7.4 Horizon 380hp compression test numbers on '01 Sea Ray 340

low944

New Member
Sep 2, 2018
4
Boat Info
2001 340 Sundancer
Engines
7.4 Horizon
Hello all,
I am in the process of buying an 01 340 with 7.4 Horizon 380hp engines. I got a compression test done today and below are the results. They seem kinda high to me and the one cylinder that is lower than the rest (150) is concerning me. This boat has done a lot of sitting in the last few years. It has always been in fresh water in the upper Potomac. We did a sea trial on it a week ago and the compression test was done today. Not sure if it was done after running it or cold. On the sea trial the starboard motor only got up to 4400 and the port got up to 4800 at WOT. Both motors have 375 hrs. They also have the hybrid exhaust and look to have the baffle / resonator in all 4 exhausts. What do you guys think? ...

cylinder - psi
starboard
1 - 160
2 - 170
3 - 160
4 - 150
5 - 160
6 - 170
7 - 170
8 - 175

port
1 - 175
2 - 160
3 - 175
4 - 165
5 - 160
6 - 160
7 - 165
8 - 175
 
Those are actually good number and still in range for what is considered very acceptable. I just had a compression check done on my 7.4 380 HP's and it was 160 across the board with 590 hours.
 
Thanks frtbroker. I'm sure you have heard of the water reversion issues with these motors. That's in the back of my mind while looking at these numbers. What exhaust system do you have on yours? Water lift mufflers?
 
Hello all,
I am in the process of buying an 01 340 with 7.4 Horizon 380hp engines. I got a compression test done today and below are the results. They seem kinda high to me and the one cylinder that is lower than the rest (150) is concerning me. This boat has done a lot of sitting in the last few years. It has always been in fresh water in the upper Potomac. We did a sea trial on it a week ago and the compression test was done today. Not sure if it was done after running it or cold. On the sea trial the starboard motor only got up to 4400 and the port got up to 4800 at WOT. Both motors have 375 hrs. They also have the hybrid exhaust and look to have the baffle / resonator in all 4 exhausts. What do you guys think? ...

cylinder - psi
starboard
1 - 160
2 - 170
3 - 160
4 - 150
5 - 160
6 - 170
7 - 170
8 - 175

port
1 - 175
2 - 160
3 - 175
4 - 165
5 - 160
6 - 160
7 - 165
8 - 175
Compressions look pretty good. I had the 7.4L 380HP Horizons in my 2000 340. I cannot find the tech bulletin right now but IIRC the difference specified by Mercury Marine was 20% between top and bottom compression.

I also think the RPM number to achieve is 4600 so you might want to check the starboard motor again.
 

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