Compression for 1998 350 mag mpi's with 330 hours

[powerguy]

any advice as to what the compression should be for healthy motors. How long do these motors have until a rebuild is needed?

[powerguy]

ttt

[Cord]

Didn't you have a thread on this in the General Discussion? If not, then there is one very simular to it that will answer all your questions and then some.

[Reed Jensen]

Compression numbers vary depending on compression ratio... I'm guessing your motors are 8.5 to 1 compression.... should have new around 125 lbs inches/2 . Cylinder pressures should not vary more than 10 percent... better to do a "leak down" test. More time consuming... but will tell you more accurately what has happened to the motors in the last 300 hrs. A standard compression test will tell you quickly if you have a dead cylinder ," low or no compression". So really the exact numbers don't mean that much... if all seven of the cylinders are 110 lbs and one is 65lbs... you know there is a problem with that one cylinder. I've had customers bring me motors that are idling rough,,, and request a "carb adjustment" to smooth out the engine... I do a quick compression test and find a dead cylinder... Sorry... carb adjustment won't smooth out that dog.

[powerguy]

Just got compression numbers from cold motor that had been fogged and winterized. The numbers were:

Port:
210
185
200
200
200
180
200
195

Starboard:
205
210
205
205
200
200
200
200

These compression numbers seem high compared to what I was expecting to see. Do these make sense?
Thanks.

[rchevelle71]

Slightly higher than what I got an my 1992 260-5.7

mine were more like 165-180

[rmbuilder]

Both those sets of numbers are right where they should be. The 5.7/260 hyd-flat cam (Merc # 431-5943) has 258* intake duration @ .006. The 350 Mag hyd-roller cam (Merc # 431-811658) has 249* intake duration @ .006. The shorter duration of the Mag cam has an earlier intake closing angle of 50.5* ABDC than the 260 cam with an intake closing angle of 57* ABDC. The later intake closing of the 260 cam bleeds of more cylinder pressure than the mag cam giving somewhat reduced cranking compression (cylinder pressure) numbers. What your readings reflect is closer to dynamic compression rather than static. As long as they are consistent it’s all good.
Bob

[rchevelle71]

Greek to me, but as long as you say its good!!!

[powerguy]

Thanks rmbuilder