I get a lot of requests for a good grooving patterns for dirt oval. Here is a good one for rear dirt oval foams. 

Notice the angle cuts don't break the side wall. Helps with chunking. Keep the depth of the grooves to about 0.060"

Suggested cutoff wheel for the cross cuts is the Dremel #545 Wheel. Lasts for ever!
http://www.dremel.com/en-us/AttachmentsAndAccessories/Pages/AttachmentsDetail.aspx?pid=545
copy and paste this link into your browser.

Click the small picture for a larger view.