'72 1150 exhaust system

[monk-monk]

I need a little help understanding the exhaust system on these early L6's. Im familiar with the exhaust porting in the powerhead, but what i need to know is: what is the function of the exhaust plate below the powerhead? Is there an exhaust baffle somewhere in the exhaust system? When the motor is trimmed in and not running, how high in the exhaust channel can the water go...I just don't understand how this all works, seems like all the water would cause a lot of back pressure in the cylinder...Please educate me on this and things i should check to be sure my exhaust is operating correctly.

 

[Dave1027]

Re: '72 1150 exhaust system

The way it was explained to me is that a 2 stroke engine requires a tuned exhaust. Diameter, length and backpressure are all calculated for a certain rpm range.

 

[Chris1956]

Re: '72 1150 exhaust system

Monk-monk, There is back pressure on the exhaust, when the motor is in neutral and in the water. That is why you have the small square ports high on the midsection. They are for exhaust relief.

The exhaust pushes the water out of the midsection, depending on how deep the boat sits, and the RPM of the motor. Once on plane, the forward motion of the boat pulls most/all the water and exhaust out of the midsection.

The exhaust extension divides the exhaust and water and sends mostly water out of the ventilation holes above the antiventilation plate. The exhaust and water exits via the prop and exhaust relief posts. There is a exhaust tuner (megaphone), however, it is not a real expansion chamber design, and likely does minimal tuning.