the 13T-U is basically a 3t but it's a "Lean burn" engine.
the chambers and pistons arent the same as a 3T.
Rumored to have stonger rods by the Peurto Rican guys.
The chamber actually has 2 chambers. One large and 1 small.
the sprak plug ignites both chambers. the small chambers flame front is aimed right into the large chamber. Its like having a flame for a spark plug. And the 13T's are the highest rated power T series engine except for twin cammers. HTH