Simplify the following text for a Grade 3 native English-speaking person.
Original: The Ilmor Mercedes V-8 engine , also a mainstay CART powerplant , was permitted , but the only time it was used was a one-off at the 1996 Indy 500 by Galles Racing .
Simplified: The Ilmor Mercedes V8 engine , was allowed . It was only used once at the 1996 Indy 500 .