RMPRam, Why did you choose # 6 for the EGT probe ?

Absolutely no reason. There are 1/4" brass pipe plugs in the bottom of ea.ex. runner just adjacent to the ex. port. I was able to drill and tap the plug for the probe and #6 was the choice.
Any better ideas as far as a cyl. to use? I suppose that I could move the probe several times to see if it made any difference. It's not that hard to move.

Pictures of the motors

In the early 90s in Grand Haven MI the local Law enforcement agencies jumped on the Fund Raising band wagon and started ticketing all performance boats for the 90 db noise law. I eliminated their ticket to my checkbook (did you get that) with two foot sections of 4 inch diameter rubber gas station exhaust vent hose and 4 hose clamps ($45.00). Attached to the exhaust tips outside the transom, the hoses hang in the water during idle and are above the water when up on plane. The set up was nicknamed "Snork Tubes" by other boaters, but it worked. Also, if anyone receives a noise ticket in Michigan, the law states the test must be run in accordance with a SAE procedure (I don't recall the number but a search should be easy) and there are so many steps involved that the Law Men never complete all of the steps, easy to win in court (but they do have a good memory)...

Who has turn downs for 5" exhaust?

G.K.,
Are you bending your own exhaust or having someone fabricate it for you. I am looking at making something similar to your previous configuration off the bottom of a set of Graffrig cans. I'll probably end up buying the pre-bent radius and straights and getting out the tig welder.

Jim

The "sweeping" downturns were made from 4" u-bends, straight sections of 4" and 90's.
The down-turns that I had on the last set of air mufflers were made by cutting out the bottom of the mufflers and welding 90's on. Even with the down-turns on the mufflers there was enough "gap" on the butterflies to cause enough rise in the db reading to make a big difference. I made some "caps" to slip in the cone ends of the mufflers that took care of the excess ex. that came by the butterflies. That worked very well, but putting the caps on every time you were going through a no-wake or in a marina was a huge pain. Hense the NEW system.

RPMbam, #6 is usually cooler than #8. Most intake manifolds shroud #8, not sure about your PSI intake.

Relative to reversion, are the tail pipes wet to the elbow ?

We have a source that will form a 90*+/- Stainless or Inconel up to 6". They will also form an inner tube and an outer tube, thereby creating a dry 90*. The 90* can then dump as a wet tail pipe or dump through an orifice. Subject to radius and pipe sizes, the outer pipe may require splitting length wise, fitting and welding. The only reason I would use the dry 90*would be to complement dry tail pipes and reversion. A fringe may be a cooler pipe for the transom and persons.

The "new", outside, turn-down pipes, are single wall. I introduce the water into the exhaust at the end of the tail pipe, just before the transom in the engine compt. The dry tail pipes are only 2' long and the inner pipe extends 6" further than the outer pipe.
So far I have not seen any indication of revision. I'm running a very mild street roller (solid lifter) cam, nothing wild at all. The engines made 1100 hp on 92 octane with a 7.5/1 compression ratio. They will idle @5-600 rpm.
I will switch the probes to #8 and see if it makes any diff. Thanks for the suggestion.

RMPRam, You explained there is a exhaust relief hole somewhere close to the transom. What is the diameter of the hole and what is the distance from the transom ? I visualize an angle cut bung welded to the elbow for directing exhaust away from the transom.

This is another visualization: The outboard pipes have an over sized,threaded ID bung. A Stainless plug screws into the bung after being drilled to create an orifice. The final orifice diameter is acceptable when the EGT's balance.

FYI, I have a New take out set of #5 near net forged upper gears. The ratio is 1.50. Bearings and cups are installed. There is a picture of them in classified,under" Huge Parts Dispersal."