Mercruiser 5.7 vortec compression

Andy456

Cadet
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May 5, 2019
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12
Hi guys some input would be great and possibly put my mind at ease. My 5.7 vortec motor has had a complete rebuild and after putting around 15hrs on it now to break it in I’ve just pulled the oil and checked the compressions and all 8 cylinders are 150psi my question is the vortec engine my higher compression than that?? Engine runs great apart from that.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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even the mercruiser manuals used to state 150 psi for cranking compression readings
 

Scott Danforth

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only if you fill the cylinders with water or oil first
 

scoflaw

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Jun 2, 2010
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962
Thanks for your reply’s, I always thought the should be around 175psi

Yup, agree with that. I've had vortec heads cracked between the inner exhaust valves 4,6 n 3,5 and those inner cylinders were 150. the outside cylinders were 175. Seen them in the 185 range as well on good running engines
 

NHGuy

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May 21, 2009
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A different cam and different oil will raise that number too. Boats need conservative cams.
 

Andy456

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May 5, 2019
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If my valves were not adjusted correctly would this cause compressions to be low? I did them with the engine running and adjusted the valves a half turn after the rockets started clacking??
 

silver_power

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Jul 8, 2009
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314
If the valves are tight adjusted enough to loose compression, then you can easily find it if the motor cranking rpms increase and desincrease.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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If my valves were not adjusted correctly would this cause compressions to be low? I did them with the engine running and adjusted the valves a half turn after the rockets started clacking??

at 150 PSI you are within spec and not low.

so what is the problem?

if you have a compression reading above 150, check your gauge or you have water or oil in the cylinder.
 
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