Normal Compression Range

chaslanier

Member
Mar 7, 2017
34
Lake Lanier GA
Boat Info
2006 Sundancer 320
Engines
Twin 350 MAG w/V Drives
What would be a normal compression range for Mag 350's? Mine on boat motors are all over the board.
Port
1- 140, 2-150, 3-110, 4-110, 5-120, 6-120, 7-130, 8-145
Starboard
1-150, 2-140, 3-120, 4-120, 5-110, 6-100, 7-150, 8-140

Any thoughts! Suggestions? Recommendations?
 
What were the circumstances when the tests were done. Has the boat been sitting for most of the season? Were the test conducted with the engines having been taken out on a substantial run that includes a few minutes ate full rpm?
 
When was the last time you inspected your risers and gaskets? Each adjacent cylinder in the middle of the head indicates you more than likely have a leaky gasket/riser.
 
When was the last time you inspected your risers and gaskets? Each adjacent cylinder in the middle of the head indicates you more than likely have a leaky gasket/riser.
I haven't since purchasing the boat in 2016. The motors were rebuilt and prior to me purchasing it. It also appears that the risers and manifolds were replaced then.
 
If the performance of the boat has dropped IE top speed then your engines may need work. If you do not run your boat at top speed every once in a while it could be just carboned upped. If you do a compression test with a warm engine you will get different results than with a cold. Consistency is in my opinion more important than high values. If the entire engines were rebuilt IE rings, bearings and heads they should be OK unless you got a leaking manifold. Rises are about $300 an engine. Complete manifold about $600 an engine. Remove a riser and see what it looks like. It is held on by four bolts. Drain the raw water first.
 
Where did you get these prices on dry joint manifolds and risers?
 

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