How can weight slow down a race car?

As we were told by Isaac newton, force equals mass times acceleration. If your engine produces a given amount of force, then the more mass you have to accelerate, the less acceleration will result. Of course, this is not the only aspect of adding mass to a race car. If the mass is added in the form of a heavier and more powerful engine, then we are no longer dealing with the same amount of force, we have increased both force and mass, so we might easily get more acceleration, not less. In addition, heavier cars get better traction. So in reality this is a complicated situation.