I gotta agree that some folks have ruined it over the years. There are so many rules now because of it. It's almost as strict as NASCAR. Once the cars are weighed and inspected, they are impounded for the remainder of the day, and the dads/scouts can't touch them.
I also insisted that once I was finished cutting the pieces, he had to do the remainder of the work (sanding, assembly, painting, axle polishing, wheel truing, etc) The only other thing I really did for him was set the wheels/axles up so it would run true.
He is in a fairly small pack (about 10% the size of the pack that I was in as a kid), so it was a lot more about the fun than winning.
We also had a blast making it, and I had a great opportunity to teach my son about the how's and why's of making a gravity-powered car go fast (friction, wheel alignment, center of mass, inertia, weight distribution)
It is always great when you can teach your kids some principals of science, and how to use tools, while having fun doing it.