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Do exhaust idle relief ports have to be sealed from the main (thru-prop) exhaust?

vznx1w

New member
I am doing an engine swap--replacing my 84 Merc 260R (350 chevy) with a late-model EFI engine. As part of the process, I am updating the old exhaust 3" risers and Y-pipe with the newer 4" y-pipe and risers.

CONCERN:
The old 3" Y has much larger idle relief ports, while the 4" pipe has a slightly larger main port. This means that when the 4" pipe is bolted to the old transom plate the ports almost overlap and the narrow land separating the idle relief and main ports will be reduced from about 5/16" to only 1/16".

QUESTION:
What happens if there is a leakage path between the main and relief ports?

I understand that the idle relief ports are designed to reduce back pressure at idle, by allowing exhaust to exit at the waterline. However, this could be accomplished by just having one large passage in the Y-pipe connect to all three transom plate ports (Main and 2 idle).

I do not fully understand why Merc took the trouble to cast separate idle relief ports and exhaust passages into the Y-pipe so that the relief gases are separated well upstream of the transom plate. I am guessing that this might be to keep the exhaust velocity up in the relief ports in order to keep lake/sea water cleared of the exhaust ports. By separating the idle relief and main ports water that is standing in the the main exhaust port (at idle) is kept from entering the relief ports and restricting exhaust flow.

If water flow from the main port to the idle bypass is the only issue, I doubt that it will cause me a problem--the potential leak gap is too small to allow significant amounts of water to enter against the engine exhaust pressure.

If necessary I can weld up the old idle bypass ports to match the new pipe, but maybe it doesn't matter?

Any opinions on this?

Thanks,

Steve​
 
Steve.... me in blue text........

CONCERN:
The old 3" Y has much larger idle relief ports, while the 4" pipe has a slightly larger main port. This means that when the 4" pipe is bolted to the old transom plate the ports almost overlap and the narrow land separating the idle relief and main ports will be reduced from about 5/16" to only 1/16".

Just to be clear.... the Y-pipe fastens to the Gimbal housing, not the inner transom plate.

QUESTION:
What happens if there is a leakage path between the main and relief ports?
Nothing happens if the leak is restricted to a path between the the two relief ports and main exhaust opening.
The low speed exhaust will take multiple paths, favoring the path or paths (plural) of least resistance.
But you certainly want the main ports to align correctly...... or you may create an exhaust restriction, causing performance issues.

However, IF these reliefs ports do not alilgn and/or seal correctly, you may end up with a water leak at the inside of the Gimbal housing where the Y-pipe mates to the housing.

Why don't you also replace the Gimbal housing with the correct unit that the larger Y-pipe belongs with?
Any and all issues would then be resolved.



I understand that the idle relief ports are designed to reduce back pressure at idle, by allowing exhaust to exit at the waterline.
Correct!

However, this could be accomplished by just having one large passage in the Y-pipe connect to all three transom plate ports (Main and 2 idle).
Not sure if any do this. Seems like there'd be a conflict at higher speeds.

I do not fully understand why Merc took the trouble to cast separate idle relief ports and exhaust passages into the Y-pipe so that the relief gases are separated well upstream of the transom plate. I am guessing that this might be to keep the exhaust velocity up in the relief ports in order to keep lake/sea water cleared of the exhaust ports. By separating the idle relief and main ports water that is standing in the the main exhaust port (at idle) is kept from entering the relief ports and restricting exhaust flow.
I believe that when the Y-pipe design changed, so did the Gimbal housing design change.......... the Merc boys can verify this for you.


If water flow from the main port to the idle bypass is the only issue, I doubt that it will cause me a problem--the potential leak gap is too small to allow significant amounts of water to enter against the engine exhaust pressure.
This is not only water flow, it's mixed exhaust and water flow.

If necessary I can weld up the old idle bypass ports to match the new pipe, but maybe it doesn't matter?
These need to match as per what the OEM designed.



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