[Tom Goldman]

Using timing retard alone to slow a car is a double edged sword.
If you only need to kill 2 or 3 tenth's it will be very effective and consistent, but if you have to kill a second or so it will play hell with with the engines bottom end, without also using a throttle stop.
Long before the Grid or even timing retard boxes, I used to use a dual point distributor and cut one set of points in and out with a timer and retarding 18-20 degrees out since I had trouble making a tunnel ram work consistently with an air stop. .... It was deadly consistent ,but it was hell on both the rod bearings and the crankshaft .