On asphalt in TC a 13.5 and stock 27 are equal from what I've seen. Actually the 27 has kicked our tales for most of the year. Now everyone is going to the 13.5 just because of the easy motor maintaince- none.Maybe for paved oval it should be a 17.5 since it's a different type of racing.
I tried a 14.5 in TC and it was slower than a stock 27.I've yet to see a 13.5 as fast as a 19t in TC.