Hello all - wondering if you can help me here with a question I have.
From elsewhere on this forum, this:


''1988 Merc 115.

Exhaust ports opens / closes: 96 deg ATDC / 98 deg BTDC

Exhaust open period: 166 deg''

ie, EO @ 84deg BBDC and EC @ 82deg ABDC (the asymmetry apparently is due to an offset gudgeon pin).

These timings suit the groups of three cylinder couplings of exhaust branches ie, are appropriate for supplying a positive exhaust pulse from a currently exhausting cylinder with the branch lengths employed and the 120deg crank phasing, to the cylinder that is closing its exhaust port.
A four cylinder can run with shorter exhaust durations than groups-of-three motors, and twin cylinder units are denied any pulse tuning of this sort.
My question is: given the above, do Mercury crossflow fours and twins have different exhaust durations from triples and sixes? And if so, does anyone know the timings for the twins in the circa 35-40hp range?
Hoping someone here can help - my proposed project hangs on it.
TIA.