Quote Originally Posted by Mark75H View Post
Merc used 3 + 3 exhaust tuning on the inline 6's starting with the first gearshift motors. Generally the firing order was alternating pairs for balance with odd/even exhaust coupling (1, 3 & 5 in one group and 2, 4 & 6 in the other). The disadvantage was length between these 3 cylinder exhaust groups favored power in the 5,000 rpm range rather than the 8,000 rpm range.

Its not that it wasn't there, it just wasn't there for super top end.

On the T-2 and T-2x race motors they welded 2 3cylinder 49ci cranks together to get 3 + 3 exhaust tuning with close spacing, 1, 2, 3 in one group and 4, 5, 6 in the other exhaust group. Again there was a problem with length ... the original T-2 set up put them TOO close for 7,000-8,000 rpm. Like these OMC side exhausts ... the T-2X used an extended exhaust chest to drop the tuning into the 7,000-8,000 rpm range and get almost 20 hp over the T-2.
As Sam points out, Mercury did use 3+3 for inline sixes, a vast improvement over the 2X3 exhaust tuning; in fact they actually patented the concept. It improved power, but was not as effective as late model triples as the cylinders were too widely spaced - every other cylinder was used - inteconnect distance was still too far apart.

Sam's continues his explantion for the T2 series Mercs which correctly indicates that closer interconnects make another huge improvement. So close interconnects are the ticket for power delivery from close spaced triples (and multiples thereof).

The question stands: Why the power difference between right and left exhaust porting as seen in the OMC FR19S -vs- FR31M, or the opposite banks in any late model V6?.