The yellow car shows good panning technique but the racing car appears a little static (like it's just parked stationary on the track.) The answer seems to be in the shutter speeds you've quoted - 1/90th for the yellow car and 1/500th for the racing car. To get the best result with long lens, hand-held shots like this, I'd personally work at 1/160. This provides the tell-tale movement in the car's wheels and streaks the background sufficiently but allows for pin sharp rendition of the car's body.
I have a few examples on my website - Go to the "Latest News" page and see the "Knockhill Racing" article
http://www.jamescrawfordphotography.com
Good luck - definitely on the right lines here.