First, all of the rubber mounting should be like new genuine VW quality or replaced with the special HD genuine VW mounts we sell. These are slightly firmer and will transfer slightly more noise to the car from the running gear, however, it is quite livable. This may also include the replacement of the spring plate torsion bar rubbers if they are more than a few years old. In minor instances of a drag type start this usually gets by, but will not help in drag race applications. What is happening is the down thrust is pushing down on the frame horns, bending them down and at some point they will spring back up, starting the continuation of an up and down bending motion. The first step for every vehicle which is higher in performance, regardless of race starts or not or a stock vehicle utilizing a race start, is to install the proper rear engine supporting device. This device must have adequate strength to stop this motion without bending and adding to the reaction.
It is often called a traction bar as it allowed us to get the car to hook up without this jumping up and down motion or wheel hop. I called it both an engine support and traction bar when I first invented it for my race car in 1964. Needless to say, the name is not as important as the application and proper strength. I experimented with bars of different weight and strength, and finally found that it requires a 1-1/2", 0.125" wall for 99% of the applications. One unfortunate problem has been our competition making a useless 1" bar so they can sell them cheaper; so be sure what you buy will do the job even for your stock car.
At one time, I thought this was not needed on a basically stock engine when hard starts were being made, as I did not feel the wheel hop. Not true! My 1967 bug was being driven hard and stop light raced, and after about six months I noticed the engine tin was no longer lined up with the rubber air seal in the engine compartment hole. Upon measuring it, I discovered that the down forces had actually bent the frame horns down from the severe driving I was doing. I promptly had them straightened at a frame shop and installed the engine support and experienced no more bending after 250,000 more miles.