I guess the obvious reason is to keep your car cooler therefore controlling KR. However the holes only reduce overall engine temps by about 5 degrees so we have a much more important reason for this.
While at the track you open the hood, maybe ice the charger and let the motor cool down between runs. This may bring your coolant temps down to very low levels, which is good however these cool temps may actually slow you down. With the coolant and engine cooled down there is no coolant circulating until the engine temps reach the temp of the Thermostat.
This would normally be OK however when you’re racing the combustion chambers heat up very quickly. So the coolant around the cylinders and heads might reach 250 degrees before the stat even begins to open. This is very bad and can cause high levels of KR in the back half of the ¼ mile. By drilling the holes in the thermostat you can eliminate this problem because you will always have a small amount of coolant flowing.
“Performance blasting down the ¼ mile isn’t the only advantage of having holes drilled in your thermostat. If you live in a colder climate this $11 investment might just save your cylinder heads from being cracked and damaged from the sudden dive in coolant temperature as your winter chilled radiator dumps it’s freezing coolant through your heads.
A drilled stat would be much more forgiving by providing a continuous flow of coolant and reducing sudden and drastic temperature changes.”
- Andy H
1999 Grand Prix GTP