Hi Trev,
i'm not too sure as I don't run in the stock class but I believe it to be a 27Turn limit. If you turn up on Friday one of the guys who race in the stock class will be able to confirm this.
My advice would be to use a 20T saturn and race in the "open class". I am personnaly not a fan of splitting the Mardave classes to suit one or two individuals. As racing with 4 cars in a heat can be a little dull compared to to a full on field of upto 10 cars.
I see the stock class suitable for beginners and the Open class suitable to more experinced drivers as the cars are a little quicker and require that little bit better setup and driving ability.
Hope this helps
Si