exhaust question????

[TTriton]

I was out in the towboat today and had time to sit and think during a long tow. Well.. I know a exhaust relieved engine or a high perfomance midsection make more power but WHY?? Thinking of the exhaust extracting device we call a prop. The prop throws out water and carries the exhaust out and I would think that it would move a bunch of exhaust out though the hub. The water moving away from the prop would make a low pressure area pulling the exhaust out like some of those fire fighter tricks they use to get smoke out of a building.

Just a quesiton of a bored towboat captain.

[us1]

It just doesn’t allow to the exhaust to escape as fast when going through the prop. Keep in mind a relieved exhaust is not always the best thing for a motor. Its a trade off give you more top end and taking away lowend torque. A motor with relieved exhaust can scavenge better at high RPM; dry stacking will improve this even more. Having a more restrictive exhaust increases the pulse charge power at lower rpm maximizing low-end torque. I guess none of this really answers your question, my guess is it has to travel through so much water and changing pressures that it just cant get out that efficiently.

[TTriton]

I was thinking some more.. The passage way though the lower unit it is fairly small and retrictive. I could see that being the problem there.

[us1]

I agree, also when you put in exhaust relief you are adding it to the lower unit exhaust escape route so the exhaust now has two ways two escape.

[MadMat]

I don't mean to be a dunce, but what is the difference between exhaust relief and dry stacking?

[us1]

Exhaust relief is drilling holes in the mid section to let out the exhaust. Dry stacking is diverting the water from the motor to dump outside the mid section. My mom is in the UK right now, just talked to her this morning.

[B.Mac]

Remember my gasket mystery, where when I changed to a stock gasket and I lost my water pressure? Well when my motor is idling on a hose, water is literally pouring out a half dozen 3/8" holes drilled into each side of the bottom of the midsection just above where the lower unit mates with the midsection. Water also blows out the two half dollar sized relief holes out the rear/bottom of midsection...Would this indicate this "drystacking" or is this normal for a relieved exhaust? What could possibly be the benefit? By the way......... I am getting 15lbs @ WOT, I've just been to freaked out to look down at my gauge at 80 mph until recently with a little more seat time. I think I must have looked like a mannequin drivin this pizza pan the first month.......Thanks B.MAC

[us1]

You have relieved exhaust without drystacking. The motor will breathe a little better to give you a little more top end. I looked into that water pressure mystery and here is what I found. I never run stock two piece exhaust plates so I was not familiar with this problem. When you run a high performance gasket and or a race cowling with the metal plate under the motor the triangular dump hole under the popet is partially blocked, there is about a 3/8 hole left for the water to escape. This is a good thing, that is why your pressure is at 15 with the race gasket and very low without. Keep running the race gasket.