exaust reversion, possable solution

[Bt Doctur]

Hello boaters, a new member with a old problem. Trying to find the answer to this problem for the owner.Boat had BB motors are they were removed for 6.2 SB W/ 3 inch riser extentions. When first serviced for a bad starter we found the manifolds heavily rusted .Motor is very poor condition and it was pulled for a rebuild at the end of 2015. For 2016 no reported problems and was winterized. Fall of 2016 port motor pulled for rebuild, same conditions of the manifold. Heavily rusted but no water in the oil.

Fast forward to spring of 2017. New port motor installed and tested on land 2X. stbd motor started and it had some stuck valves.

??
Removed heads and they were just ugly. No issues found in the heads and motor was new so new heads installed.Boat put in service.
2 hours later port motor takes a dump. 3 valve hung wide open

?

Shop keeps saying water ingestion but oil remains clean so we then suspect exaust reversion. Cannot raise the risers or use dry headers,
cannot lower the exaust or swim platform so we did the next best thing , extended the riser outlet.Not knowing how long they should be I used 12 inch tubes hammered in 2 inches leaving 10 inches exposed.

The manifold and riser pics shown only have about 2 hours run time and were loaded with what looked like rocksalt debris. So being a test engine were guessing on the length. The tubes are 2 inch OD SS thin wall 12 inches long .Our "Think Tank" says it should work to prevent the reversion .If this is true I should find nothing but carbon in the manifolds and risers when I remove one for inspection at the end of this season.

Stay tuned for a update and pictures soon







An Update 4 hours run time at power

Burned a small hole thru the exaust hose but it was expected because I`m in uncharted waters with the length of the tubes. Remove the risers and shortened then 2 inches and repaired the hole for now. You could see where the tube was pushing down on the hose because it was shiny clean.

The good news is there was no sign of any moisture and the manifolds, the extention or the risers .There was a nice black coat of carbon deposits and no white salt residue.

Finally some sucess. Pics at the end of this season

 

[NorCal Boater]

My '89 Cobalt CM23 had this same problem but the engine is mounted forward in the boat with a Mercruiser jackshaft setup. We had the reversion situation several times and actually hydro-locked the engine each time. I eventually contacted Cobalt and they finally admitted they made an error in the design of this boat by mounting the motor a bit too low in the boat. With the stock setup the exhaust came out of the manifolds, through fiberglass exhaust tubes and into the standard Alpha "Y" pipe and exited through the prop. We figured the problem was occurring when we backed off the throttle too quickly. Cobalt suggested to put 3" extensions under the elbows. Not an easy task as the entire engine cover had to be raised. This was all going on in the mid 90s.

In 1998 I sold my interest in the boat (I had a partner) and moved out of state. My partner continued to struggle with the issue. He replaced several motors and one marine motor builder theorized that water was getting into the engine during combustion. I wasn't there at the time so I was not sure how he came up with that idea. My old partner had a custom exhaust manifold made, at considerable expense, that introduced the cooling water into the exhaust stream about 10-12" further aft. This seemed to work for a while and then ruined another engine.

At this point the boat was almost 20 years old and he decided to have a lot of work done to the boat and one of the things he did was have a 383 (6.2) "Stroker" motor built for the boat. The horsepower for this motor is unknown but estimated to be somewhere north of 350. Yes, he blew the Alpha 1 drive but that's a story for another thread. When this last motor was built they added a fully closed cooling system which seems to have solved the problem. They also blocked off and bypassed the thru-hub exhaust and went to a thru-transom wet exhaust. This motor has been in the boat for about 10 years and is still in great shape. So one could assume that water was not entering through the exhaust but rather it was cooling water back through the exhaust valves during combustion Not sure why it worked but really glad it did.

BTW I bought that boat back 3 years ago and I am now restoring it.

Shawn

 

[CliffA]

.....just a minor point but the 6.2L engine is actually 377 cubic inches, not 383.....both the engines are stroked but have different displacements....

cliff

 

[NorCal Boater]

Picky, picky, picky.... my mistake. My motor is not a 6.2L then. It is a 383. It started as a 350 with a 400 crank and pistons/rods.

 

[CliffA]

Picky, picky, picky.... my mistake. My motor is not a 6.2L then. It is a 383. It started as a 350 with a 400 crank and pistons/rods.

..............

 

[Thornton69]

Was there reversion issues with the original BB's?
Were the 6.2's properly cam'd for a marine application?
Seems to be a fairly common problem when guys try to get the most out of their SB's.

 

[Bt Doctur]

the motors were rebuilt with the same cam , nothing exotic. No history from the PO about the running of the BB`s, met the customer with the 6.2 already installed. Reversion was already present given the condition of the manifolds and risers, heavily rusted but no water in the oil or combustion chambers. Just heavily rusted exaust ports and exaust valves.

The salt in the pics was after a run time of approx 1.5 hours. on the new motor with brand new heads. It seems it`s involved with the "Silent Thunder" swim platform exaust system using straight thru hull exausts into the swim platform.

Inspection of the manifolds and risers on the port motor that burned a hole thru the wet exaust showed nothing buy nice carbon black, no salt crystals.
So, I`m on the right track, we just dont know how long the tube must be.

 

[Bt Doctur]

Quick update after a 4 hour run. Burned a small hole in 1 tube but that was expected because the tube was pushing down in the hose. Removed risers and confirmed no water reversion or salt deposits.
Shortened tubes 2 inches and reinstalled .Will be remove risers for pic at end of this season should find no reversion problems and then will shorten the stbd motors tubes to the same length and winterize it till next year.
pics at end of season,

 

[Bt Doctur]

As promised the engines were inspected and nothing but nice black carbon deposits, port motor has 10 inch tubes and stbd motor still has the 12 inch tubes. Port motors tubes will be getting a 10 or 15 deg kick up and weld to center then in the 4 inch exaust tube. This is a pre shortened tube.

 

[joeyleggz]

Genius !!!!

 

[370Dancer]

Any update to the update? As an owner of the infamous Gen VI Mag MPI engines (set #4), I wonder if this would help the apparent water ingestion issues these motors constantly had/have. Makes sense to run the exhaust outlet further past the cooling water outlet in the elbow. any guesses on the diameter for a 454 4"? Probably 3"? These also have water lift muffler systems.

 

[370Dancer]

@Bt Doctur : Did this work out long term? I'm getting ready to replace all maifolds and risers, and would like to extend mine as well.