i thrw in a 540 8.5 today to see how it was. more top end than my 6.5 550, smoother acceleration and lower temps. i'm guessing longer run times too without using a timer. whats the benefit of using the 550's over 540's? the truck is light enough for a 540 and can be overpowered with a 550. guess i'll run the 540's for now and see how it goes in race conditions.