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
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