameter. If you want
to keep the chamber wall from exceeding the cylinder wall, creating a ledge on the deck of the block,
then stay within the scribe line by the appropriate amount. Or scribe the
cylinder line from the crankcase with the head placed on the block.
Most folks will just lay the wall back to near the scribe line.
It is very important that you do not cut the chamber wall back past the head gasket scribe line.
head gasket in the chamber could cause detonation and/or damage the gasket.
Clearance is checked with the valve raised to about .5". Valves
that are slanted will shift in relation to the chamber wall as they
are raised. With this valve lifted to about .5", it moved out of
the recessed area. More material could be removed from this area
A very small stone can blend the wall into the valve
I really got too close to the scribe line here. A small margin
should be maintained for missalignment or gasket expansion.
This is probably much more important on the exhaust side than
on the intake side.
Hemi combustion chambers owe a lot of their ability to make power from the fact that
valve shrouding is greatly reduced by the fact that the valve moves away from the
chamber wall as it rises.
More good stuff!