An educated guess, but really only a guess, 28.375 from the piston face to the end of the converging cone. That's where my "SL" bullet pipe ran best. It was more of a top end pipe. The pipe was 20 inches and the header was 8 3/8. The RLV L4 used straight sided cones whereas my SL bullet pipe used spun cones, very similar effects.
The RLV website has all the numbers you need.