This short page describes my version of the famous Stage 1.
It actually explains how easily the standart 998 A+ head can be modified to greatly
improve the power output at the cost of a Stage 1 needle and a new head gasket.
I bought one of my 998 Mini with a blown head gasket. I did not realize it at once because in the beginning the leak to the cooling system was very low. But with hotter days the leak had started growing up so that I had to replace the head gasket. Meanwhile I've had enough time to tune up the engine which was working quite well with a standart carb needle.
While the head was off I'd decided to do a job that's usually designated as "valve unshrouding".
The goal is to create more room around valves in engines with valves located too close to
chamber walls. In theory the cross-section of an open valve should be limited by the area of the
valve seat less the section of the stem, or if the lift isn't high enough, by the area of the
cylinder that would link the seat edge to the valve edge.
However if the valve is too close to the wall the cross-section is limited by the area of the
surface between the valve and the wall. For every given valve lift there exist a minimum distance
that should be available between the valve edge and the wall.
This means the wall does not need to be far for any valve lift, it's even better that distance
increases for higher lifts.
On the left of the following picture is a not modified chamber, on the right is au unshrouded chamber.
No more than 2-3 mm were removed on both intake and exhaust sides.
There is no need to remove valves to do the job, just take care to not damage the valves.
After the mod the engine was running so lean that I had to buy a Stage 1 needle to make it run properly. And of course once tunned up it was pulling much stronger.