Ok so my diferential has the little bar that goes thru it, but you can see thru it... I believe that is the stock open diferential...

why is it sometimes both wheels spin and some times only one spins... Today i did a burn out and it left two equally dark lines.... wtf...

But when im at the track im pretty sure only one is spinning...?>

 

My old y8 would light up both for maybe 50 feet and then one would start to fade. Once you break them loose they just let go. I think the reason one grips at the track is because the traction or coefficient of friction is higher so its harder to get them both going. Just part of having an open diff.